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Author Topic: DIY Ghz sampling head for <100Mhz scopes  (Read 13614 times)

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Offline Mosaic

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DIY Ghz sampling head for <100Mhz scopes
« on: May 12, 2016, 10:37:31 AM »
Hi all:
This is a project I've been giving some thought to and I wanted to open a conversation on it.

1Ghz being the first target or merit, 3% accuracy a figure of merit. An overall stretch goal of 3Ghz to cover wifi etc.
Hopefully the outcome can be a kit or a DIY project etc.

I feel there is a need for this as >  1Ghz  scopes tend to stretch the hobbyist budget compared to the sub 100Mhz Rigol, Siglents etc.

So there's sequential sampling, random  sampling and Coherent Interleaved sampling methods I know of.

To me the trick is in the strobe triggering, triggering reliably (with known delays) off the signal itself  and the jitter management in a cost effective and DIY manner.
Balancing the  detector bridge strobe with a balun might be a bit interesting as well.




 

Offline rfeecs

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #1 on: May 12, 2016, 10:56:41 AM »
 

Offline Mosaic

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #2 on: May 12, 2016, 11:22:28 AM »
Yes I have, thx.

It is interesting and can no doubt be improved now.

It might be a reasonable starting point from a bock diagram or perhaps a Tektronix S1 or S3 approach as well can be looked at from the sampling POV.

Triggering and timing precision though needs thought given component availability like an ADCMP820 etc.
 

Offline Earendil

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #3 on: May 12, 2016, 05:33:49 PM »
I don't think there's a real need for a sampling scope front-ends.
There was a guy here who tried to do a 10 Ghz sampling scope. He had a working prototype yet he didn't manage to get the necessary funding on Kickstarter.
It's pretty much limited what you can do with it anyway. Most hobbyists don't do multi gigabit digital boards so they don't need it to verify signal integrity.
 

Offline Mosaic

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #4 on: May 13, 2016, 12:23:59 AM »
Maybe so:
But 10 Ghz sits squarely in the commercial arena of folks like Picoscope and may not be commercially viable given that mainly corporate and lab interests require proper warranty support and service and therefore will not support a KS project over an established product line.

Speed of data is guaranteed to improve all the time. Therefore instrumentation to monitor  such data is also required, maybe this project is before its time. I don't mind developing it for non commercial purposes.

 
 

Online Kalvin

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #5 on: May 13, 2016, 01:09:01 AM »
Things may be changing as we now have USB 3.0. Also, using and probing FPGA LVDS signals in hobby projects would be possible, too.
« Last Edit: May 13, 2016, 01:44:15 AM by Kalvin »
 

Offline rfeecs

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #6 on: May 13, 2016, 03:14:26 AM »
... There was a guy here who tried to do a 10 Ghz sampling scope. He had a working prototype yet he didn't manage to get the necessary funding on Kickstarter.

It did eventually become an actual product.

The previous thread discussing that project:
http://www.eevblog.com/forum/testgear/10-ghz-usb-oscilloscope-by-darwin-sabanovic/
 

Offline Mosaic

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #7 on: May 13, 2016, 04:38:22 AM »
The approach of using a fast comparator with a DAC for a trigger/sample is interesting as a means to side step strobing a quad detector bridge.
This approach requires a means to track the trigger time of the comparator in the picosecond range ideally. For now I'd settle for the 20 pico second stepping range as a first run.
 Thus two comparators,  one to define the  zero time trigger , then one to trigger on +ve and then -ve going transitions and then use sin(x)/x interpolations for the inflection interpolation. Time stamping the triggering of the 2nd comparator is the key to reconstructing the signal.

Any suggestions for a low jitter, picosecond scale delay timing chip?

I can step back a bit in time and use a slotted line approach with the 2nd comparator. Then derive the signal reconstruction based on signal propagation and a capacitive vernier distance encoder. This would perhaps reduce semiconductor jitter issues, but enlarge the device and add mechanical issues. Advantage here is possible oversampling at any position along the wave front for better resolution of aberrations or ringing. Maybe 300 dpi as a realistic lower stepping precision.

