A small useful RF probe for the scope

On a sunny day (Mon, 22 Oct 2012 10:01:59 GMT) it happened Jan Panteltje wrote in :

PS, you can actually poke around with this on the ground plane and find the hot spots, where the RF goes so to speak, few cm makes all the difference. Find bad capacitators.. etc etc etc

Is the coaxial shield of the coax and the 3n3 capacitor supposed to create some virtual RF ground on the PCB, since there seems to be no real RF ground connection ?

What is the purpose of the 1/4 wavelength tip, is it some kin of 1/4 impedance transformer, transforming an open circuit to short circuit and visa versa ? With 1 Mohm scope input, the load at the tip would be nearly 0 ohms.

On a sunny day (Mon, 22 Oct 2012 16:11:46 +0300) it happened snipped-for-privacy@downunder.com wrote in :

Yes, at these frequencies, the capacitance of 'the rest' against ground seems to be enough for reliable measurements, as capacitive mains hum (50Hz here) does not pass a lot through the input cap. The 3nF close to the diode makes sure it actually sees a RF short.

It is some experiment with field strength measurement, say 'antenna'.

Main purpose it to be able to adjust things for maximum, and test some phase shift things I am into trying. And at the same time get an idea from radiation emitted by parts of the circuit, and if decoupling caps are actually decoupling.

Reminds me of my old school days with a RF voltmeter, but now at a frequency where a testpoint tip can be a significant part of a wavelength. Fun!

Do any digital scopes have an RF detector function built into their firmware? A 100 or 200 MHz scope wouldn't really need a diode detector... the function could be faked within the scope's bandwidth.

That BAT15 is a pretty nice diode.

I generally just use the envelope mode.

Cheers

Phil Hobbs

to be enough for reliable

the input cap.

circuit,

I have used similar circuits (series capacitor and 1N914/1N4148 to ground) to tune RF stages up to 1.3 GHz. At those days the standard OpAmp (741) had far too big bias/offsert currents, making RF probe + amplifier + multimeter useless, so I had to use the quite expensive 25 uA instrument as a signal strength indicator. Removing any active components from the system also removed quite a lot stray capacitance issues.

On a sunny day (Mon, 22 Oct 2012 19:57:15 +0300) it happened snipped-for-privacy@downunder.com wrote in :

Wow, 1N914, that is a high frequency for that part! And same for 1N4148! These BAT15/99 diodes are really nice, for mixers etc up to 12 GHz,

And dirt cheap, got them from ebay, guy send me a lot more than I payed for :-) The low knee voltage of 200mV at 10uA makes it possible to measure very small signals.

And still a visible size!

signals.

Eh, 100mV, close to zero if heated.

I take it that the 3.3nf cited is the capacitance of "the blue wire" coax.

Bet you do not need a (postage) stamp..

On a sunny day (Mon, 22 Oct 2012 22:17:59 -0800) it happened Robert Baer wrote in :

You took it wrong.

Wow, that would have to be some special cable to have that much capacitance. There is some spiral delay cable that has a lot of capacitance, the center conductor is actually a helix inside the sheath, but typical coax is about

Jon