Author Topic: DIY active probe - another approach (BUF802)  (Read 4646 times)

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Offline IAmBackTopic starter

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DIY active probe - another approach (BUF802)
« on: November 21, 2022, 08:48:49 pm »
Hi.
If anyone would like to see BUF802-based active probe - here are my preliminary results.
If anyone has his own experiences - You are welcome to share.
I've used circuit published in application note, with minor changes of the input part (including additional protection with PIN diodes, biased by zener diodes).
Results looks promising, IMO.
I attached plot from spectrum analyzer. Tracking generator was connected directly to the high impedance probe's input, so some impedance mismatch effects are obvious.
Best.

 
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Online tautech

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Re: DIY active probe - another approach (BUF802)
« Reply #1 on: November 21, 2022, 09:11:49 pm »
Why a photo and not a nice clear png screenshot from your analyser ?  :-//
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Offline IAmBackTopic starter

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Re: DIY active probe - another approach (BUF802)
« Reply #2 on: November 21, 2022, 09:15:13 pm »
I was in a hurry, sorry.
 

Online tautech

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Re: DIY active probe - another approach (BUF802)
« Reply #3 on: November 21, 2022, 09:22:46 pm »
I was in a hurry, sorry.
It don't show.....much.  :P

Yeah I know it's often more convenient to take pics and then post and often the whole lot from a phone however if you want to convey best info from the instrument it's best to even capture an appropriate menu that helps convey even more info.  ;)
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Offline Someone

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Re: DIY active probe - another approach (BUF802)
« Reply #4 on: November 21, 2022, 10:14:22 pm »
I was in a hurry, sorry.
It don't show.....much.  :P

Yeah I know it's often more convenient to take pics and then post and often the whole lot from a phone however if you want to convey best info from the instrument it's best to even capture an appropriate menu that helps convey even more info.  ;)
+ some cursors to make it clear what you think is important.
 
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Online tautech

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Re: DIY active probe - another approach (BUF802)
« Reply #5 on: November 21, 2022, 10:19:54 pm »
I was in a hurry, sorry.
It don't show.....much.  :P

Yeah I know it's often more convenient to take pics and then post and often the whole lot from a phone however if you want to convey best info from the instrument it's best to even capture an appropriate menu that helps convey even more info.  ;)
+ some cursors to make it clear what you think is important.
;D Markers, as these SA's don't do cursors.
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Online nctnico

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Re: DIY active probe - another approach (BUF802)
« Reply #6 on: November 21, 2022, 10:37:10 pm »
Maybe it is time to stop obsessing over equipment use and discuss the actual circuit at hand?

A better test is to terminate the generator at the probe's input. Probe performance is typically measured from a 25 Ohm source (which is a 50 Ohm cable terminated with a 50 Ohm resistor). Adding an attenuator (say 10 dB to 20 dB) at the probe side of the cable from the generator helps a bit to dampen cable reflections due to impedance mismatch.
« Last Edit: November 21, 2022, 10:46:38 pm by nctnico »
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Offline IAmBackTopic starter

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Re: DIY active probe - another approach (BUF802)
« Reply #7 on: November 22, 2022, 09:30:19 am »
Maybe it is time to stop obsessing over equipment use and discuss the actual circuit at hand?

A better test is to terminate the generator at the probe's input. Probe performance is typically measured from a 25 Ohm source (which is a 50 Ohm cable terminated with a 50 Ohm resistor). Adding an attenuator (say 10 dB to 20 dB) at the probe side of the cable from the generator helps a bit to dampen cable reflections due to impedance mismatch.
Thank you. This is helpful.
 

Online 2N3055

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Re: DIY active probe - another approach (BUF802)
« Reply #8 on: November 22, 2022, 10:08:40 am »
...
A better test is to terminate the generator at the probe's input. Probe performance is typically measured from a 25 Ohm source (which is a 50 Ohm cable terminated with a 50 Ohm resistor). Adding an attenuator (say 10 dB to 20 dB) at the probe side of the cable from the generator helps a bit to dampen cable reflections due to impedance mismatch.

