[sdiy] sniffing

[Magnus Danielson]

From: "Czech Martin" <Martin.Czech at Micronas.com>
Subject: RE: [sdiy] sniffing
Date: Thu, 24 Apr 2003 15:11:26 +0200

> I bought a styro foam ball (will cover with alu foil) for E-field
> and a toroid (some copper windings on it and alu foil
> E-field shielding) for B-field (H-field).
> Some early experiments show n x 10mV of AC line noise (50 Hz
> fundamental, but much overtones) for the E-field (or
> voltage, as J.H. pointed out) probe.
> I try to build a jfet decoupling amp for the E-field
> sensor, the other one will go into 50 Ohm scope input.

You probably want a buffer amp regardless. It raises the effective noisefloor
of your system.

> Interesting side effect: I tryed to put the scope ground clamp/tip
> loop into a metal case in order to see the shielding effect...
> I found out that the 107 MHz FM stuff will disappear if the loop
> is far away from metal (table etc.). So I think that some capacitanc
> is needed in order to tune the loop for this frequency.
> Since I could'nt close the metal case completely, no shielding
> was observed....

"Completely" is allways a relative term in these cases. Allways look for how
much damping of offending frequencies you acheive and also watch out so that
you do not happend to reduce the damping of some other frequencies.

Try by changing the terminating impedance. This will give you a good hint if
you have mainly magnetically or mainly capacitive coupling.

> btw.: I learned that the Hameg 1505 scope has no 50 Ohm
> termination as built in switch (which I thought it had,
> bacause all the Tek scopes have it and I'm used to it).
> So a clear reflection could be seen when using RG58
> 50 Ohm lines and a square wave generator.
> You have to buy a special through-termination-resistor-BNC-thing
> for a ridiculous price.
> I then opted for a BNC-T and a BNC-50-Ohm termination piece,
> that is ok, at 1/10 of the cost.

BNC-T's are horrible things at higher frequencies...

The impedance-missmatch is just not nice. However, if you put it right up on
the oscilloscope the stub into the scope will act as a capacitive load at
lower frequencies. Probably works well enought for you anyway.

Built-in 50-ohm terminators or through-terminators is really the best thing.

You can build yourself a through-terminator quite easilly.
49.9 Ohm chip-resistors are a lovely invention ;O)

Cheers,
Magnus