Single PCB Carlson Super-Probe catches radio channels - Page 1
Forum section: electronics / projects

author: cincin
author_rank: Regular Contributor
timestamp: March 22, 2023, 03:39:31 am
content: I built a Carlson Super Probe (see video) but theSingle PCB version.I rigged it for headphones, put it in a metal case, added copper tape to the tip to complete the shielding.It seems to be working, but it really likes to catch radio stations whenever I point it a large metallic object (the chassis of what I'm working on). It's is an RF probe... but listening to the news while troubleshooting can be a bit distracting.Any ideas on how I could modify it to prevent it from catching radio signals?Cheers!

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 22, 2023, 04:08:32 am
content: If you have a strong AM radio station in your area, then there's probably not much you can do except build a shielded room (Faraday cage).

author: cincin
author_rank: Regular Contributor
timestamp: March 22, 2023, 04:40:35 am
content: I seem to have a few! the station changes depending on what metal object I point the probe at.I know little to none about radio, and I'm still very much a newb at electronics, but I guess I was hoping there might be some way of repressing those specific frequencies.Cheers!

author: Gyro
author_rank: Super Contributor
timestamp: March 22, 2023, 09:34:48 am
content: You could try adding a capacitor of a few pF between the base and emitter of the first transistor. The value would have to be the minimum possible to remove the radio pickup. You will be reducing the RF sensitivity of the probe - but that's kind of what you're asking for.

author: jonpaul
author_rank: Super Contributor
timestamp: March 22, 2023, 12:40:39 pm
content: ferrite shield bead.Probe body in pix is the 1960s Heathkit RF probe for the class V-7A VTVM!Jon

author: cincin
author_rank: Regular Contributor
timestamp: March 22, 2023, 04:03:52 pm
content: Quote from: Gyro on March 22, 2023, 09:34:48 amI will try that, thanks!Quote from: jonpaul on March 22, 2023, 12:40:39 pmInteresting. First time needing one of those. And where would I apply this ferrite bead? Are you suspecting the headphone cable?Cheers!

author: jonpaul
author_rank: Super Contributor
timestamp: March 22, 2023, 06:39:38 pm
content: a powerful nearby AM radio station RF is entering the probe tip.any semiconductor junction in transistors, diodes or IC will detect the AM into audio like a crystal radio set.Use a large low freq bead, one hole cylenders at the wire from probe tip to circuitMay need several turns on the bead.Bon courageJon

author: RF-Engineer
author_rank: Contributor
timestamp: April 25, 2023, 01:56:04 pm
content: An alternative to Carlson's device is a telecom inductive probe. See attached photo. It offers the same high gain as Carlson's probeand detects passive component noise very well. Add a switchable 1N34A diode as an RF detector and you're done.Much of Carlson's device uses multiple stages of 2N3904 amplification. Not necessary with op-amps. The telecom probe also has the advantage of detection, amplification, and speaker built into the probe and doesn't require a separate box. Add a normally-closed 3.5mm jack if you really want to use it with an optional external speaker.

author: MathWizard
author_rank: Super Contributor
timestamp: April 27, 2023, 10:10:29 pm
content: I made one of these, and it will also output the local radio stations sometimes, and be quite listenable. But I made a 1/4W AM transmitter, and I never could get it to detect that at all.

author: coppercone2
author_rank: Super Contributor
timestamp: April 27, 2023, 10:27:45 pm
content: Hmm RF engineer you gave me an idea, I will try to use the telecom probe to see if I can figure out which transistor is making noise in my audio amp.I was going to try freeze spray first.