With a modern uC the, slotted line / microstrip/ coplanar waveguide can be 'calibrated' for amplitude flatness and % signal transfer efficiency given a known signal reference, reducing mechanical precision requirements and wear effects over time. Add SOL cals and the device can also become a standing wave analyzer and impedance matcher with TDR benefits.

http://www.ultracad.com/mentor/microstrip%20propagation.pdf

Averaging propagation at, say, 150 ps per inch of FR4. With 300 dpi resolution we have 1/2 pico sec resolution. With enough oversampling jitter noise can be substantially reduced.





 

Online tggzzz

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #8 on: May 13, 2016, 04:43:09 AM »
Any suggestions for a low jitter, picosecond scale delay timing chip?

Turn the problem on its head and look for time-to-digital (TTD) devices.

Or there are ECL delay lines for aligning clocks.
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Offline NiHaoMike

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #9 on: May 13, 2016, 12:44:51 PM »
One that can go up to 5.8GHz or so would be very useful for analyzing 802.11.
Cryptocurrency has taught me to love math and at the same time be baffled by it.
 

Offline Lukas

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #10 on: May 13, 2016, 05:33:29 PM »
Some years ago, I've been into building a sampling scope using the comparator-as-sampler approach. Unfortunately, I never finished it :(
Sampler comparator was an ADCMP582.
For trigger generation, I used a startable ring oscillator (basically an and gate and transmission lines on the PCB) and an MC100EP196 delay line with analog fine tuning. The ring oscillator's output is divided by 2 to to reduce the length of the delay line. This signal is fed an ECL counter, that generates a pulse every 256(?) clock cycles. By presetting this counter, I can control the sampling instant with a granularity of approx. 10ns. To get down to ~1ps, the delay line is used.
Since both of these delay elements are rather inaccurate, the MCU measures the frequency of the ring oscillator. To calibrate the MC100EP196, it can be configured as ring oscillator as well, so the MCU can measure its frequency as well.

Unfortunately, I haven't been to characterize the start-up behaviour of the ring oscillator yet.

In general the approach of using a startable ring oscillator has the disadvantage, that for fast start up, the oscillator should have low Q, but for low jitter it should have high Q.
Anyone got any expericene with going the analog way? I.e. steering currents in and out of a capacitor for ramp generation
 

Offline Earendil

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #11 on: May 13, 2016, 07:01:28 PM »
One that can go up to 5.8GHz or so would be very useful for analyzing 802.11.

How would you use a sampling oscilloscope to analyze 802.11?
 

Offline Marco

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #12 on: May 13, 2016, 09:56:22 PM »
In general the approach of using a startable ring oscillator has the disadvantage, that for fast start up, the oscillator should have low Q, but for low jitter it should have high Q.

Why would that matter? You aren't relying on noise to start the oscillation are you? You said AND but I assume you meant NAND, the trigger makes one input high, output goes low and cycle starts ... how does high Q matter?

The problem is that your first cycle starts with a gate signal shape different from the steady state, so it will take a while to settle. A high Q simply means insensitivity to such effects, but it would still instantly start up. The higher the Q the better.
« Last Edit: May 13, 2016, 10:07:18 PM by Marco »
 

Offline Mosaic

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #13 on: May 14, 2016, 01:11:41 AM »
Some years ago, I've been into building a sampling scope using the comparator-as-sampler approach. Unfortunately, I never finished it :(
Sampler comparator was an ADCMP582.
For trigger generation, I used a startable ring oscillator (basically an and gate and transmission lines on the PCB) and an MC100EP196 delay line with analog fine tuning. The ring oscillator's output is divided by 2 to to reduce the length of the delay line. This signal is fed an ECL counter, that generates a pulse every 256(?) clock cycles. By presetting this counter, I can control the sampling instant with a granularity of approx. 10ns. To get down to ~1ps, the delay line is used.
Since both of these delay elements are rather inaccurate, the MCU measures the frequency of the ring oscillator. To calibrate the MC100EP196, it can be configured as ring oscillator as well, so the MCU can measure its frequency as well.

