First post - New RF Sniffer Project! - Page 1
Forum section: electronics / projects

author: Bartronics
author_rank: Newbie
timestamp: January 18, 2022, 02:10:06 am
content: Hello, this is my first post... I should have joined this forum a long time ago, I love the video series, I've been watching it for a few years, and I've been looking for an (active) electronics forum, and I think this is it!!My most recent project is this RF Sniffer I'm in the process of building, its not done yet but getting there. Hard to see here but all of the resistors and non electrolytic caps are SMD components. I'm using a schematic I found online... I hope it works cause you never know for sure using random schematics you stumble across online.I'm kind of new to Manhattan style PCB builds, but I'm pretty much sold on this method! It may not be pretty, but so fast in building prototypes, and I like being able to mix and match SMD parts and through hole since I've got a bunch of both in my parts boxes. I use cut pieces of clad pcb board, but I also like to scratch away lines forming islands on the copper surface.So one question, C5 on the schematic looks like it was added later? I'm guessing that that cap is only connected to the vertical trace that goes to ground. Also would you think all the .1 caps are showing 100pf caps?? Because that's what I'm assuming, and C7 doesn't show any number at all, but I'm also going with 100pf there too. Any further thoughts?Love the EEVblog! Glad I finally joined!!

author: fourfathom
author_rank: Super Contributor
timestamp: January 18, 2022, 03:40:49 am
content: Yes, C5 is connected between switched V+ and ground. It's poorly drawn on the schematic, but it's a necessary part.The ".1" caps are 0.1 uF. Should be small ceramic caps. 100 pF is going to be too small.C7 is 0.001 uF (the associated test is inside the amplifier symbol -- again, poorly-drawn schematic. This provides high-frequency rolloff for the audio channel, and there will be more "hiss" if you use 100 pF for this one.The circuit looks like it's going to work, but I have no idea how sensitive it's going to be, Only AM modulation will be detected as audio, but the meter should indicate any modulated or unmodulated signal.What are you using for L1 and D1?

author: David Hess
author_rank: Super Contributor
timestamp: January 18, 2022, 05:50:05 am
content: I designed and built a very similar RF sniffer except for an added AGC stage for greater dynamic range. I did not bother cutting islands into the ground plane but just wired above it. The original design used a square-law balanced bridge for detection, but I never got it working correctly, (1) so as shown below I replaced it with a half-wave voltage doubler using 2N270 germanium diodes which worked pretty well. Useful detection range with the antenna was fractions of a milliwatt to 10s of watts.I did not do it as shown below, but I think a better way is to punch little pads out of copper clad board and glue them in place as needed. This also works to make controlled impedance microstrips.(1) I did not know exactly what I was doing then. I could get it working now. The advantage is being able to using better diodes without the disadvantage of their high voltage drop.

author: fourfathom
author_rank: Super Contributor
timestamp: January 18, 2022, 05:58:40 am
content: Quote from: David Hess on January 18, 2022, 05:50:05 amYou probably noticed this, but FWIW the OP's circuit has an adjustment to pre-bias the diode so the voltage drop isn't an issue.[EDIT:] No, I take that back, the adjustment looks like it is for zeroing the meter. The small voltage change of perhaps 6 mV (depending on supply voltage) isn't enough to forward-bias the diode.

author: David Hess
author_rank: Super Contributor
timestamp: January 18, 2022, 06:25:41 am
content: Quote from: fourfathom on January 18, 2022, 05:58:40 amA bridge allows biasing the diode with a current for maximum sensitivity, while removing the temperature dependent forward voltage drop with the second identically biased diode that has no RF applied to it. It is mostly useful because it allows sensitive detection without a less available "zero bias" detector diode like an RF schottky or germanium part.

author: Bartronics
author_rank: Newbie
timestamp: January 18, 2022, 01:59:15 pm
content: Thanks for the tips and information. There was some text that went with the schematic talking about the parts you mentioned...As for L1.... isn't it similar to a loading coil? I'm not sure yet exactly how many turns I'll use for it, but the documentation gives a hint and mentions,"You can screen out domestic AM radio signals by making L1 small enough." I've seen that same basic element on a lot of RF Sniffer schematics and figured I could match it to a similar schematic that gives more details if I had any problems.For the diode, D1 it says "RF is rectified by D1, which can be a 1N914, but a 1N34A gives the sniffer much greater sensitivity. A hot-carrier diode, such as 1SS99 or NTE112, works even better."I have a few large, old style D18 Glass NOS Russian Soviet Military Germanium Diodes which would probably work, but I also have some 1SS99 hot-carrier diodes, so was planning to use one of those most likely.There's an electronics channel on Youtube, I think the channel is called Leo's Bag of Tricks. Guy shows you how to make a copper PCB scratching tool by cutting a hook shape out of the back side of an exact-o blade using a Dremmel tool. You can score straight lines through the copper super easily with it. Good for making small pads for component legs directly on the copper, maybe not as fast as dead bug though?

author: Terry Bites
author_rank: Super Contributor
timestamp: January 18, 2022, 03:45:38 pm
content: Have you seen Mr Carlson's super probe...?

author: Bartronics
author_rank: Newbie
timestamp: January 20, 2022, 07:55:53 pm
content: The suggestion of Mr. Carlson's Super Probe is excellent!I'm definitely building one of those as an upcoming project! Seems like a super useful tool or piece of gear to have in your lab!I've never seen Mr. Carlson's videos... Really great stuff there! I've been watching many of the videos he posted ever since your suggestion. Guy is so calm. Thanks for the suggestion!!