Author Topic: Scope and Probes - Measuring Crystal  (Read 2382 times)

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Offline WanaGo

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Scope and Probes - Measuring Crystal
« on: August 16, 2019, 02:02:16 am »
Hi,

Bit of a noob question, but I am wanting to measure/visualise the oscillations of a number of crystals on some products we have, to try and tune the optimal capacitors for those crystals. Things were designed some years ago, and over the years the suppliers have changed, but the loading caps have never changed, and I am wanting to check to ensure we are using the right caps for the crystals we now use.

I have a Hantek DSO-1152S scope, 150Mhz (hacked to be a DSO-1202S thanks to this forum).
I have the standard 1x/10x probes, and when looking at the output side of the xtal on the 10x setting, it seems to load the crystal somewhat, and instead of showing 12Mhz crystal, it's much lower. On some boards, it's around 460Hz and some are 1.2Khz.
I am guessing this is from the capacitance of the probes?
Will a 100x probe solve this ?
Is there such a thing as a cheap 100x probe? I don't need it for high voltage etc, its purely for low power things like this.

Thanks
 

Offline tautech

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Re: Scope and Probes - Measuring Crystal
« Reply #1 on: August 16, 2019, 02:32:10 am »
I am guessing this is from the capacitance of the probes?
Yep.
Quote
Will a 100x probe solve this ?
Partially as there still will be some tip capacitance just less so.
Quote
Is there such a thing as a cheap 100x probe? I don't need it for high voltage etc, its purely for low power things like this.
I've got some Pintek 100x 300MHz probes for $75 + GST
http://www.pintek.com.tw/files/pintek/CP-3308R-spec.pdf

Send PM if interested.
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Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #2 on: August 16, 2019, 03:28:55 am »
I am guessing this is from the capacitance of the probes?
Yep.
Quote
Will a 100x probe solve this ?
Partially as there still will be some tip capacitance just less so.
Quote
Is there such a thing as a cheap 100x probe? I don't need it for high voltage etc, its purely for low power things like this.
I've got some Pintek 100x 300MHz probes for $75 + GST
http://www.pintek.com.tw/files/pintek/CP-3308R-spec.pdf

Send PM if interested.

Thank you.
$75 + GST is in NZD I assume, you are local to me it seems - nice

So if 100:1 is better but still not perfect, what is the ideal solution for this sort of measurement?

Thanks
 

Offline KaneTW

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Re: Scope and Probes - Measuring Crystal
« Reply #3 on: August 16, 2019, 03:39:35 am »
Intuitively, I'd impedance match for the crystal frequency so that the complex impedance component cancels out.
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #4 on: August 16, 2019, 03:55:33 am »
Hi,

Bit of a noob question, but I am wanting to measure/visualise the oscillations of a number of crystals on some products we have, to try and tune the optimal capacitors for those crystals. Things were designed some years ago, and over the years the suppliers have changed, but the loading caps have never changed, and I am wanting to check to ensure we are using the right caps for the crystals we now use.

I have a Hantek DSO-1152S scope, 150Mhz (hacked to be a DSO-1202S thanks to this forum).
I have the standard 1x/10x probes, and when looking at the output side of the xtal on the 10x setting, it seems to load the crystal somewhat, and instead of showing 12Mhz crystal, it's much lower. On some boards, it's around 460Hz and some are 1.2Khz.
I am guessing this is from the capacitance of the probes?
Will a 100x probe solve this ?
Is there such a thing as a cheap 100x probe? I don't need it for high voltage etc, its purely for low power things like this.

Thanks

A while back I was fixing something with an intermittent starting crystal oscillator just under 5 MHz.  Granted, my oscillator was weak to begin with, but I couldn't scope it with anything without killing it, even after I had repaired it to work reliably.  Finally, I tried my 100X 100M (not all 100X are 100M...) 4pf probe from eBay--which is surprisingly good-- and it worked, or at least it didn't kill the oscillator.  However, that's nowhere near the goal of not significantly loading the circuit.  I was just trying to tell the difference between working or not. If you want an accurate picture, even just connecting the ground lead will significantly change the circuit, let alone the probe.  To optimize and adjust, you will need a very low capacitance, as in <1pf, active differential probe like the Tek P6247. 
 

Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #5 on: August 16, 2019, 04:06:27 am »
Finally, I tried my 100X 100M (not all 100X are 100M...) 4pf probe from eBay--which is surprisingly good-- and it worked, or at least it didn't kill the oscillator.  However, that's nowhere near the goal of not significantly loading the circuit.  I was just trying to tell the difference between working or not. If you want an accurate picture, even just connecting the ground lead will significantly change the circuit, let alone the probe.  To optimize and adjust, you will need a very low capacitance, as in <1pf, active differential probe like the Tek P6247.

Thanks. Yeah, seems it is all down to the capacitance of the probe. The one Tautech offered above looks to be 5.5pF (http://www.pintek.com.tw/files/pintek/CP-3308R-spec.pdf), which is lower than some of the other ones I have seen locally which were all well over 10pF. I cant justify spending big money on an Active probe though, and that Tek P6247 looks like crazy money.

Thanks for your help
 

Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #6 on: August 16, 2019, 04:07:27 am »
Intuitively, I'd impedance match for the crystal frequency so that the complex impedance component cancels out.

Sorry can you say that again in layman's terms for me :) Thanks
Im not quite sure what to do with that information, but thank you
 

Offline tautech

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Re: Scope and Probes - Measuring Crystal
« Reply #7 on: August 16, 2019, 04:12:12 am »
I am guessing this is from the capacitance of the probes?
Yep.
Quote
Will a 100x probe solve this ?
Partially as there still will be some tip capacitance just less so.
Quote
Is there such a thing as a cheap 100x probe? I don't need it for high voltage etc, its purely for low power things like this.
I've got some Pintek 100x 300MHz probes for $75 + GST
http://www.pintek.com.tw/files/pintek/CP-3308R-spec.pdf

Send PM if interested.

Thank you.
$75 + GST is in NZD I assume, you are local to me it seems - nice

So if 100:1 is better but still not perfect, what is the ideal solution for this sort of measurement?

Thanks
Yep NZD, replied via your email.

Best solution is a active probe......but cost $ $ %
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Offline David Hess

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Re: Scope and Probes - Measuring Crystal
« Reply #8 on: August 16, 2019, 04:19:10 am »
Is there such a thing as a cheap 100x probe? I don't need it for high voltage etc, its purely for low power things like this.

There is something even better, an RF JFET like a 2N4416.  Design it into the circuit as a source follower to buffer the signal to the oscilloscope probe.  Or build a low capacitance active probe like that shown below.

But better is to measure the signal at the output of the amplifier where a typical probe will have no effect.  Besides frequency, the startup time after power is applied will give some idea of the operating margin.
 
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Offline vk6zgo

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Re: Scope and Probes - Measuring Crystal
« Reply #9 on: August 16, 2019, 04:21:21 am »
I've found with discrete crystal oscillators that just resting the probe tip on the outside of the crystal is enough for quite a good display.
This should have very little loading effect ------whether it works with packaged oscillators, I don't know.

In analog stuff, there is usually a "buffer" stage following the crystal, & with digital circuitry, there will sutely be places where the clock, (or some derivative of it) is isolated from the oscillator, & which can be probed without affecting it.
 

Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #10 on: August 16, 2019, 04:38:57 am »
These circuits are simple SMT 4 pad 12Mhz crystals with currently 2x 22pF caps to Gnd, and crystal directly into a microcontroller of sorts, that run between 40Mhz and 80Mhz, depending on the product they are in.

Based on the reading I have done, the loading caps (currently 22pF) should be selected based on a formula similar to
Caps = 2x CL - Cstray
Where Cstray is somewhere between 2-5pF, and CL comes from the Crystal datasheet.
I then found variations on this, where its 2x CL - 2x Cstray
And then another variation which ends up much the same as the second one, which is CL = ((Cap x Cap) / (Cap + Cap)) + Cstray

Either way, the current values we have been using I am imagining are on the fridge of being correct now, and are not ideal at all. Assuming those formulas are anything like being correct.

22pF was selected way back in 2010 and the supplier of the crystals has changed a few times, and staff also have changed, so the spec has somewhat slipped.

Looking on the scope, one product the scope loads down to about 400Hz instead of 12Mhz, but the sine wave is beautiful. Another one also loads it down about the same, but the Sine wave is not very Sine wavey at all. Another product it loads it down to 1.2Khz and is also beautiful. So I really just want to see what its like when its running at 12Mhz if I can, to hopefully then be able to judge which is a 'better' cap, if we have a few variants side by side and view them all.

Money is an issue, and so is time, so I would prefer to be able to just get a 100x probe off the shelf and quickly see if we are in the right zone. Active probe is cost-prohibitive, as is building a circuit I think (although this would be neat to try next if the probe doesnt do what I want). Because its all low voltage I deal with, many of these probes seem specific to HV stuff, which I guess is where the $$ comes in.
« Last Edit: August 16, 2019, 04:42:03 am by WanaGo »
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #11 on: August 16, 2019, 05:01:59 am »
You are saying your 12MHz oscillator is stable at 400Hz with the scope probe connected???   :scared:
 

Offline vk6zgo

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Re: Scope and Probes - Measuring Crystal
« Reply #12 on: August 16, 2019, 05:15:53 am »


Looking on the scope, one product the scope loads down to about 400Hz instead of 12Mhz, but the sine wave is beautiful. Another one also loads it down about the same, but the Sine wave is not very Sine wavey at all. Another product it loads it down to 1.2Khz and is also beautiful. So I really just want to see what its like when its running at 12Mhz if I can, to hopefully then be able to judge which is a 'better' cap, if we have a few variants side by side and view them all.

Please tell me that what you wrote there is a typo, & what you meant was " the scope loads down by "400Hz" rather than "to 400Hz"!

And again, " by 1.2kHz", rather than " to 1.2kHz"!

A frequency shift of 400Hz or even 1.2kHz at 12MHz is possible, but if you are seeing an actual signal at 400Hz or 1.2kHz, either the xtal isn't oscillating, & you are seeing some other random signals, or you have major aliasing problems with a DSO.
 
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Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #13 on: August 16, 2019, 05:53:01 am »
Yep wasnt a typo. Auto freq was showing 400hz and 1.2Khz.
I need to test this again as you are right, that is rubbish.
These are on the standard probes, of which I have no idea what the capacitance is, likely 20-40pF
 

Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #14 on: August 16, 2019, 06:01:21 am »
Yeah not sure what happened there, its 12Mhz now on the 10x probe.
I found an older 100x probe which I will test out tomorrow and see how it goes
 

Offline MosherIV

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Re: Scope and Probes - Measuring Crystal
« Reply #15 on: August 16, 2019, 07:10:34 am »
Hi. As vk6zgo said, it is better to measure the buffered output of the xtal oscillator.
Some micros have a dedicated gpio that can be set to output the crystal frequency.

If it does not you just toggle a gpio pin either in code (you will need to work out the number of assembley instructions to work out number clock cycles between each toggle) or use a timer/counter peripheral being clocked by the xtal.

This will of course need some special test code.
 

Offline mjs

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Re: Scope and Probes - Measuring Crystal
« Reply #16 on: August 16, 2019, 07:36:09 am »
Oscillator output is a good place to measure without loading the crystal. I've also used HP 3586C selective level meter to measure oscillators inside encapsulated devices. 32.768kHz RTC crystal can easily be received with a piece of wire or small coil.
 

Offline Ice-Tea

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Re: Scope and Probes - Measuring Crystal
« Reply #17 on: August 16, 2019, 04:30:44 pm »
Are you aware your measurements are mostly meaningless anyway as your scope has a Timebase Accuracy of ±50ppm?

In general your better of with a field probe so you can up the frequency without loading the circuit.
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Offline fourfathom

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Re: Scope and Probes - Measuring Crystal
« Reply #18 on: August 16, 2019, 05:49:19 pm »
I use a spectrum analyzer and a little wire coil probe (a few turns about 1/16" diameter) soldered on a SMA connector, or at the end of the coax cable.  Just bring it in close to the xtal traces and I can measure frequency without significantly loading the circuit.
 

Online CDaniel

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Re: Scope and Probes - Measuring Crystal
« Reply #19 on: August 16, 2019, 08:09:04 pm »
12MHz is not that high , I can measure this with no problems using a Siglent SDS1202X-E and 200MHz probes , clean signal on both pins ... That microcontroller oscillator is weak or the quartz needs other capacitors values . Some microcontrollers have  fuse settings for the oscillator proper operation ... Or you should check if your oscilloscope is to blame and the oscillator is actually still running. Even with an old 20MHz analog oscilloscope I don't remember having problems with microcontroller oscillators  :-// These oscillators don't die so easily if you measure the frequency .
« Last Edit: August 16, 2019, 08:45:14 pm by CDaniel »
 

Offline vk6zgo

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Re: Scope and Probes - Measuring Crystal
« Reply #20 on: August 17, 2019, 01:49:24 am »
Yep wasnt a typo. Auto freq was showing 400hz and 1.2Khz.
I need to test this again as you are right, that is rubbish.
These are on the standard probes, of which I have no idea what the capacitance is, likely 20-40pF

At the risk of sounding "snotty", based on your revealed knowledge, & the probable limitations of your DSO1152s, I don't think you should be contemplating making changes to equipment which is working OK as it is.

Everybody, except bdunham7, read your "400Hz & 1.2kHz" as variations of the nominal 12MHz frequency, because that was the obvious, logical conclusion.
Some still haven't caught on, & are posting good ideas to minimise any "loading" causing such error.

Honestly, you should have immediately realised that a few tens of pF loading could not cause a frequency change of four orders of magnitude, & that your DSO was lying to you!

In fact, nothing that you could do would cause a 12MHz oscillator of any kind to oscillate stably at 400Hz or 1.2KHz.

There are circuits which oscillate at audio & RF frequencies simultaneously, but they are designed that way.
It doesn't "just happen!"
 
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Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #21 on: August 17, 2019, 03:10:12 am »
If it is about "tuning" frequency of crystal oscillators, only way of probing is JFET probe. DIY is fine, just do not mess-up input tip capacitance. You *NEVER* shall measure "IN" pin of the oscillator, only "OUT". Use short as possible connection to nearest ground (ground spring clip) to avoid interference pickup by probe and "feedback" into oscillator.

