Impedance matching test equipment - Page 1
Forum section: electronics / projects

author: Kraythorne
author_rank: Contributor
timestamp: June 11, 2021, 11:54:48 am
content: HiI have a test probe for radio IF alignment that was originally designed for test equipment with 1M Ohm impedance input. I want to put the probe signal output through a circuit that will match the impedance to my 50Ohm Scope and spectrum analyser.Does anyone know of any existing circuit designs that would do this?I'm thinking some form of Source follower might do it, but not sure of how to design the circuit to manage IF frequencies, possibly need a couple of circuits and a band switch to flick between them depending upon the range.I can find plenty of info on source follower circuits, but very little on how they interact with frequency.Any help or pointers in the right direction would be appreciatedThanks!

author: srb1954
author_rank: Super Contributor
timestamp: June 11, 2021, 12:23:21 pm
content: Quote from: Kraythorne on June 11, 2021, 11:54:48 amEasiest, but not necessarily the cheapest, solution is to just get a commercial FET input scope probe. These are available in very wide bandwidths up to more than 1GHz so will easily cover the frequency range you need.Such a probe is also a useful as a general purpose probe for your scope or spectrum analyser where you want to probe circuits with minimum loading, especially capacitive loading.

author: Kraythorne
author_rank: Contributor
timestamp: June 11, 2021, 02:13:35 pm
content: Quote from: srb1954 on June 11, 2021, 12:23:21 pmThanks, but the existing probe has lots of DC protection build in to it that I would rather not lose and, like you say, probes aren't cheap.I don't mind putting the work into building something (I would quite enjoy the challenge), I just need some pointers

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2021, 06:55:43 pm
content: Quote from: Kraythorne on June 11, 2021, 11:54:48 amA source follower is exactly how it is done, however small signal RF FETs cannot drive a 50 or 100 ohm load without some help so a bipolar transistor or two is included to help drive the transmission line.The easiest way to do this now is probably a fast JFET or CMOS input operational amplifier configured as a voltage follower, like the LTC6268.The old way, which has some advantages, is to use a JFET source follower to directly drive bipolar emitter follower which drives the transmission line. Then use a matched JFET to control the channel current.

author: bdunham7
author_rank: Super Contributor
timestamp: June 11, 2021, 07:01:43 pm
content: What bandwidth do you need? This is a simple-ish plan for a 10X probe to 50R output for exactly the purpose you state, good to 200MHz or so.https://www.edn.com/high-impedance-fet-probe-extends-rf-spectrum-analyzers-usable-range/It's on my "to-do" list, but pretty far down. Let me know if you build one.One of the reasons it isn't a high priority is that I have a Tek A6902 isolator that has 10M probes and isolates in addition to having a 50R output. It only has 60MHz bandwidth, but that's worked so far. It is another huge box, though.

author: Kraythorne
author_rank: Contributor
timestamp: June 11, 2021, 08:10:03 pm
content: Quote from: bdunham7 on June 11, 2021, 07:01:43 pmStandard Radio Ham stuff up to say 146MHz , probably won't bother with the 70cm band (430-440Mhz)I'll take a look and see how it goes, thanksThanks all for your input, I'll let you know how I get on.If anyone else can contribute I would be very interested