On Tue, 15 Sep 1998, Quentin wrote:

> I guess this is more for the hobbyist, but will give you a indication of
> emissions:

A small correction: EMI/RFI professional test gear comes with just
exactly such probes except they cost an arm and a leg ;) They do however
match the loop to the gear.

> Take a Scope probe's co-ax cable (cut the probe off). Strip back the screen
- snip -

Or better, just buy a piece of BNC coax and use that with one end
snipped off... it's significantly cheaper.

> the center wire so that you have a loop of about 1 inch diameter. Solder
> the wire end to the shield.

And to have good performance with this, connect the cable to the scope
using a 'T' BNC adapter and stick a termination resistor that matches the
cable's Z into it, if the cable is longer than 2 meters. That's what I do
at least. Without this, the loop + cable is tuned by the scope input C and
resonates somewhere in VHF-UHF which can be a real pain sometimes. It's a
good idea to take a temp. reading (finger) of the termination when
troubleshooting power leaks from Txs, and to have some spares...

> Now if you run this loop over your circuit you will see any emissions on
> the scope. Test this by running it over any RF circuit like a radio.

But not over a high Q LC oscillator that may stop and steer CLEAR of RF Tx
stages, i.e. do not move in very close to them (I mean power in excess of
2 Watts or so - the loop may start a self-oscillation or collect enough
energy to destroy the scope input). Superreactive receivers also loathe
the loop...

Anyway, there is a variant of this scheme that uses a hot carrier diode to
rectify the loop output for use with frequencies above the scope's.

And, last, this loop can be used as an ESR tester. Since it is current
coupled, if it will not read a strong signal on the suspected cap, vs, a
strong reading on the resp. power supply line, or choke then the cap is
probably dead.

$0.02,

Peter