If I would need to check oscillators, I would make special firmware which output XTAL frequency on other than oscillator, pin. If no access to firmware then I would make this probe design:

https://www.instructables.com/id/DIY-1GHz-Active-Probe-for-Under-20/

« Last Edit: August 17, 2019, 03:12:10 am by ogden »
 

Offline vk6zgo

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Re: Scope and Probes - Measuring Crystal
« Reply #22 on: August 17, 2019, 03:45:27 am »
One way to determine if your crystal oscillator has any discernible frequency change when you probe it with a normal probe is as follows :-

(1) Obtain a HF communications receiver.

(2) Attach a loose wire to the receiver's antenna input, & rest the other end fairly near the oscillator
Note:- this is just an antenna, & is not directly electrically connected to the oscillator wiring.

(3) As radios are highly sensitive, they don't need a lot of pickup, so your "antenna" will normally be far enough away from the oscillator circuitry that it doesn't add any significant loading.

(4) Assuming the OP's 12MHz frequency, tune the radio to that frequency with its mode set  to CW, USB or LSB.
As you approach the frequency, you will hear a high pitched tone, dropping in frequency as you get closer to the oscillator frequency, eventually being so low in frequency as to become inaudible--this is "zero beat"

(5) Now probe your oscillator--- any change in its frequency will be shown by the presence of an audible tone, as the radio will no longer be "zero beat".
 
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Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #23 on: August 17, 2019, 04:02:44 am »
I think we may be missing the point here.  I think the OP is concerned about the reliability of the oscillator, not its precise frequency.  The load capacitors will change many characteristics, including the voltage across the crystal and the startup time, or if they are off enough, they may prevent reliable startup at all.  I think you would select the capacitors by starting with the datasheets and a little math, then doing final testing with various values and testing both the crystal voltage to make sure it isn't too high and then the startup characteristics over the range of temperatures you expect the device to be subjected to.  If you want to measure and view those things, you need a probe such as the one I mentioned--and even that is going to have at least a small effect. 

This isn't tuning a transmitter.  How accurate do you thing the average internal MCU oscillator circuit is anyhow, even with a proper crystal? 
« Last Edit: August 17, 2019, 04:06:56 am by bdunham7 »
 

Offline Kean

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Re: Scope and Probes - Measuring Crystal
« Reply #24 on: August 17, 2019, 04:15:53 am »
How about one of these probes?
https://www.ebay.com/itm/332393676082
 
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Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #25 on: August 17, 2019, 04:20:28 am »
I think we may be missing the point here.  I think the OP is concerned about the reliability of the oscillator, not its precise frequency.  The load capacitors will change many characteristics, including the voltage across the crystal and the startup time, or if they are off enough, they may prevent reliable startup at all.

If you are concerned about reliability of oscillator due to load capacitors - you badly messed-up something. Usually you just open datasheet of the crystal and know load capacitor values. You want to measure if you have either quality concerns of crystal, doubt about drive strength or want to check/tune frequency.
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #26 on: August 17, 2019, 04:31:29 am »
How about one of these probes?
https://www.ebay.com/itm/332393676082

Um, yeah!  $12?  I'm going to have to get one.  I suspect calibration will be an issue, but for many applications--including the one in this thread-- 10% is plenty good.

EDIT:  It's not differential.  Thats going to be an issue because you generally can't ground either side of an XO circuit....
« Last Edit: August 17, 2019, 05:25:58 am by bdunham7 »
 
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Offline vk6zgo

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Re: Scope and Probes - Measuring Crystal
« Reply #27 on: August 17, 2019, 05:53:08 am »
I think we may be missing the point here.  I think the OP is concerned about the reliability of the oscillator, not its precise frequency.  The load capacitors will change many characteristics, including the voltage across the crystal and the startup time, or if they are off enough, they may prevent reliable startup at all.  I think you would select the capacitors by starting with the datasheets and a little math, then doing final testing with various values and testing both the crystal voltage to make sure it isn't too high and then the startup characteristics over the range of temperatures you expect the device to be subjected to.  If you want to measure and view those things, you need a probe such as the one I mentioned--and even that is going to have at least a small effect. 

This isn't tuning a transmitter.  How accurate do you thing the average internal MCU oscillator circuit is anyhow, even with a proper crystal?

If the OP is worried about the reliability of the oscillator, surely "the proof of the pudding is in the eating"?
The thing has been out in the market for some time --have they had complaints about the reliability with the later crystal type?

In any case, crystal manufacturers usually give suggested values for components like the "load" capacitors.
If they suggest a different value--- change to that value!

As I pointed out earlier in a more diplomatic manner, the OP doesn't seem to have the first clue about what he/she is trying to achieve.
It certainly looked like they are concerned with frequency stability, but the fact that they thought 400Hz & 1.2kHz were possible outcomes :palm: raises a serious doubt.

Ultimately, as you pointed out, it is questionable how accurate the thing will be, or needs to be, for that matter.
By & large  crystal oscillators are not particularly critical, & millions of them operate reliably every second of the day.
 
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Offline Kean

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Re: Scope and Probes - Measuring Crystal
« Reply #28 on: August 17, 2019, 06:16:00 am »
The OP made it clear that it was a "noob" question, and they were looking for some advice.  It sounds like they've inherited a design and they want to learn how to ensure it is still appropriate with various component/supplier changes.  Seems like a reasonable question, and we've all had to learn things like this ourselves in the past.  No need to get rude.
 
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Offline MosherIV

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Re: Scope and Probes - Measuring Crystal
« Reply #29 on: August 17, 2019, 08:21:38 am »
One thing that can be done is to have the xtal manufacturer verify the design will work over a wide range of evironmental conditions. Need to supply the relevant part of the schematic and samples of the circuit.

It is something done for the automotive industry, I never knew this until I join my current company and the electronics engineers told me it is standard practise for xtals used in automotive designs.
No idea how much it costs.
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #30 on: August 17, 2019, 02:56:54 pm »
EDIT:  It's not differential.  Thats going to be an issue because you generally can't ground either side of an XO circuit....

There's no need for differential probe to measure crystal oscillator. Idea that part of the circuit must be grounded for single-ended probe is bizarre  :-BROKE

Here's Epson tech papers regarding crystal oscillator measurements:

https://www5.epsondevice.com/en/information/technical_info/pdf/tech_notes_e201302.pdf
https://www5.epsondevice.com/en/information/technical_info/pdf/tech_notes_e_oscillator_circuit_evaluation_method_2.pdf
 
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Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #31 on: August 17, 2019, 07:43:09 pm »
EDIT:  It's not differential.  Thats going to be an issue because you generally can't ground either side of an XO circuit....

