RF probe - can you recommend any? - Page 1
Forum section: electronics / rf-microwave

author: Jane
author_rank: Frequent Contributor
timestamp: October 11, 2018, 06:14:22 pm
content: Which probe would you recommend when working with CB ( city band) and HAM radio repairs? Or is there a better solution than using RF probe( also as AM probe) nowadays?

author: radioactive
author_rank: Regular Contributor
timestamp: October 12, 2018, 03:15:26 pm
content: I've found the Auburn Technology P20-A probe to be very useful for debugging prototypes and general purpose probing around in the frequency domain (spectrum analyzer 50 ohm). I use it more as a relative measurement, so I don't even worry much about the ground clips. You usually get a pretty good idea of what is going on just probing coupling caps between stages, etc. Looks like they have a new model out (P20-B). I don't know how it compares to the A model.

author: KJDS
author_rank: Super Contributor
timestamp: October 12, 2018, 08:21:32 pm
content: Make one, put a 430 or 470 ohm resistor on the end of a length of thin coax to give a 20dB probe. The last one I made was good to about 2GHz.

author: bd139
author_rank: Super Contributor
timestamp: October 12, 2018, 09:37:43 pm
content: I just use a 10x probe on scope. Most of the diagnostics are “is the signal getting to X”. Rest of the work done is with a calibrated signal generator and attenuators on the antenna port and the radio itself.

author: DaJMasta
author_rank: Super Contributor
timestamp: October 12, 2018, 11:09:02 pm
content: Even inexpensive 10x probes are going to be very high impedance and very low capacitance in those bands and should be just fine for looking at your signal.Just terminate the like properly, then probe the signal pin, ideally using a spring ground clip to minimize parasitic inductance, but at these frequencies even the flying ground lead isn't too bad.

author: TheSteve
author_rank: Unknown
timestamp: October 12, 2018, 11:21:26 pm
content: I use an Agilent 1163a 10:1 50 ohm probe. They occasionally pop up cheap on eBay with pouch and accessories. The BNC might not plug into everything though. They are auto sensed as a 10:1 50 ohm probe with HP/Agilent/Keysight scopes.

author: radioactive
author_rank: Regular Contributor
timestamp: October 12, 2018, 11:23:08 pm
content: Agree that a good 10x probe should be fine for most CB/Ham probing. I just took some measurements to compare the aforementioned "RF probe" to a Tek 500 MHz probe.

author: bson
author_rank: Unknown
timestamp: October 13, 2018, 04:40:09 am
content: If it's the 27MHz band, yeah, any old passive 10x probe is fine. UHF is a different matter.

author: G0HZU
author_rank: Super Contributor
timestamp: October 13, 2018, 11:47:57 am
content: Yes, for CB servicing you just need a scope probe if connecting to a scope. You can also make an RF detector probe using a detector diode and feed this to a DMM. This would replace the traditional VTVM although a basic diode detector probe won't have a linear response for small RF signals.The simple homebrew -20dB probe using a 470R resistor and some decent RF coax works well as long as you use it with a decent RF cable and an analyser that has a 50R test port with low VSWR. Otherwise you end up with a wonky and ripply response. With a 470R resistor the probe should give about a -20.7dB response and if you manage the inherent mismatch uncertainty with this type of probe you can get a very flat response to about 2GHz even with a basic homebrew probe. See below for my version of this probe.This can be made in a few minutes and costs nearly nothing to make. You can see it shows -20.7dB at 100MHz as expected and the response is flat to 2GHz. There is a tiny compensation cap to really flatten the response but this isn't essential.However, this type of probe will be of limited use for CB repair. I'd suggest using a scope and a scope probe or the diode probe. It's also fairly easy to make a basic active probe that will have high impedance and with the ability to drive a 50R spectrum analyser. It can also be used to drive a counter without loading/pulling the oscillator circuit in the CB too much.A regular x10 scope probe will have maybe 11-16pF of capacitance and this is often enough to pull a crystal oscillator slightly in frequency. Usually, the CB will have a suitable test point for these measurements where the scope probe will only pull the frequency slightly.