Can you use a scope probe to make RF probe? - Page 1
Forum section: electronics / beginners

author: Bhutt
author_rank: Contributor
timestamp: May 10, 2020, 11:01:19 pm
content: Hi, Can you use a scope probe to make a RF detector/demodulator probe? Typically the probing tip comes right off the detector circuit but what if you used a scope probe, cut the coax and inserted the detector circuit in line closer to a millivolt meter or scope? Could this work?I just want to use this to peak a RF signals during alignment procedures for a shortwave radio I previously described/posted here. I have received a long awaited LEADER AC millivolt meter without probe. And the scope probes I have come with the clip and ground connector neatly attached and isolated and thought it could be a cool way to make a RF probe.I have watched a YouTube video from W2AEW which describes a detector and want to add the scope probe to it. See my attached drawing.Your thoughts appreciated,Cheers

author: ArthurDent
author_rank: Super Contributor
timestamp: May 10, 2020, 11:56:31 pm
content: What you want to do won’t work that well because the loading of a tuned circuit by the probe you show will prevent you from finding the ‘peak’. Here is an article from 73 Magazine, pages 46-47, 1985 that described the problem in detail and shows how to construct a simple active R.F. probe with almost no loading that does work and is built in a retractable ball point pen plastic case. You do have to supply the circuit with 5 to 15VDC at a few mA but that shouldn’t be a problem. Here is a link to the article. I know this works well because I built a few.https://www.americanradiohistory.com/Archive-DX/73-magazine/73-magazine-1985/73-magazine-11-november-1985.pdf

author: Bhutt
author_rank: Contributor
timestamp: May 11, 2020, 12:33:53 am
content: Hi,I looked through the 1985 magazine you sent but did not find what you are referring to. What do you mean by loading of a tuned circuit?Thank you,

author: ArthurDent
author_rank: Super Contributor
timestamp: May 11, 2020, 12:46:52 am
content: Pages 46-47.

author: intmpe
author_rank: Contributor
timestamp: May 11, 2020, 12:52:39 am
content: Well that old jfet on the front end of the probe has a input capacitance of about 5 puff. Fine for fixing an old cb radio but useless in upper UHF and microwave regions unless you don't mind factoring that into what you are seeing.

author: Bhutt
author_rank: Contributor
timestamp: May 11, 2020, 01:02:12 am
content: Right, pages 46-47, Thank you, a clear case of not seeing the forest for the trees.. LolBut I am still not seeing how basically adding coax to either end of the circuit would not work, remember, beginner here. What I was looking for an easy way to free up a hand and fingers by using a pre made well shielded grab hook.

author: ArthurDent
author_rank: Super Contributor
timestamp: May 11, 2020, 01:11:57 am
content: Quote from: intmpe on May 11, 2020, 12:52:39 amThat wasn't what the OP described he wanted and the article clearly explains the limitations of the circuit. It was designed to be better than than the simple diode detector probe, up to over 30 Mhz, not to replace a more expensive UHF or microwave instrument. The jfet used in this 1985 article is old because the article is from 1985.

author: Bhutt
author_rank: Contributor
timestamp: May 11, 2020, 01:28:25 am
content: Okay, now that that's outta the way. Can a surplus O scope probe be used to make a relative reading for RF amplitude based on the design presented by W2Aew?Cheers

author: ArthurDent
author_rank: Super Contributor
timestamp: May 11, 2020, 01:33:52 am
content: Quote from: Bhutt on May 11, 2020, 01:02:12 amIf you have a tuned circuit in your shortwave receiver it might have a variable capacitor of maybe 25 pF across the coil. Coax has a certain capacitance per foot that might be 75pF as shown in the diagram below. If you listen to the hiss when you try to tune an LC R.F. ampl circuit and peak it by listening to the noise, you might find the variable capacitor give the peak when it is at about 1/2 the max value, or 12 pF. If you connect your scope probe coax to the tuned circuit you will have added perhaps 75 pF to the circuit so you now have 87 pF across the coil pulling the tuning way outside of where it needs to be and you won't be able to get a peak.

author: Bhutt
author_rank: Contributor
timestamp: May 11, 2020, 01:48:22 am
content: I see, if I kept detector circuit very close to the O scope probe end then would that limit the loading? Would there be loading from the O Scope probe itself?I looked at taking the probe apart but it seems sonically welded. So it could not be used as the ball point pen idea in the article you referenced.Does it matter the length of coax after the probe to the millivolt meter?Cheers

author: StuartA
author_rank: Regular Contributor
timestamp: May 11, 2020, 02:31:26 am
content: Key questions for the OP here are what RF frequency you want to go to and what kind of sensitivity do you need?Even FET's can load an RF circuit to an unacceptable degree at low signal levels. Many designs have been published and most are pretty poor unless you are talking of signal levels of 10mV and aboveThe Leader AC millivoltmeter presumably only operates to quite low frequencies (100kHz)? Once you've got the diode in the line, you just need a DC millivoltmeter.I think you need to spell out precisely what your requirement / objective is here.

author: Bhutt
author_rank: Contributor
timestamp: May 11, 2020, 11:13:10 am
content: As I mentioned in my original post " I just want to use this to peak a RF signals during alignment procedures for a shortwave radio I previously described/posted here."It is an old general coverage 10khz to 31000khz receiver, I have a few of them. I have attached a picture that shows part of the adjustment procedure. And again, will be using to show a relative reading of RF amplitude. If there is other information you need beyond this, please let me know. Thanks

author: Bhutt
author_rank: Contributor
timestamp: May 11, 2020, 01:31:58 pm
content: Well with some gentle persuasion I was able to disassemble a scope probe and can clearly see why it would not work as a RF probe. Now need to figure out how to get a detector circuit inside. There is a variable capacitor at the other end of this probe where it attaches to the scope, will this cause problems, should it be removed?Cheers,

author: RoGeorge
author_rank: Super Contributor
timestamp: May 11, 2020, 01:39:49 pm
content: Oscilloscope probes are well designed and expensive pieces of equipment. Treat them with care. Don't ruin an oscilloscope probe to put a detector inside.The detector can be easily made as an adapter at the tip of the probe.