There's no need for differential probe to measure crystal oscillator. Idea that part of the circuit must be grounded for single-ended probe is bizarre  :-BROKE

Here's Epson tech papers regarding crystal oscillator measurements:

https://www5.epsondevice.com/en/information/technical_info/pdf/tech_notes_e201302.pdf
https://www5.epsondevice.com/en/information/technical_info/pdf/tech_notes_e_oscillator_circuit_evaluation_method_2.pdf

I've read other such papers.  Putting a current probe on the xtal is inconvenient at best in a repair situation or when evaluating an already assembled circuit.  The same goes for bodging in test resistors, although to be fair, since the OP is arguably in the design/redesign phase, this might not be an unreasonable way to test.  As far as grounding, are you proposing not using the probe ground lead?  If not, where do you propose connecting it?  Some XOs, like the last one I worked on, are in isolated circuits.  Every circuit is different and IMO the easiest, quickest way of measuring startup and drive level characteristics would be an active differential probe right across the crystal.  I'm not sure where your "bizarre" comment comes from--a single ended probe connected to a regular oscilloscope has a ground lead that when connected will ground that part of the circuit.  Now it may be the case that many XOs can be measured easily with a single-end low capacitance probe, but I assure you that some can't.
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #32 on: August 17, 2019, 09:01:35 pm »
As far as grounding, are you proposing not using the probe ground lead?  If not, where do you propose connecting it? Some XOs, like the last one I worked on, are in isolated circuits.  Every circuit is different and IMO the easiest, quickest way of measuring startup and drive level characteristics would be an active differential probe right across the crystal.  I'm not sure where your "bizarre" comment comes from--a single ended probe connected to a regular oscilloscope has a ground lead that when connected will ground that part of the circuit.  Now it may be the case that many XOs can be measured easily with a single-end low capacitance probe, but I assure you that some can't.

Any XO circuit needs power, it has positive supply rail and negative supply rail which usually is considered as ground in unipolar circuits. So any XO already has ground where it's signals are referenced-to. Connect ground clip of your probe there. Saying "you generally can't ground either side of an XO circuit" does not even makes sense or we got some strange language barrier problem here. [edit] By grounding any side of XO I understand - short it's output or input to ground. Obviously such suggestion is bizarre to me.

Most of the oscillators are either Colpitts or cmos inverter topology, boths has grounds, both can be measured using fet probe. No need for fancy FET differential probes I am not sure even exist. Could you show pointers to differential probe you are talking about?
« Last Edit: August 17, 2019, 09:07:35 pm by ogden »
 

Offline David Hess

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Re: Scope and Probes - Measuring Crystal
« Reply #33 on: August 17, 2019, 09:47:17 pm »
One thing that can be done is to have the xtal manufacturer verify the design will work over a wide range of evironmental conditions. Need to supply the relevant part of the schematic and samples of the circuit.

The most common problem is the gain element used for the oscillator.  CMOS gate oscillators are especially problematical because MOSFET transconductance on a digital process is not well controlled and batches of microcontrollers have been occasionally released which would not start at low temperatures.

This makes oscillators using discrete transistors more reliable but it is a good idea to test operation beyond the full temperature range.
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #34 on: August 18, 2019, 01:33:00 am »
As far as grounding, are you proposing not using the probe ground lead?  If not, where do you propose connecting it? Some XOs, like the last one I worked on, are in isolated circuits.  Every circuit is different and IMO the easiest, quickest way of measuring startup and drive level characteristics would be an active differential probe right across the crystal.  I'm not sure where your "bizarre" comment comes from--a single ended probe connected to a regular oscilloscope has a ground lead that when connected will ground that part of the circuit.  Now it may be the case that many XOs can be measured easily with a single-end low capacitance probe, but I assure you that some can't.

Any XO circuit needs power, it has positive supply rail and negative supply rail which usually is considered as ground in unipolar circuits. So any XO already has ground where it's signals are referenced-to. Connect ground clip of your probe there. Saying "you generally can't ground either side of an XO circuit" does not even makes sense or we got some strange language barrier problem here. [edit] By grounding any side of XO I understand - short it's output or input to ground. Obviously such suggestion is bizarre to me.

Most of the oscillators are either Colpitts or cmos inverter topology, boths has grounds, both can be measured using fet probe. No need for fancy FET differential probes I am not sure even exist. Could you show pointers to differential probe you are talking about?

I agree that any XO circuit needs positive and negative supply and if you wish you can assert that convention implies the negative to be "ground".  However, that may be isolated from or at a different potential than other grounds, including the case of your DUT or the shell of the oscilloscope BNC connector.  Isolation isn't all that rare, especially in test instruments.  In one particular case, I was able to see the XO signal simply using a single 100x 100M probe with no ground lead.  However, to accurately measure the crystal drive voltage, you have to either know the particulars of the circuit and plan it out carefully--or use a low capacitance active differential probe.

They exist.

https://www.tek.com/datasheet/differential-probes-2

I've been stalking them on eBay, but they still go for big bucks unless you buy them as-is, which means someone probably burned it out--they're quite sensitive.
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #35 on: August 18, 2019, 07:01:21 am »
They exist.

https://www.tek.com/datasheet/differential-probes-2

I've been stalking them on eBay, but they still go for big bucks unless you buy them as-is, which means someone probably burned it out--they're quite sensitive.

It is widely known that differential probes exist. I did ask you differential FET probe. Hint: one with FET transistor input. TEK differential probes (P624*) are *not* FET probes because they have differential 200k input resistance and 100k common mode resistance (attach) - as soon as you connect such to your oscillator, it (oscillator) will stop operation.
« Last Edit: August 18, 2019, 08:25:26 am by ogden »
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #36 on: August 18, 2019, 05:14:32 pm »
They exist.

https://www.tek.com/datasheet/differential-probes-2

I've been stalking them on eBay, but they still go for big bucks unless you buy them as-is, which means someone probably burned it out--they're quite sensitive.

It is widely known that differential probes exist. I did ask you differential FET probe. Hint: one with FET transistor input. TEK differential probes (P624*) are *not* FET probes because they have differential 200k input resistance and 100k common mode resistance (attach) - as soon as you connect such to your oscillator, it (oscillator) will stop operation.

 I didn't say FET, although I do believe the P6247 does use them, just not in the high-impedance direct input configuration that typical single-ended active-FET probe might have.  I'm fully aware of the input impedances of these probes.  I am also aware of the significant voltage limitations, which is why the eBay ones may be commonly burned out.  I don't have one to try yet, but I don't think they will kill a typical XO because 200K is still much higher than the impedance of the small input capacitance at most XO frequencies I'm worried about.  It is also much less than the typical impedance of the crystal itself.  I think it would work fine for a ground-referenced or truly isolated circuit, but the common-mode voltage limitation would prevent using it on a circuit that was offset from ground unless you have a separate isolator and float the probe power supply.

As for the existence of a "high-impedance differential active FET" probe, I can't say I know of one.  I did have the idea of taking two of the devices that Kean posted and summing the outputs.  And that is what you would essentially need to build such a probe--a dual input active-FET and a summing circuit.  I'd be surprised if someone somewhere hasn't already made such a thing, but as I said, I've not seen one.  I suspect that it would not be easy to achieve a decent CMRR.
« Last Edit: August 18, 2019, 05:33:15 pm by bdunham7 »
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #37 on: August 18, 2019, 06:19:55 pm »
200K is still much higher than the impedance of the small input capacitance at most XO frequencies

 :palm: Before we continue this debate, you better show ANY mention of using differential probes for XO measurements in papers from reputable sources, preferably crystal/oscillator manufacturers. I did show Epson papers suggesting that only single FET probe and little brain cells needed to check XO.
« Last Edit: August 18, 2019, 06:59:44 pm by ogden »
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #38 on: August 18, 2019, 11:33:07 pm »
200K is still much higher than the impedance of the small input capacitance at most XO frequencies

 :palm: Before we continue this debate, you better show ANY mention of using differential probes for XO measurements in papers from reputable sources, preferably crystal/oscillator manufacturers. I did show Epson papers suggesting that only single FET probe and little brain cells needed to check XO.

