| |
G8MNY > TECH 24.03.90 09:31z 122 Lines 4888 Bytes #999 (0) @ WW
BID : 54775_GB7CIP
Subj: Clip on QRM Probe
Path: HB9ON<IW8PGT<IZ3LSV<IW0QNL<JH4XSY<N3HYM<GB7YEW<GB7CIP
Sent: 260318/1048Z @:GB7CIP.#32.GBR.EURO #:54775 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Nov 19)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Wanting to find which lead on a devices like TV and STB set up, has the most
RFI current in it, I made a simple current probe from a clip in ferrite choke.
It can be used with a Rx or spectrum analyser. And for Tx RF work too, finding
if balanced line is balanced, coax line is really RF free etc.
THE CURRENT PICK UP
/~~~~\
QRM cable Hinge / /~~\ \ Locking
================= ùÃÄ´ ÃÄ´û Catch
:::::-¿screen Rx_________\ \__/ /
10 ())))_³____ Coax~~~~~~~~\\±±±±/
turnsÀÄÄÄÄÄÄÄÄÄÄ50ê Coax Heat Glue^ Coil
The clip on ferrite is about 2cm dia with a single locking catch, the catch
guard I cut off making it easy to unclip. I removed 1 of the ferrite halves
from the hinged plastic casing by levering a plastic protrusion out of the way.
Then using some very fine enamelled wire I wound 10 turns on it, made a hole in
the case, passed the wire ends through it, and then put back the ferrite
against the hidden spring inside the case until locked in place. (depends on
make)
I glued a thin coax to the case and connected the coil, after DC & RF testing,
I soldered some aluminium foil (or copper foil if you have no aluminium solder)
to the coax outer and heat glue a narrow strip of it through the centre hole
shielding the coil 1/2 a turn from the QRM cable, do not short the search coil
with a full turn of screening! Then I put a layer of thin tape (parcel tape)
over this to protect the coil.
I have seen other design use a cloths peg to house the 2 ferrite cores.
10 turns was used to give some calibration, and not put on too much wire that
the frequency response will be affected. With 50R load only 0.5R is effectively
inserted in QRM line.
0.5R 1T
1A============
:::::10T
())))_____100mA
³_________50 ohm = 0.5W = 5V = 74dBmV = 134dBuV
WHERE TO TEST
The clip probe will show the highest current point along a cable and the worst
cable in a set up (where a ferrite choke core will be most effective!)
Here is a typical TV set up...
SCART SCART
ÚÄÄÄÄÄÄÄÄÄÄÄ¿ RGB ÚÄÄÄÄÄÄÄÄÄ¿ RGB ÚÄÄÄÄÄÄÄÄÄÄ¿ \|/UHF
³ TV ÃðÄÄÄÄÄÄÄð´ D.V.D. ÃðÄÄÄÄÄð´ S.T.B. ÃðÄÄÄÄÄÙAerial
³ DISPLAY ÃðÄ¿ ÀÄÄÄÄÄÄÄÂÄÙ ÀÄÂÄÄÄÄÄÄÂÄÙ
ÀÄÄÄÄÄÄÄÄÂÄÄÙ ³ ÚÄÄÄÄÄÄÄÄ¿ ÀðÄMains ³UHF DC³ ÚÄÄÄÄ¿
MainsÄÄÄÄðÙ ÀÄð´ V.C.R. ÃðÄÄÄÄÄÄÄÄÄÄÄÄðÙ ÀðÄÄÄÄÄÄÄð´Plugà Mains
ÀÄÄÂÄÄÄÄÄÙ ³PSU ³
MainsÄÄÄÄðÙ ðtest points ~~~~
The clip probe will show which of these 14 places may the best place for a
choke. And you can even check an added choke is having some effect..
QRM Test
==========ððð=ð====
SOURCE choke³
ÀÄÄRx
As single clip on choke is not very effective (5dB!). More of them or a few
turns on one is much better. You will also soon know if you have an AF "steel
tape core" and not an RF ferrite core as it will have little effect at RF!
SPECTRUM
This is typical..
CRT TV SMPSU PC
³ ³º| ³ ³ ³
³± ³ººº| ³ ³ ³ ³ ³
³±± ³ººººº| ³ ³ ³ ³ ³
³±±±==.._ݳ______±_ ³ººººººº|:. ³ ³ ³ ³ ³
15kHz 4,4 6 38MHz ÀÐÐÐÐÐÐÐÐÐÐÐÐÐÐÁÄÄÄÄ ÀÄÄÁÄÄÁÄÄÁÄÄÄÁÄ
lines 45kHz 50Hz FM hummy lines Clock lines
Now as most things have SMPSU & PC architecture in them, so combinations occur.
Even a USB keyboards show up like this...
³
³ ººº ºººº
³ ºººººººººººººº
³ ºººººººººººººººº
ÀÄÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÄÄ
5 10 15 20 25 MHz
TIPS
Some devices may put out a lot of RF current on some leads and not on others!
Not all these currents lead to radiation that affects ham bands!
It often depends on the wiring and wire aerials at that location. E.g. a few uA
of current into a 1/4 wave long TV aerial lead, may cause havoc, but into a
small loop of wires around the TV none at all.
UHF \³/ 10m
1/4 WAVE TV ³ SMALL
VERTICAL ANT ³ 37uW EQUIPMENT 0pW
AERIAL ³ ERP on O R LOOP ANT ERP on
³ 40m __ 40m but
³ _³ ³ high
QRMÄðÄÄÄÙZ=37 ohm QRMÄðÄÄÙ on 6m
1mA 1mA
See also Tech Bul "Reducing Electronic RF QRM", "Stopping HF Tx/Rx SMPSU QRM" &
"Analyser SWIRES RESEARCH SA87".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