Unfortunately, I haven't been to characterize the start-up behaviour of the ring oscillator yet.

In general the approach of using a startable ring oscillator has the disadvantage, that for fast start up, the oscillator should have low Q, but for low jitter it should have high Q.
Anyone got any expericene with going the analog way? I.e. steering currents in and out of a capacitor for ramp generation


So you're doing random, non sync'd triggering? Then using known sequential time slices to sample the waveform for up to 10nS? But the EP196 isn't giving a reliable, known, delay and needs calibration?
The Ep196 appears to have a 16.7 pS  timing drift per °C rise. That is a lot for the app. Looks like you might need to do a 0.1°C accurate Oven controlled  box for it.

Then it has > 120ps Tr so  how accurate is this going to be for the app?
 

Offline Lukas

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #14 on: May 14, 2016, 02:44:52 AM »
In general the approach of using a startable ring oscillator has the disadvantage, that for fast start up, the oscillator should have low Q, but for low jitter it should have high Q.

Why would that matter? You aren't relying on noise to start the oscillation are you? You said AND but I assume you meant NAND, the trigger makes one input high, output goes low and cycle starts ... how does high Q matter?

The problem is that your first cycle starts with a gate signal shape different from the steady state, so it will take a while to settle. A high Q simply means insensitivity to such effects, but it would still instantly start up. The higher the Q the better.
Since I'm using an differential AND gate, I can make it a NAND gate just by swapping differential pairs. I'm not relying on noise for startup, the circuit is inherently unstable.
IIRC, I read that fact about Q factor in some paper about sampling scope timebases and considered ist plausible. Maybe I can find that paper again.

random, non sync'd triggering? Then using known sequential time slices to sample the waveform for up to 10nS? But the EP196 isn't giving a reliable, known, delay and needs calibration?
The Ep196 appears to have a 16.7 pS  timing drift per °C rise. That is a lot for the app. Looks like you might need to do a 0.1°C accurate Oven controlled  box for it.

Then it has > 120ps Tr so  how accurate is this going to be for the app?
No, I'm using the circuit to generate a known delay from the trigger event. Yes, the EP196 needs calibration, that's why I included circuitry to use it in an ring oscillator so that I can calibrate for non-linearity and drift. The idea was to calibrate for nonlinearity once, and do linear calibration every few seconds or minutes.

The rise time will only affect jitter since pretty much all of the trigger path is fully differential.
 

Offline Marco

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #15 on: May 14, 2016, 04:21:30 AM »
IIRC, I read that fact about Q factor in some paper about sampling scope timebases and considered ist plausible. Maybe I can find that paper again.

Hmm, I guess high Q higher order oscillators (a delay line oscillator being infinite order) are forced to run near linear ... so it will only reach steady state when the delay line is filled with a half period sine voltage.

Also it only just occurred to me that the inverter for a non-linear delay line oscillator needs to have a schmitt trigger input.
 

Offline LaserSteve

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #16 on: May 14, 2016, 06:00:35 AM »
I'd love to have a 1 to 1.5 Ghz sampling head. Especially if it were a dual  head for comparative phase measurements.
One of my friends has an ancient   TEK  1 Ghz sampler and it has turned out to be quite useful...

My contribution to the initial research reading  is attached, it makes a nice test pulse. It was designed by Dr. Houtman who did the delay line sampling scope plans mentioned earlier. When one does a little digging into Dr. Houtman's academic work, its very clear he knows his way around very fast phenomena.

I love the colliding wave  architecture he used to make a symmetrical pulse..