Oh dear!  Papers from reputable sources?  I'm not that invested in this teapot tempest.   And as I read it, the paper you cited directed the use of a FET probe, a current probe which has to be installed in a test circuit and some math.  I know that method and I've read it before. I'm trying to substitute direct measurement across the xtal and knowledge of it's characteristics to avoid the current probe necessity in repair and analysis of the drive levels.  If I happen to acquire a suitable probe, I will try it and report back. I'm not going to write a paper, but I'll let you know either way! 
 

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Re: Scope and Probes - Measuring Crystal
« Reply #39 on: August 18, 2019, 11:46:21 pm »
If I happen to acquire a suitable probe, I will try it and report back.

Good luck with that. Before you invest into probe - solder some 100k load resistors to various crystals and see how it goes. Start with 32KHz tuning fork crystal.
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #40 on: August 19, 2019, 12:09:10 am »
If I happen to acquire a suitable probe, I will try it and report back.

Good luck with that. Before you invest into probe - solder some 100k load resistors to various crystals and see how it goes. Start with 32KHz tuning fork crystal.

How did we go from discussing a 12MHz MCU crystal to a 32kHz low-power unit with an ESR that is probably 1000X higher?  Yes, obviously I'd have to adapt my method--but it doesn't take much imagination to see a way to do it.
 

Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #41 on: August 19, 2019, 02:24:20 am »
I think we may be missing the point here.  I think the OP is concerned about the reliability of the oscillator, not its precise frequency.  The load capacitors will change many characteristics, including the voltage across the crystal and the startup time, or if they are off enough, they may prevent reliable startup at all.  I think you would select the capacitors by starting with the datasheets and a little math, then doing final testing with various values and testing both the crystal voltage to make sure it isn't too high and then the startup characteristics over the range of temperatures you expect the device to be subjected to.  If you want to measure and view those things, you need a probe such as the one I mentioned--and even that is going to have at least a small effect. 

This isn't tuning a transmitter.  How accurate do you thing the average internal MCU oscillator circuit is anyhow, even with a proper crystal?

Yeah I think a number of replies are going way too complex for what I am trying to do.

I am not suggesting I am the best at electronics or testing anything, and the fact I missed the 'glaringly obvious' fact that xtals cant work at 400hz or 1.2khz when they are meant to be 12Mhz, I apologise. You are superior than me.
Turns out I was looking too far out on the scope and had to 'zoom in' a bit, and the 12Mhz then came up fine. I did find it odd that it wasn't locking on to the 400hz very well, despite my trigger, and I guess it explains why.
I don't do this type of work daily.

I have a goal I am trying to reach and I am doing the best I can to try and achieve that goal.

We have systems which are all running fine. The 12Mhz isn't in question really, the products work. I am just trying to determine if the brand/model of crystals we have currently are running the best they can based on the loading capacitors in the circuit. That is it.
I know crystals work at quite a lot of values within reason, but then become problems when the temperature goes up or down, and other factors like that. Nearby noise etc.
We have had some problems in the past when our product is used in particular environments where there are high noise devices, and our products can go a bit weird. I am just trying to work out if a factor like our loading caps on the particular brand of xtal we are running at the moment, is contributing to these edge cases.

IF working it out from the xtal datasheet, is it a case of "Capacitor Value = 2x CL - 2x Stray" ? or something close to that sort of formula?

This is a 12Mhz xtal running a microprocessor. Not RF/Radio/Wizardry

Thanks everyone for trying to help, or point out how useless I am.
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #42 on: August 19, 2019, 04:19:23 am »
You are actually on the right track.  Datasheets only go so far.  Also, you need both the XTAL datasheet and the MCU datasheet and you have to know approximately what the stray capacitance of the circuit board is (hint: keep the traces absolutely as short as possible...)

I'm not really an expert and I'm learning too--I just fix things and have run into an occasional issue with this.  Some XO designs, especially MCUs, are not particularly robust.  The two tests I think matter most are the negative resistance test, where you add a series resistance until the oscillator fails, and measuring the drive level.  The idea is that you want the XO design to work even if a particular crystal is a bit high ESR or the MCU is a bit weak.  However, you don't want to overdrive the crystal or it won't last and the output will suffer distortion and phase noise.

I don't know how many products you are dealing with, but specifics might help--including closeup photos of the actual board layout.
 

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Re: Scope and Probes - Measuring Crystal
« Reply #43 on: August 19, 2019, 05:55:19 am »
Without knowing for sure how much the probes will effect the result , the best way to test it is to write a little program to toggle a pin with the maximum frequency possible and measure there ... You can change then the oscillator components and see the tolerances margin . Ok , you could buy some good active probes , or other equipment ,  but you need a time to learn how to use them properly ...

 

Offline MosherIV

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Re: Scope and Probes - Measuring Crystal
« Reply #44 on: August 19, 2019, 07:10:12 am »
Quote
the best way to test it is to write a little program to toggle a pin with the maximum frequency possible and measure there ...
Already mentioned that.

I already said that some microcontrollers have a gpio pin dedicated to outputting the oscilator so that you can see what freq the xtal is running at, you can probe this output with oscilloscope without affecting the xtal.
You have to write some code to enable that pin.
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #45 on: August 19, 2019, 08:16:21 am »
How did we go from discussing a 12MHz MCU crystal to a 32kHz low-power unit with an ESR that is probably 1000X higher?

We discussed crystal oscillator measurements, suggested methods that work for most of the crystals. Fact that you push towards HF 12MHz crystal to make your invention of using differential probe work, is your problem. I already said that - good luck with that.

OP needs just one thing - ensure that load capacitors have right capacitance. Easy way is to measure frequency of XO w/o loading it much with probe. If you have option to output 1:1 or divided frequency to dedicated pin - then you can use 1:10 generic scope probe and call it a day, considering that your scope have good enough frequency counter. Otherwise you need to use dedicated counter. If there is no pin to output MCU clock - you better use FET probe to measure XO output. You need to check couple of devices/crystals and ensure that they fall into specified frequency tolerance, their "ppm specs".

Drive strength measurements are needed when you need to find lowest possible, still stable drive strength in low power application. Obviously it would be every real-time clock that uses low frequency crystal (32KHz). When you use HF crystals and saving 1mW is not important - just drive your crystal at max as whole industry do, that's it.
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #46 on: August 19, 2019, 02:39:52 pm »
How did we go from discussing a 12MHz MCU crystal to a 32kHz low-power unit with an ESR that is probably 1000X higher?

We discussed crystal oscillator measurements, suggested methods that work for most of the crystals. Fact that you push towards HF 12MHz crystal to make your invention of using differential probe work, is your problem. I already said that - good luck with that.

OP needs just one thing - ensure that load capacitors have right capacitance. Easy way is to measure frequency of XO w/o loading it much with probe. If you have option to output 1:1 or divided frequency to dedicated pin - then you can use 1:10 generic scope probe and call it a day, considering that your scope have good enough frequency counter. Otherwise you need to use dedicated counter. If there is no pin to output MCU clock - you better use FET probe to measure XO output. You need to check couple of devices/crystals and ensure that they fall into specified frequency tolerance, their "ppm specs".

Drive strength measurements are needed when you need to find lowest possible, still stable drive strength in low power application. Obviously it would be every real-time clock that uses low frequency crystal (32KHz). When you use HF crystals and saving 1mW is not important - just drive your crystal at max as whole industry do, that's it.

