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G8MNY > TECH 23.07.05 00:19l 119 Lines 5210 Bytes #999 (0) @ WW
BID : 55228_GB7CIP
Read: GUEST OE7FMI
Subj: Reducing Electronic RF QRM 2/2
Path: DB0FHN<DB0RGB<DB0AAB<F6KFT<LX0PAC<ON0LGE<ON0DXC<WA7V<CX2SA<GB7CIP
Sent: 050722/1236Z @:GB7CIP.#32.GBR.EU #:55228 [Caterham] $:55228_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Mar 05)
2/ Applies the last filtering principle to the whole noise source..
RF proof
Metal Box
Filter ÚÄÄÄÄÄÄÄÄ¿ little
All leads ³ NOISE ³ \³/ RF
ÄÂÄ()))Ä´ SOURCE ÃÄ()))ÄÂÁÄÄ
=== ÀÄÄÄÄÂÄÄÄÙ ===
ÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄ
You can see this approach used on all PCs nowadays in an attempt to meet the
RFI laws. Ferrites are used on all leads close to any noise source box to
reduce any currents flowing into the leads & acting as aerials.
Full screening is often not used due to dangerous voltages etc. But it may
also be left off due to cost, & design / aesthetics / sales considerations.
eg. a metal box around an RF lit economy lamp. :-)
For TV -> HF QRM, consider the UHF aerial as a HF aerial, put a HF braid
breaker in the aerial & ferrites on the mains lead etc. as these filter
work both ways.
If QRM source items in 1 or 2 are not yours, then modification can be risky
& unadvisable!
I have heard of hams & interference source cases, who are prepared to buy a
neighbour a new TV etc to solve the problem. But note well, modern items can
be a lot worse than some older designs! eg low voltage 12v lighting now uses
80KHz 150W square wave power inverters rather than a straight 50Hz
transformer!
3/ Reducing the QRM seen by the Rx can be simple or hard to achieve.
a) If it is yours turn QRM source off. (TV / PC / RF lamp)
b) Prove path of QRM to RT, eg not 12V pick up/mains pick up. Rx on battery,
or via wound up extension lead. If not aerial filter leads.
For UHF/VHF QRM try braid cover leads of 1/4 wave long bonded to Rx earth.
\³/ ÚÄÄÄÄ¿
ÀÄÄÄÄÄÍÍÍ͵ Rx ÆÍÍÍÍÄÄÄDC
ÀÄÄÄÄÙ
c) Increase the distance from Rx aerial to QRM source. Move TV/ Aerials.
True RF fields die away @ the square law eg. 6dB per doubling distance.
But near fields with transformer/stray capacitance coupling like TVs
timebase etc. obey a cube law. eg 9dB per doubling of distance.
d) If QRM is steady, null out QRM with 2nd aerial & phase/amplitude system.
Noise Cancel
Main \³/ ÚÄÄÄÄÄ¿
Ant ÀÄÄÄÄÄÄÄÄÄÄÄ´Phase³ ÚÄÄÄÄ¿
Noise \³/ ³ & ÃÄ>´ Rx ³
Ant ÀÄÄÄ´Level³ ÀÄÄÄÄÙ
ÀÄÄÄÄÄÙ
4/ Operate when the QRM is not there. Operate on bands/frequencies unaffected.
Or filter it out after in the Rx. Impulse Noise gates or blankers were the
1st attempt to do this, they remove car ignition & spiky mains noise quite
well on SSB mode.
20KHz 2.3KHz
ÚÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿
SSB +> ³ MixerÃ>Ä´ WIDE ÃÄÂÄÄÄÄ>ÄÄ´ GATE ÃÄ>Ä´ SSB ÃÄÄ> No
Spikes ÀÄÄÄÄÄÄÙ ³ IF ³ ³ _ ÀÄÄÂÄÄÄÙ ³ IF ³ Spikes
ÀÄÄÄÄÄÄÙ ³ ³\ /| === ÀÄÄÄÄÄÙ
NoiseÀÄ´/`>ÄDETÂÙSpike
amp /³/ÄÄÄÄÄÄÙ
Fast AGC
The noise amp AGC is fast enough (300uS) to follow a single SSB voice
envelope, maintaining a fairly steady RF signal. When a wideband spike
occurs the rise time is too fast & it produces an AGC spike that is used
to turn off the IF gate to the narrow SSB filter. The result is very narrow
holes (eg 1mS) in the audio rather than loud clicks, that could also have
turned the Rx AGC down as well. Drawbacks are if there are too many stations
in the passband or big signals (eg contests) the noise amp miss operates
the gate making signals break up or sound very wide.
After the Rx. With modern noise DSP processing much of the QRM can be
removed with too much loss of intelligence in the signal with many types of
QRM noise. One external "add on DSP LS system" I tested with a steady
carrier noise was reduced by 65dB automatically at the max noise reduction
setting, or 35dB for more broadband noises.
________
Signal > \³/ ÚÄÄÄÄ¿ ³ D.S.P. ³ 99% of
+ QRM > ÀÄÄ´ Rx ÃÄÄ´NOISE LS³ Signal >
ÀÄÄÄÄÙ ³ SYSTEM ³
ÀÄÄÄÄÄÄÄÄÙ
It works by digitally analysing the AF spectrum into 100s of narrow
frequency bands, with an "FM noise squelch" system on each one, deciding if
the signal in the band is significantly above the noise to let through. If
the level on any band has not changed after 2 seconds ignore that band.
The drawback is that a voice with high levels of noise will sound like a
nasty cellphone call at high noise reduction levels, as the voice pitch to
allow through may be the wrong one!
When living with QRM, remember that our hobby is just a good a pastime a any
other. So you don't have to put up with it really, if you can get help to get
the problem sorted. Try an intermediary, local radio club, radio authority,
other radio users affected? (eg police, ambulance, aircraft, broadcast bands
etc). This can all help if you can find the right person to take up the case.
Why don't U send an interest bul?
73 de John G8MNY @ GB7CIP
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