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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated May 13)
(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. 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
================= ∙├─┤ ├─┤√ Catch
:::::-┐screen Rx_________\ \__/ /
10 ())))_│____ Coax~~~~~~~~\\▒▒▒▒/
turns└──────────50Ω 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, & 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 the centre 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 50Ω load a 0.5Ω is effectively inserted in QRM line.
0.5Ω 1T
1A============
:::::10T
())))_____100mA
│_________50Ω = 0.5W = 5V = 74dBmV = 134dBuV
WHERE TO TEST
The clip probe 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
┌───────────┐ RGB ┌─────────┐ RGB ┌──────────┐ \|/UHF
│ TV ├≡───────≡┤ D.V.D. ├≡─────≡┤ S.T.B. ├≡─────┘Aerial
│ DISPLAY ├≡─┐ └───────┬─┘ └─┬──────┬─┘
└────────┬──┘ │ ┌────────┐ └≡─Mains │UHF DC│ ┌────┐
Mains────≡┘ └─≡┤ V.C.R. ├≡────────────≡┘ └≡───────≡┤Plug├ Mains
└──┬─────┘ │PSU │
Mains────≡┘ ≡test 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
==========≡≡≡=≡====
SOURCE choke│
└──Rx
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" & not an RF ferrite core as it will have little effect at RF!
SPECTRUM
This is typical..
CRT TV SMPSU PC
│ │║| │ │ │
│▒ │║║║| │ │ │ │ │
│▒▒ │║║║║║| │ │ │ │ │ │▒▒▒==.._▌│______▒_ │║║║║║║║|:. │ │ │ │ │
15kHz 4,4 6 38MHz └╨╨╨╨╨╨╨╨╨╨╨╨╨╨┴──── └──┴──┴──┴───┴─
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...
│
│ ║║║ ║║║║
│ ║║║║║║║║║║║║║║
│ ║║║║║║║║║║║║║║║║
└─╨╨╨╨╨╨╨╨╨╨╨╨╨╨╨╨╨╨──
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 aerial lead, may cause havoc, but into a small loop of wires around the TV none at all.
UHF \│/ 10m
1/4 WAVE TV │ SMALL
VERTICAL ANT │ 37uW EQUIPMENT 0pW
AERIAL │ ERP on O R LOOP ANT ERP on
│ 40m __ 40m but
│ _│ │ high
QRM─≡───┘Z=37Ω QRM─≡──┘ on 6m
1mA 1mA
See also Tech Bul "Reducing Electronic RF QRM", & "Stopping HF Tx/Rx SMPSU QRM"
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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