That is the standard test and would make for results that are comparable to others..

But in all reality, probe should be tested with VNA, to get both gain and phase response...
Even NanoVNA or LiteVNA would be more than enough for this task if done right..

"Just hard work is not enough - it must be applied sensibly."
Dr. Richard W. Hamming
 

Offline IAmBackTopic starter

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Re: DIY active probe - another approach (BUF802)
« Reply #9 on: November 22, 2022, 04:08:45 pm »
...
A better test is to terminate the generator at the probe's input. Probe performance is typically measured from a 25 Ohm source (which is a 50 Ohm cable terminated with a 50 Ohm resistor). Adding an attenuator (say 10 dB to 20 dB) at the probe side of the cable from the generator helps a bit to dampen cable reflections due to impedance mismatch.

That is the standard test and would make for results that are comparable to others..

But in all reality, probe should be tested with VNA, to get both gain and phase response...
Even NanoVNA or LiteVNA would be more than enough for this task if done right..

Thank You.
Yes, I know, but I don't have VNA near...
Instead, I have a plan check step response...

 

Offline noisyee

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Re: DIY active probe - another approach (BUF802)
« Reply #10 on: November 23, 2022, 01:36:20 am »
Hi.
If anyone would like to see BUF802-based active probe - here are my preliminary results.
If anyone has his own experiences - You are welcome to share.
I've used circuit published in application note, with minor changes of the input part (including additional protection with PIN diodes, biased by zener diodes).
Results looks promising, IMO.
I attached plot from spectrum analyzer. Tracking generator was connected directly to the high impedance probe's input, so some impedance mismatch effects are obvious.
Best.

Probe loading effect may be quite high using the stand circuit in the BUF802 datasheet. Specified input capacitance of the chip alone is 2.4 pF, which turn to be 66 ohm @ 1 GHz, not to mention the PCB parasitic. That will make the probe useless in real world HF circuit.
A common practice for HF active probe is adding a high impedance passive attenuator in front of the buffer to isolate the capacitance.
Like nctnico has mentioned, using a matched impedance test fixture is also a good practice.
Also, measure S11 of a known good termination and then probe it to see how the S11 degrade. If don't have a VNA, try measure S21 and see how the probe influence transmission. The goal is to design a probe have minimum effect on the DUT.
 

Offline Berni

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Re: DIY active probe - another approach (BUF802)
« Reply #11 on: November 23, 2022, 07:40:24 am »
The easiest way to do high impedance RF probing is to place a SMD resistor (around 1KOhm) between the probe tip and a 50 Ohm amplifier. Sure 1KOhm might not sound like very high impedance but for GHz it is(most other probes might cause loading into below 10 Ohms).

Getting into GHz parasitics become a huge problem, every picofarad can wreck things. So even connectors cause parasitics problems. The probe tip is ideally part of the probe and as small as possible.

The way you then test the probe is to terminate your signal generator into a 50 Ohm resistor and poke the resistor with your probe. this makes sure that the source signal is stable over frequency rather than looking at all the reflections of the path to the signal generator (that happen if you end any coax into high impedance)
 

Offline luudee

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Re: DIY active probe - another approach (BUF802)
« Reply #12 on: November 23, 2022, 12:35:39 pm »

Is this a regular FR4 ?

May be something like a Megtron-6 would be a better choice?


luudee
 

Offline IAmBackTopic starter

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Re: DIY active probe - another approach (BUF802)
« Reply #13 on: November 23, 2022, 02:38:21 pm »
Probe loading effect may be quite high using the stand circuit in the BUF802 datasheet. Specified input capacitance of the chip alone is 2.4 pF, which turn to be 66 ohm @ 1 GHz, not to mention the PCB parasitic. That will make the probe useless in real world HF circuit.
A common practice for HF active probe is adding a high impedance passive attenuator in front of the buffer to isolate the capacitance.
Like nctnico has mentioned, using a matched impedance test fixture is also a good practice.
Also, measure S11 of a known good termination and then probe it to see how the S11 degrade. If don't have a VNA, try measure S21 and see how the probe influence transmission. The goal is to design a probe have minimum effect on the DUT.
My current 500M passive probe is 10+pf and has 10x attenuation.
Active probe with 5x divider on input (which gives 10x attenuation of whole probe) should have even less...
I know, that there's quite a lot of work to do, but - as I mentioned - these are early results.
 