The OP stated 12MHz and an MCU, IIRC.  My ramblings henceforward reflected this, that is something we call context. 

As for the rest of your statements, that is the exact thinking that has yielded products that work most of the time but sometimes they don't and nobody knows why.  If you are in the least concerned about XO reliability, you have to do the stability and drive level tests.  If you are stuck with a particular MCU and production changes, you may need to change the circuit or the crystal to get reliable operation, and if  you can't get reliable operation with a particular MCU under test conditions, you may need to use an external oscillator.  These are the things the OP needs to know, since he is clearly concerned with reliability beyond the "hey, it works OK" level.
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #47 on: August 19, 2019, 03:33:15 pm »
The OP stated 12MHz and an MCU, IIRC.  My ramblings henceforward reflected this, that is something we call context. 

Context is "Scope and Probes - Measuring Crystal" - subject of this thread.

Quote
If you are stuck with a particular MCU and production changes, you may need to change the circuit or the crystal to get reliable operation, and if  you can't get reliable operation with a particular MCU under test conditions, you may need to use an external oscillator.  These are the things the OP needs to know, since he is clearly concerned with reliability beyond the "hey, it works OK" level.

Right. Using unproven way of measuring crystal oscillators suggested by "expert" who can't find any 3rd party document of proposed method, nor confirm that he tested it himself is proper way of ensuring product reliability. Good luck with that.
 

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Re: Scope and Probes - Measuring Crystal
« Reply #48 on: August 19, 2019, 04:27:42 pm »
What exactly is method of measuring drive strength with differential probe? Please describe steps in enough details.
 

Offline David Hess

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Re: Scope and Probes - Measuring Crystal
« Reply #49 on: August 19, 2019, 05:55:33 pm »
What exactly is method of measuring drive strength with differential probe? Please describe steps in enough details.

You could use the differential probe to measure the supply current into the drive amplifier and calculate the crystal current it from there.  Trying to measure it directly with a current shunt would be an interesting exercise.
 

Online Howardlong

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Re: Scope and Probes - Measuring Crystal
« Reply #50 on: August 19, 2019, 06:09:01 pm »

They exist.

https://www.tek.com/datasheet/differential-probes-2

I've been stalking them on eBay, but they still go for big bucks unless you buy them as-is, which means someone probably burned it out--they're quite sensitive.

Although of little practical use for this thread, I did just pick up a P6330 for $200 on eBay (3.5GHz differential) on a punt... and it works! It did mean I needed to pick up a Tekprobe to TekVPI adapter, which cost more than the probe. Item was 123822379720, on a BiN price of $225.

Condition was the standard "item may have some signs of cosmetic wear, but is fully operational and functions as intended" but then stated "untested" in the body text. I suspect they had no means of testing it judging by their inventory.
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #51 on: August 19, 2019, 07:17:49 pm »
What exactly is method of measuring drive strength with differential probe? Please describe steps in enough details.

You could use the differential probe to measure the supply current into the drive amplifier and calculate the crystal current it from there.  Trying to measure it directly with a current shunt would be an interesting exercise.

Unfortunately there is no current measurement tap for oscillator of the MCU. Thou we may get some clue measuring MCU consumption in stop mode. If it consumes 1mW, then crystal drive can't he higher than that. According to @bdunham7 statements, FET probe does not do the job but differential probe allows to 1) accurately measure the crystal drive voltage 2) analysis of drive levels w/o bodging in additional resistors, so I am looking forward to see his comment.

Although of little practical use for this thread, I did just pick up a P6330 for $200 on eBay (3.5GHz differential) on a punt... and it works! It did mean I needed to pick up a Tekprobe to TekVPI adapter, which cost more than the probe. Item was 123822379720, on a BiN price of $225.

Wow, fine deal indeed. Congrats! BTW if possible, could you stick your diff.probe across 8...20MHz crystal and see how oscillator behaves in result? I say it will stop for <= 1MHz LF oscillators, but not sure about >= 8MHz HF.
 

Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #52 on: August 19, 2019, 09:09:54 pm »
Are we all talking about the same thing, or are some of you in some sort of pissing contest with each other to show who thinks they know more about something we are not even discussing here?

Please take your contest somewhere else.

Digesting what has been said, ill come back with another post if required.
Thanks for all the helpful responses.
 

Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #53 on: August 19, 2019, 11:23:48 pm »
So for an example of setting up a MCU with a 12Mhz Crystal, if I was to buy 2 different crystals, how would 'YOU' go about selecting the correct caps for this.

I acknowledge some of you have said to read the DS and use the values they state. From what I have seen in the last 5 datasheets I have read, they don't give you a cap value, they give you some specs of the crystal only.
As I have mentioned 2 or 3 times already, there are some formulas I have come across in order to select the cap value based on the crystal CL value, and whatever stray capacitance your circuit etc might have.
I dont believe anyone has commented on this. Does anyone have any input on this?
Here is one post on Adafruit blog, take what you will from it, but here is the source I found: https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/

So if we take 1 crystal for example. Abracon ABM8-12.000MHZ-20-D1-T3
This has a CL of 20pF
https://abracon.com/Resonators/abm8.pdf

What capacitors would you aim for on this?

Based on the formulas I have found, it would be somewhere between 35pF and 38pF.
Does this seem correct?

Another one, a chinese branded Yangxing Tech X322512MMB4SI
Found a DS here: https://datasheet.lcsc.com/szlcsc/Yangxing-Tech-X322512MMB4SI_C50430.pdf
This particular one has a CL of 10pF

What capacitors would you aim for on this one?
15-18pF?

Do these seem reasonable?
If they are not reasonable, can you please make a suggestion as to what you think is reasonable, as a place to start.

Trying to learn here.

Thanks
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #54 on: August 19, 2019, 11:29:57 pm »
Can you specify the MCU?
 

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Re: Scope and Probes - Measuring Crystal
« Reply #55 on: August 19, 2019, 11:39:18 pm »
For one of the products, its a PIC24EP128GP204
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #56 on: August 20, 2019, 12:01:27 am »
So there's an application note for that and it basically doesn't give Cin values and tells you to use the same formulas you already have.  Some MCUs specify a Cin (extal) and Cin (xtal) and you subtract that from your C1 and C2 values.  In this case it is all rolled into Cstray, I guess. So you are on the right track.

That MCU has a very complex set of oscillator modules and all sorts of configurations one might not see on other models.  Apparently there is even a register for setting the OSC GAIN value of the inverter.  That's good from one standpoint because you can more readily configure a robust product.  OTOH, it's much more complex...

Here's the application note, your info starts on page 21, although the whole thing is worth a read.

http://ww1.microchip.com/downloads/en/devicedoc/70005131a.pdf
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #57 on: August 20, 2019, 12:25:13 am »
That MCU has a very complex set of oscillator modules and all sorts of configurations one might not see on other models. Apparently there is even a register for setting the OSC GAIN value of the inverter.

Right. Oscillator module have very complex config of exactly one bit (mentioned OSC GAIN) which shall be set to HS mode for 12MHz crystal.
 

Offline David Hess

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Re: Scope and Probes - Measuring Crystal
« Reply #58 on: August 20, 2019, 03:03:34 am »
You could use the differential probe to measure the supply current into the drive amplifier and calculate the crystal current it from there.  Trying to measure it directly with a current shunt would be an interesting exercise.

