DIY active probe - another approach (BUF802) - Page 1
Forum section: electronics / testgear

author: IAmBack
author_rank: Regular Contributor
timestamp: November 21, 2022, 08:48:49 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: November 21, 2022, 09:11:49 pm
content: Why a photo and not a nice clear png screenshot from your analyser ?

author: IAmBack
author_rank: Regular Contributor
timestamp: November 21, 2022, 09:15:13 pm
content: I was in a hurry, sorry.

author: tautech
author_rank: Super Contributor
timestamp: November 21, 2022, 09:22:46 pm
content: Quote from: IAmBack on November 21, 2022, 09:15:13 pmIt don't show.....much.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.

author: Someone
author_rank: Super Contributor
timestamp: November 21, 2022, 10:14:22 pm
content: Quote from: tautech on November 21, 2022, 09:22:46 pm+ some cursors to make it clear what you think is important.

author: tautech
author_rank: Super Contributor
timestamp: November 21, 2022, 10:19:54 pm
content: Quote from: Someone on November 21, 2022, 10:14:22 pmMarkers, as these SA's don't do cursors.

author: nctnico
author_rank: Super Contributor
timestamp: November 21, 2022, 10:37:10 pm
content: 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.

author: IAmBack
author_rank: Regular Contributor
timestamp: November 22, 2022, 09:30:19 am
content: Quote from: nctnico on November 21, 2022, 10:37:10 pmThank you. This is helpful.

author: 2N3055
author_rank: Super Contributor
timestamp: November 22, 2022, 10:08:40 am
content: Quote from: nctnico on November 21, 2022, 10:37:10 pmThat 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..

author: IAmBack
author_rank: Regular Contributor
timestamp: November 22, 2022, 04:08:45 pm
content: Quote from: 2N3055 on November 22, 2022, 10:08:40 amThank You.Yes, I know, but I don't have VNA near...Instead, I have a plan check step response...

author: noisyee
author_rank: Contributor
timestamp: November 23, 2022, 01:36:20 am
content: Quote from: IAmBack on November 21, 2022, 08:48:49 pmProbe 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.

author: Berni
author_rank: Super Contributor
timestamp: November 23, 2022, 07:40:24 am
content: 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)

author: luudee
author_rank: Frequent Contributor
timestamp: November 23, 2022, 12:35:39 pm
content: Is this a regular FR4 ?May be something like a Megtron-6 would be a better choice?luudee

author: IAmBack
author_rank: Regular Contributor
timestamp: November 23, 2022, 02:38:21 pm
content: Quote from: noisyee on November 23, 2022, 01:36:20 amMy 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.

author: IAmBack
author_rank: Regular Contributor
timestamp: November 23, 2022, 02:43:59 pm
content: Quote from: Berni on November 23, 2022, 07:40:24 amThanks.

author: IAmBack
author_rank: Regular Contributor
timestamp: November 23, 2022, 02:45:23 pm
content: Quote from: luudee on November 23, 2022, 12:35:39 pmIt's thin (1mm) FR4.

author: colorado.rob
author_rank: Frequent Contributor
timestamp: November 23, 2022, 07:35:31 pm
content: Quote from: luudee on November 23, 2022, 12:35:39 pmWouldn't this turn a $3 PCB into a $300 PCB (minimum)?

author: nctnico
author_rank: Super Contributor
timestamp: November 23, 2022, 07:43:49 pm
content: Quote from: colorado.rob on November 23, 2022, 07:35:31 pmAnd 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.

author: Berni
author_rank: Super Contributor
timestamp: November 24, 2022, 06:08:40 am
content: Quote from: colorado.rob on November 23, 2022, 07:35:31 pmYep 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.

author: nctnico
author_rank: Super Contributor
timestamp: November 24, 2022, 11:37:06 am
content: Quote from: Berni on November 24, 2022, 06:08:40 amHaving 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.

author: tggzzz
author_rank: Super Contributor
timestamp: November 24, 2022, 01:48:06 pm
content: Quote from: nctnico on November 24, 2022, 11:37:06 amOne 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.

author: IAmBack
author_rank: Regular Contributor
timestamp: November 24, 2022, 02:36:28 pm
content: Quote from: tggzzz on November 24, 2022, 01:48:06 pmInstead of 4 layer expensive board probably better solution is 2-side assembly.

author: noisyee
author_rank: Contributor
timestamp: November 28, 2022, 07:53:56 am
content: Quote from: IAmBack on November 24, 2022, 02:36:28 pmI 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.