Offline IAmBackTopic starter

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Re: DIY active probe - another approach (BUF802)
« Reply #14 on: November 23, 2022, 02:43:59 pm »
The easiest way to do high impedance RF probing is to place a SMD resistor (around 1KOhm) between the probe tip and a 50 Ohm amplifier. Sure 1KOhm might not sound like very high impedance but for GHz it is(most other probes might cause loading into below 10 Ohms).

Getting into GHz parasitics become a huge problem, every picofarad can wreck things. So even connectors cause parasitics problems. The probe tip is ideally part of the probe and as small as possible.

The way you then test the probe is to terminate your signal generator into a 50 Ohm resistor and poke the resistor with your probe. this makes sure that the source signal is stable over frequency rather than looking at all the reflections of the path to the signal generator (that happen if you end any coax into high impedance)
Thanks.
 

Offline IAmBackTopic starter

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Re: DIY active probe - another approach (BUF802)
« Reply #15 on: November 23, 2022, 02:45:23 pm »

Is this a regular FR4 ?

May be something like a Megtron-6 would be a better choice?

luudee

It's thin (1mm) FR4.
 

Offline colorado.rob

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Re: DIY active probe - another approach (BUF802)
« Reply #16 on: November 23, 2022, 07:35:31 pm »
Is this a regular FR4 ?

May be something like a Megtron-6 would be a better choice?

Wouldn't this turn a $3 PCB into a $300 PCB (minimum)?
 

Online nctnico

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Re: DIY active probe - another approach (BUF802)
« Reply #17 on: November 23, 2022, 07:43:49 pm »
Is this a regular FR4 ?

May be something like a Megtron-6 would be a better choice?

Wouldn't this turn a $3 PCB into a $300 PCB (minimum)?
And it is not really necessary. With a good PCB design, the traces will be very short anyway. Lots of designs run several GHz over FR4.
« Last Edit: November 23, 2022, 07:47:19 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline Berni

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Re: DIY active probe - another approach (BUF802)
« Reply #18 on: November 24, 2022, 06:08:40 am »
Is this a regular FR4 ?
May be something like a Megtron-6 would be a better choice?
Wouldn't this turn a $3 PCB into a $300 PCB (minimum)?

Yep FR4 is perfectly fine for it and it is dirt cheep from china. You can do most high speed things on it, especially on 4 layer boards that let you keep a ground plane nice and close.

Excuses for fancy PCB core materials might be very large loads that make short traces impossible but need very stable conditions or when building various filter and coupler structures into the PCB as distributed elements. The sort of designs where you do an EM simulation before even manufacturing the board.
 

Online nctnico

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Re: DIY active probe - another approach (BUF802)
« Reply #19 on: November 24, 2022, 11:37:06 am »
Is this a regular FR4 ?
May be something like a Megtron-6 would be a better choice?
Wouldn't this turn a $3 PCB into a $300 PCB (minimum)?

Yep FR4 is perfectly fine for it and it is dirt cheep from china. You can do most high speed things on it, especially on 4 layer boards that let you keep a ground plane nice and close.
Having a 4 layer board is not always a blessing for RF stuff. First the stackup needs to be controlled as in the PCB needs to be build according to the dielectric thicknesses you specify. Otherwise the impedances may be off. Secondly, with a thinner dielectric, the traces get thinner so variation in width of the trace gives you a larger relative error. For my 1.4GHz differential probe design I have used a 2 layer board; it is good enough and keeps things simple.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Online tggzzz

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Re: DIY active probe - another approach (BUF802)
« Reply #20 on: November 24, 2022, 01:48:06 pm »
Is this a regular FR4 ?
May be something like a Megtron-6 would be a better choice?
Wouldn't this turn a $3 PCB into a $300 PCB (minimum)?