Unfortunately there is no current measurement tap for oscillator of the MCU. Thou we may get some clue measuring MCU consumption in stop mode. If it consumes 1mW, then crystal drive can't he higher than that.

I have done it before but cheated by implementing the MCU's gate oscillator externally using comparable CMOS logic so I could measure the supply current to the driver.  My conclusion was what I stated earlier; discrete transistor crystal oscillators are more reliable than CMOS gate crystal oscillators.

Quote
According to @bdunham7 statements, FET probe does not do the job but differential probe allows to 1) accurately measure the crystal drive voltage 2) analysis of drive levels w/o bodging in additional resistors, so I am looking forward to see his comment.

The only way I have seen it done directly is with an AC current transformer.  Jim Williams wrote an application note about this method for the extreme case of a 32kHz crystal oscillator.

I think I could manage it with a custom low voltage differential probe and current shunt resistor in the straightforward way but with a not so straightforward low voltage differential probe.
 
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Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #59 on: August 20, 2019, 03:33:06 am »
So there's an application note for that and it basically doesn't give Cin values and tells you to use the same formulas you already have.  Some MCUs specify a Cin (extal) and Cin (xtal) and you subtract that from your C1 and C2 values.  In this case it is all rolled into Cstray, I guess. So you are on the right track.

That MCU has a very complex set of oscillator modules and all sorts of configurations one might not see on other models.  Apparently there is even a register for setting the OSC GAIN value of the inverter.  That's good from one standpoint because you can more readily configure a robust product.  OTOH, it's much more complex...

Here's the application note, your info starts on page 21, although the whole thing is worth a read.

http://ww1.microchip.com/downloads/en/devicedoc/70005131a.pdf

I think we are fine with the MCU configuration side of things, the processor itself is set up correctly and working fine. It's more case now of ensuring the external stuff is optimal so if it is to fail in the field at least its not related to the crystal.
I will however look more into the DS of this particular processor and see if there are any things which need modifying. Thank you.

Another part of the equation on one of our other products is the xtal output drives an NXP 74LVC1G08 gate used as a buffer, which is used to drive the clock input of another part of the system.
This product was designed about 2008 and we have made large numbers of them, so the overall concept works fine, however I just had a look at the DS of the 74LVC1G08 and it looks to have an input capacitance of 5pF.
The question I have is, would this 5pF them be biasing the output side of the xtal, while the input side is still the base 22pF? So effectively like having a 22pF on the input side, and 27pF on the output side? Or is that not how it works.
https://assets.nexperia.com/documents/data-sheet/74LVC1G08.pdf
Should the circuit take the 5pF into account, to have say 5pF less on the output side of the xtal so its balanced?

Again - just learning here, would love to know.
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #60 on: August 20, 2019, 04:48:34 am »

This product was designed about 2008 and we have made large numbers of them, so the overall concept works fine, however I just had a look at the DS of the 74LVC1G08 and it looks to have an input capacitance of 5pF.
The question I have is, would this 5pF them be biasing the output side of the xtal, while the input side is still the base 22pF? So effectively like having a 22pF on the input side, and 27pF on the output side? Or is that not how it works.
https://assets.nexperia.com/documents/data-sheet/74LVC1G08.pdf
Should the circuit take the 5pF into account, to have say 5pF less on the output side of the xtal so its balanced?

Again - just learning here, would love to know.

If the 74LVC1G08 is very near the crystal and the trace is short and neat and not subject to interference, then yes, the capacitance would be figured in on that side of the circuit just like Cin and Cout would be if specificed.  If any of the conditions I mentioned aren't met, then I don't know the answer.  B/t/w, I think I've seen warnings not to drive other circuits off of MCU XO circuits in that way, but if it has been working for you then don't worry.

 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #61 on: August 20, 2019, 07:57:45 am »
I have done it before but cheated by implementing the MCU's gate oscillator externally using comparable CMOS logic so I could measure the supply current to the driver.  My conclusion was what I stated earlier; discrete transistor crystal oscillators are more reliable than CMOS gate crystal oscillators.
Most MCU oscillators and especially watch crystal oscillators, are more than simple inverting CMOS gate - due to same reason you mention. In this regard TI msp430 clocks and their documentation are excellent. Check for example oscillator specs/documentation of msp430f5 and you will see.
 

Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #62 on: August 20, 2019, 09:07:53 am »
If the 74LVC1G08 is very near the crystal and the trace is short and neat and not subject to interference, then yes, the capacitance would be figured in on that side of the circuit just like Cin and Cout would be if specificed.  If any of the conditions I mentioned aren't met, then I don't know the answer.  B/t/w, I think I've seen warnings not to drive other circuits off of MCU XO circuits in that way, but if it has been working for you then don't worry.

Yeah it is a curious one, but it has been used actually on a number of products in all sorts of environments over the years, and it seems to be fine.
The distance between MCU and buffer is short and direct, typically within about 5-10mm of the MCU, then typically the next chip using the output is 10-20mm from the output, but it does depend somewhat.
So you would suggest trying say 5pF less for the capacitor on the XO side, so they are then a closer match? Up till now, this has never been done. But in the name of optimising things, if you or anyone else think it is worth doing, then I will note it down to get that tested out.

Thanks
 

Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #63 on: August 20, 2019, 09:18:30 am »
While looking at the buffer, I stumbled on this...
Seems to be more specific to buffering crystals.
It would be applied to the crystal quite differently, but I wonder if this would be 'better'...

http://www.ti.com/lit/ds/symlink/sn74lvc1gx04.pdf
 

Online Howardlong

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Re: Scope and Probes - Measuring Crystal
« Reply #64 on: August 20, 2019, 09:29:44 am »
While looking at the buffer, I stumbled on this...
Seems to be more specific to buffering crystals.
It would be applied to the crystal quite differently, but I wonder if this would be 'better'...

http://www.ti.com/lit/ds/symlink/sn74lvc1gx04.pdf

I'm not clear, are you suggesting that you're in a position to be able to re-spin the board? If you are that opens up many other possibilities.
 

Offline bdunham7

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Re: Scope and Probes - Measuring Crystal
« Reply #65 on: August 20, 2019, 10:26:19 pm »
While looking at the buffer, I stumbled on this...
Seems to be more specific to buffering crystals.
It would be applied to the crystal quite differently, but I wonder if this would be 'better'...

http://www.ti.com/lit/ds/symlink/sn74lvc1gx04.pdf

Yes, IMO, better designs that need a separate clock signal will expressly design in a buffered output rather than tap into the existing XO and subject it to additional component variability and potential interference.  This is still a CMOS gate oscillator, however, and I have no experience with it and can't tell you if it is more or less reliable than your MCU. FWIW, I think the most reliable variant are the high quality external self contained XOs.

However, I would hope that you aren't going to redesign your product based on these conversations!  I would caution you that you need to go over a lot more material before you can start down that road.  I fix things more than design them, so I wouldn't do it lightly either.  Although I'm sure I can put together a reliable XO from scratch, I would not want to unnecessarily redesign a proven reliable product even if I thought I was improving it because there is always a possibility of introducing new issues--you can't anticipate everything.  I wouldn't hesitate to redesign an UNreliable product, though.   ;)
« Last Edit: August 20, 2019, 10:30:52 pm by bdunham7 »
 

Offline WanaGo

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Re: Scope and Probes - Measuring Crystal
« Reply #66 on: August 20, 2019, 11:39:42 pm »
Yep I appreciate that.
We have designed many products over the years, and yes we can make new revisions as we see fit, prototype them, have them running for weeks/months in various situations to try and see if they fail etc. Its just time, but yes we can change the design if there is a problem.

