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DIY active probe - another approach (BUF802)
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noisyee:

--- Quote from: IAmBack 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.

--- End quote ---

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.
Berni:
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)
luudee:

Is this a regular FR4 ?

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


luudee
IAmBack:

--- Quote from: noisyee on November 23, 2022, 01:36:20 am ---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.

--- End quote ---
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.
IAmBack:

--- Quote from: Berni 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)

--- End quote ---
Thanks.
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