Yep FR4 is perfectly fine for it and it is dirt cheep from china. You can do most high speed things on it, especially on 4 layer boards that let you keep a ground plane nice and close.
Having a 4 layer board is not always a blessing for RF stuff. First the stackup needs to be controlled as in the PCB needs to be build according to the dielectric thicknesses you specify. Otherwise the impedances may be off. Secondly, with a thinner dielectric, the traces get thinner so variation in width of the trace gives you a larger relative error. For my 1.4GHz differential probe design I have used a 2 layer board; it is good enough and keeps things simple.

One trick would be to have the Vcc and Gnd planes on layers 3 and 4, and the signals on layers 1 and 2.

The minimal distance between Vcc and Gnd would improve the distributed decoupling.

The maximal distance between signals and layer 3 would minimise the consequences of prepreg thickness variation. It would be no worse than a 2-layer board, but still suffer from FR4 weave variations.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline IAmBackTopic starter

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Re: DIY active probe - another approach (BUF802)
« Reply #21 on: November 24, 2022, 02:36:28 pm »
Is this a regular FR4 ?
May be something like a Megtron-6 would be a better choice?
Wouldn't this turn a $3 PCB into a $300 PCB (minimum)?

Yep FR4 is perfectly fine for it and it is dirt cheep from china. You can do most high speed things on it, especially on 4 layer boards that let you keep a ground plane nice and close.
Having a 4 layer board is not always a blessing for RF stuff. First the stackup needs to be controlled as in the PCB needs to be build according to the dielectric thicknesses you specify. Otherwise the impedances may be off. Secondly, with a thinner dielectric, the traces get thinner so variation in width of the trace gives you a larger relative error. For my 1.4GHz differential probe design I have used a 2 layer board; it is good enough and keeps things simple.

One trick would be to have the Vcc and Gnd planes on layers 3 and 4, and the signals on layers 1 and 2.

The minimal distance between Vcc and Gnd would improve the distributed decoupling.

The maximal distance between signals and layer 3 would minimise the consequences of prepreg thickness variation. It would be no worse than a 2-layer board, but still suffer from FR4 weave variations.
Instead of 4 layer expensive board probably better solution is 2-side assembly.
 

Offline noisyee

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Re: DIY active probe - another approach (BUF802)
« Reply #22 on: November 28, 2022, 07:53:56 am »
Is this a regular FR4 ?
May be something like a Megtron-6 would be a better choice?
Wouldn't this turn a $3 PCB into a $300 PCB (minimum)?

Yep FR4 is perfectly fine for it and it is dirt cheep from china. You can do most high speed things on it, especially on 4 layer boards that let you keep a ground plane nice and close.
Having a 4 layer board is not always a blessing for RF stuff. First the stackup needs to be controlled as in the PCB needs to be build according to the dielectric thicknesses you specify. Otherwise the impedances may be off. Secondly, with a thinner dielectric, the traces get thinner so variation in width of the trace gives you a larger relative error. For my 1.4GHz differential probe design I have used a 2 layer board; it is good enough and keeps things simple.

One trick would be to have the Vcc and Gnd planes on layers 3 and 4, and the signals on layers 1 and 2.

The minimal distance between Vcc and Gnd would improve the distributed decoupling.

The maximal distance between signals and layer 3 would minimise the consequences of prepreg thickness variation. It would be no worse than a 2-layer board, but still suffer from FR4 weave variations.
Instead of 4 layer expensive board probably better solution is 2-side assembly.

I think 2 layer is enough for BUF802, it's an open loop buffer tend to be quite stable. It's not that sensitive to power decoupling or parasitic parameter on signal trace.
FR-4 is good enough for 2 GHz with short trace. With passive high impedance attenuator, you will have plenty of headroom to compensate for bandwidth or response. Just remember to cut out copper plane under the attenuator and BUF802 input pin to minimize capacitance, or you may have trouble compensating low frequency response.
 


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