But as I have said, there is not a specific problem. The boards are not unreliable. There is nothing specifically wrong.

I am just exercising the process of determining if things are as good as they can be, rather than just in a 'working state'. If some of the fringe cases where we have reliability issues, such as if our product is used where there are TIG welders or other things like that, we have had the odd situation where the product will self restart etc. We normally come up with a solution, but if we can avoid that happening at all then it would be great.

It may not be related to the crystal setup at all.

I was just initially trying to find out a way to view/measure the current situation, to see if what we have is OK or if its on the edge of being OK.
I then went down this buffer path to see if that might also be a contributor to some of these edge cases.

I say again, we dont have faulty product, I was just trying to determine if we can figure out if what we have is optimal or if there is room for improvement which may provide benefit.

Thanks
 

Offline David Hess

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Re: Scope and Probes - Measuring Crystal
« Reply #67 on: August 21, 2019, 02:21:37 am »
I have done it before but cheated by implementing the MCU's gate oscillator externally using comparable CMOS logic so I could measure the supply current to the driver.  My conclusion was what I stated earlier; discrete transistor crystal oscillators are more reliable than CMOS gate crystal oscillators.

Most MCU oscillators and especially watch crystal oscillators, are more than simple inverting CMOS gate - due to same reason you mention. In this regard TI msp430 clocks and their documentation are excellent. Check for example oscillator specs/documentation of msp430f5 and you will see.

Oh, for sure.  But the characteristics of the gate oscillator can be measured without the crystal in place to make an accurate external reproduction.

The common problem I mentioned is low transconductance, which is not well controlled, at low temperatures which cause the oscillator not to start with some crystals.  I do not know if this situation has improved but it used to be a big problem in applications which exercised the microcontroller over the lower end of its temperature range.

Discrete bipolar oscillators can completely avoid this problem making them more reliable over wide temperature ranges but they generally perform better anyway.

 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #68 on: August 21, 2019, 12:42:27 pm »
The common problem I mentioned is low transconductance, which is not well controlled, at low temperatures which cause the oscillator not to start with some crystals.

Please check your notes. As far as I know, CMOS gain *increase* at low temperatures and decrease at high. At low temperatures there's risk of crystal overdrive if you aim for huge safety margin. Yes, I agree that Colpitts oscillator is better from temperature point of view. It is advised to use crystal oscillators in demanding applications - you have no concerns of temperature, layout, load capacitors, noise immunity. Win win win
 

Offline David Hess

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Re: Scope and Probes - Measuring Crystal
« Reply #69 on: August 21, 2019, 02:52:17 pm »
The common problem I mentioned is low transconductance, which is not well controlled, at low temperatures which cause the oscillator not to start with some crystals.

Please check your notes. As far as I know, CMOS gain *increase* at low temperatures and decrease at high. At low temperatures there's risk of crystal overdrive if you aim for huge safety margin. Yes, I agree that Colpitts oscillator is better from temperature point of view. It is advised to use crystal oscillators in demanding applications - you have no concerns of temperature, layout, load capacitors, noise immunity. Win win win

Well, they were failing at low temperatures but within their specified operating temperature range.  Most microcontroller manufacturers had problems at one point or another.

Some crystal oscillator modules did as well so that was no solution.
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #70 on: August 21, 2019, 03:18:44 pm »
Well, they were failing at low temperatures but within their specified operating temperature range.  Most microcontroller manufacturers had problems at one point or another.
Some crystal oscillator modules did as well so that was no solution.

Well, I do not doubt they failed, yet reason most likely was not like you say. What temperatures are you talking about?
 

Offline David Hess

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Re: Scope and Probes - Measuring Crystal
« Reply #71 on: August 21, 2019, 03:44:43 pm »
Well, I do not doubt they failed, yet reason most likely was not like you say. What temperatures are you talking about?

They failed to start at low temperatures within their specified operating temperature range.  The common solution when there was no better option was to qualify specific crystals but this was not always a cure.

I have occasionally had the same problem with CMOS gate oscillators but not TTL or properly designed discrete transistor oscillators.
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #72 on: August 26, 2019, 09:22:29 pm »
Well, I do not doubt they failed, yet reason most likely was not like you say. What temperatures are you talking about?
They failed to start at low temperatures within their specified operating temperature range.  The common solution when there was no better option was to qualify specific crystals but this was not always a cure.
For those unaware - he is talking about 20+ years old events & tech which is long gone. Oscillators and crystals are way different today.
 

Offline radiolistener

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Re: Scope and Probes - Measuring Crystal
« Reply #73 on: August 27, 2019, 01:36:20 am »
I think oscilloscope is a bad choice to test crystals. It's better to use VNA. With VNA you can see parallel and serial resonance frequency, Q factor of crystal and it's equivalent circuit values and all parasitic resonances. With oscilloscope it will be almost impossible.
« Last Edit: August 27, 2019, 01:40:01 am by radiolistener »
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #74 on: August 27, 2019, 07:38:16 am »
I think oscilloscope is a bad choice to test crystals. It's better to use VNA. With VNA you can see parallel and serial resonance frequency, Q factor of crystal and it's equivalent circuit values and all parasitic resonances. With oscilloscope it will be almost impossible.

I am afraid that you have to read more than just (quite misleading) subject of the thread. Measuring oscillators is actual topic here and VNA is wrong tool for that.
 

Offline tautech

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Re: Scope and Probes - Measuring Crystal
« Reply #75 on: August 27, 2019, 07:57:56 am »
I think oscilloscope is a bad choice to test crystals. It's better to use VNA. With VNA you can see parallel and serial resonance frequency, Q factor of crystal and it's equivalent circuit values and all parasitic resonances. With oscilloscope it will be almost impossible.

I am afraid that you have to read more than just (quite misleading) subject of the thread. Measuring oscillators is actual topic here and VNA is wrong tool for that.
Well yes and no.
An in-circuit active crystal, well yes a scope can give you information to see it's working  as expected.

Yet SFRA or a VNA can also give you info on the crystal outside the circuit.
From this post:
https://www.eevblog.com/forum/testgear/siglent-sds1204x-e-released-for-domestic-markets-in-china/msg2447832/#msg2447832
Avid Rabid Hobbyist
 

Online ogden

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Re: Scope and Probes - Measuring Crystal
« Reply #76 on: August 27, 2019, 08:09:32 am »
Yet SFRA or a VNA can also give you info on the crystal outside the circuit.

That post is about measuring *crystals*. I already said that you do not measure *oscillator* using VNA. How crystal measurements you mention helps to pick proper drive level and load capacitors? Please elaborate.
 

Offline David Hess

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Re: Scope and Probes - Measuring Crystal
« Reply #77 on: August 27, 2019, 11:29:32 pm »
For those unaware - he is talking about 20+ years old events & tech which is long gone. Oscillators and crystals are way different today.

The only thing which might have improved is the awareness of the IC designers and I am dubious about that when it involves analog circuits.  Crystals are the same but with a smaller selection and CMOS oscillators on lower voltage processes are worse.

It still happens that designers report with crystal oscillator problems on new microcontrollers.  Some have even done so on EEVBlog.
 


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