OpenBCM V1.07b12 (Linux)

Packet Radio Mailbox

IW8PGT

[Mendicino(CS)-Italy]

 Login: GUEST





  
G8MNY  > TECH     27.03.24 11:00l 121 Lines 4870 Bytes #999 (0) @ WW
BID : 4172_GB7CIP
Read: GUEST
Subj: Clip on QRM Probe
Path: IW8PGT<LU4ECL<JE7YGF<JH4XSY<N3HYM<W0ARP<CX2SA<GB7CIP
Sent: 240327/0942Z @:GB7CIP.#32.GBR.EURO #:4172 [Caterham Surrey GBR] $:4172_GB
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 & 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, & 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, 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, & not put on too much wire that the
frequency response will be affected. With 50ê load only 0.5ê is effectively
inserted in QRM line.

    0.5ê 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 & 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" & 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 & 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 \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


 25.12.2024 06:09:26lGo back Go up