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Old 14th November 2014, 08:30 AM   #1
Bonsai is offline Bonsai  Taiwan
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Default Oscillation Sniffer

Hope this helps
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File Type: pdf Oscillation Sniffer by Bonsai.pdf (309.2 KB, 143 views)
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Old 14th November 2014, 05:28 PM   #2
Elvee is offline Elvee  Belgium
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It is indeed a useful piece of kit.

I personally use slightly different versions of detectors, see below.

Here are some pics of the voltage-doubler version. It uses point-contact Ge diodes and is extremely sensitive.

I generally do not connect physically the probe to the DUT: it can upset the circuit, inject LF perturbations, or even cause oscillations because of the capacitances or ground loops.
I prefer to use a small sniffing antenna made of a centimeter or two of insulated wire.
I also use inductive sniffing loops, when mainly currents are involved (sometimes, oscillations generate rather low voltages on accessible nodes, and inductive sensing is preferable; it also pinpoints the radiating tracks more accurately).
Attached Images
File Type: png Sniff1.png (60.8 KB, 215 views)
File Type: jpg Sniff2.jpg (51.5 KB, 213 views)
File Type: png Sniff3.png (803.8 KB, 222 views)
File Type: png Sniff4.png (958.5 KB, 210 views)
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Old 14th November 2014, 05:31 PM   #3
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What happened to the true and trusted RF detector scope probe??

Jan
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Old 20th November 2014, 08:21 PM   #4
rsavas is offline rsavas  Canada
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Quote:
What happened to the true and trusted RF detector scope probe??
Scope probe method is okay for VHF range, not so good for UHF+, unless you have a GHz scope and a fet probe = very expensive.
The passive diode detector will go into the GHz range but has limited dynamic range!!
Now add a wide band sensitive rf amp, (BF998 or j310 in common gate config for example) to that passive diode detector and you really are getting much better for a very cheap solution.
We used to use a vhf/uhf rf pager at Motorola to detect rf leakage from rf signal generators. Damn those pagers were sensitive.

Last edited by rsavas; 20th November 2014 at 08:32 PM. Reason: sniffer
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Old 20th November 2014, 09:21 PM   #5
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by rsavas View Post
Scope probe method is okay for VHF range, not so good for UHF+, unless you have a GHz scope and a fet probe = very expensive.
The RF detector probe mentioned by Jan is just that: a RF (diode) detector, very much like the first version of the circuits I showed. It was designed precisely to be able to detect tens or hundreds megahertz signals using a 2 or 4MHz oscilloscope.
However, for the more modern use of sniffing parasitic oscillations, this kind of old style probe is not ideal: the shapes described by Bonsai and myself are more adapted and flexible
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Old 21st November 2014, 05:37 AM   #6
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There are several, commonly available, Schottky diodes intended for UHF detector applications. They should equal the venerable 1N34 (or 1N21) for sensitivity, even when you aren't looking for signals at 1 GHz and above. Designations include the BAT68, HSMS2850 and HSMS2860, and MBD101.

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Old Yesterday, 08:34 AM   #7
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Quote:
Originally Posted by dchisholm View Post
There are several, commonly available, Schottky diodes intended for UHF detector applications. They should equal the venerable 1N34 (or 1N21) for sensitivity, even when you aren't looking for signals at 1 GHz and above. Designations include the BAT68, HSMS2850 and HSMS2860, and MBD101.
Dale, glad you posted that. I didn't want to order some diodes
for the OA81/OA91 That Bonsai posted in is sniffer.

But the good news is I have a supply of 1N34 that I don't use much.
So it looks like it should work. I'm going to be swapping some opamps
around and the sniffer looks like it will do the trick.

Thanks, y'all.
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