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G8MNY > TECH 05.12.10 11:16l 117 Lines 4641 Bytes #999 (0) @ WW
BID : 1821_GB7CIP
Read: GUEST
Subj: Clip on QRM Probe
Path: IZ3LSV<IK2XDE<IR2UBX<IW2OHX<UA6ADV<GB7CIP
Sent: 101205/0008Z @:GB7CIP.#32.GBR.EU #:1821 [Caterham] $:1821_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New June 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Wanting to find which lead on a devices like TV & STB set up has the most RFI
current it, I made a simple current probe from a clip in ferrite choke. it can
be used with a Rx or spectrum analyser. It can be use for Tx RF work too
finding if balenced 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
I used a clip on ferrite 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 & 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 & 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 & heat glue a narrow strip of it through hole shielding the
coil 1/2 a turn from the QRM cable. 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, & not put on too much wire that the
frequency response will be affected. With 50ê load a 0.5ê is effectively
inserted in QRM line.
0.5ê 1T
1A============
:::::10T
())))_____100mA
³_________50ê = 0.5W = 5V = 74dBmV = 134dBuV
WHERE TO TEST
The clip on will show the highest current point along a cable & the worst cable
in a set up (where a ferrite choke will be most effective).
Here is a typical TV set up...
SCART SCART
ÚÄÄÄÄÄÄÄÄÄÄÄ¿ RGB ÚÄÄÄÄÄÄÄÄÄ¿ RGB ÚÄÄÄÄÄÄÄÄÄÄ¿ \|/
³ TV ÃðÄÄÄÄÄÄÄð´ D.V.D. ÃðÄÄÄÄÄð´ S.T.B. ÃðÄÄÄÄÄÙ UHF
³ DISPLAY ÃðÄ¿ ÀÄÄÄÄÄÄÄÂÄÙ ÀÄÂÄÄÄÄÄÄÂÄÙ Aerial
ÀÄÄÄÄÄÂÄÄÄÄÄÙ ³ ÚÄÄÄÄÄÄÄÄ¿ ÀðÄMains ³UHF DC³ ÚÄÄÄÄ¿
MainsÄðÙ ÀÄð´ V.C.R. ÃðÄÄÄÄÄÄÄÄÄÄÄÄðÙ Àð´Plugà Mains
ÀÄÄÂÄÄÄÄÄÙ ³PSU ³
MainsÄÄðÙ ðtest points ~~~~
The clip probe will show which of these 13 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 or a few turns on
one is much better. You will also soon know if you have a steel tape core & not
a 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 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 not on others!
Not all these currents lead to radiation that affects ham bands!
It often depends on the wiring & wire aerials at that location. E.g. a few uA
of current into a 1/4 wave long TV 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=37ê QRMÄðÄÄÙ on 6m
1mA 1mA
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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