Steve
« Last Edit: May 14, 2016, 06:20:41 AM by LaserSteve »
 
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Offline Mosaic

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #17 on: May 14, 2016, 06:44:05 AM »
I'd love to have a 1 to 1.5 Ghz sampling head. Especially if it were a dual  head for comparative phase measurements.
One of my friends has an ancient   TEK  1 Ghz sampler and it has turned out to be quite useful...

My contribution to the initial research reading  is attached, it makes a nice test pulse. It was designed by Dr. Houtman who did the delay line sampling scope plans mentioned earlier. When one does a little digging into Dr. Houtman's academic work, its very clear he knows his way around very fast phenomena.

I love the colliding wave  architecture he used to make a symmetrical pulse..

Steve


I've added that doc to my collection of research docs related to this subject so far.

To keep the thread current I'm sharing my drop-box link of what I have collated so far. It is not very well curated yet but, I'm still gathering.
https://www.dropbox.com/sh/kmdbbjrzofdxmkb/AADKVlHctfqbE8CNwntixXnza?dl=0'
 

Offline Marco

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #18 on: May 14, 2016, 07:03:22 AM »
I'm not sure a balanced sampling gate with balanced sample pulses is really necessary if you see what LLNL did with an extremely simple two diode sampling gate ... it just makes it harder to go faster.
 

Offline xygor

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #19 on: May 14, 2016, 08:43:13 AM »
HMC661LC4B DC-18 GHz, ultra-wideband, 4 gs/s track-and-hold amplifier
HMC1061LC5 DC-18 GHz, ultra-wideband, dual rank 4 gs/s track-and-hold amplifier
LTC6950 1.4GHz Low Phase Noise, Low Jitter PLL with Clock Distribution
 
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Offline Marco

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #20 on: May 14, 2016, 08:58:15 AM »
They want 600$ for it ...
 
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Offline xygor

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #21 on: May 14, 2016, 09:06:06 AM »
I assumed you wanted something that worked. :)

Some more papers I didn't see in your list:

Random Sampling Oscilloscope for the
Observation of Mercury Switch Closure
Transition Times (1973)
JAMES R. ANDREWS, MEMBER, IEEE
http://w140.com/andrews_random_sampling_observe_mercury_switch.pdf

Random Sampling Oscillography (1964)
G. J. FRYE, MEMBER, IEEE, AND N. S. NAHMAN, SENIOR MEMBER, IEEE
http://w140.com/frye_random_sampling_oscillography.pdf

 
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Offline Marco

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #22 on: May 14, 2016, 09:24:13 AM »
Tektronix/HP mainframes with sampling heads work for less than that.
 
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Offline Mosaic

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #23 on: May 14, 2016, 09:47:30 AM »
Perhaps I should  indicate that since this is targeted for folks who can afford modern 100Mhz - 200Mhz scopes etc, perhaps in the $400 to $1K range, the DSO/PC compatible sampling head should also be limited to that range.

While using vintage tek/HP equipment and  CRT photography to do the job is fun, something a bit more PC and DSO friendly is warranted for modern signal analysis convenience.


 

Offline Earendil

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Re: DIY Ghz sampling head for <100Mhz scopes
« Reply #24 on: May 14, 2016, 10:22:56 AM »
Perhaps I should  indicate that since this is targeted for folks who can afford modern 100Mhz - 200Mhz scopes etc, perhaps in the $400 to $1K range, the DSO/PC compatible sampling head should also be limited to that range.

While using vintage tek/HP equipment and  CRT photography to do the job is fun, something a bit more PC and DSO friendly is warranted for modern signal analysis convenience.

What are you talking about? The Tek 11800 series has RS232 interface. You can grab the screen digitally. You can also transfer all your data.
Such a system can be bought under $500 with a 12.5 Ghz low noise sampling head (+ shipping cost, which admittedly can be high if you're not in the US). For $100 more you can have the 20 Ghz version.

Edit: quote added.
« Last Edit: May 14, 2016, 10:26:21 AM by Earendil »
 


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