| |
G8MNY > TECH 06.07.06 17:17l 211 Lines 9135 Bytes #999 (0) @ WW
BID : 37231_GB7CIP
Read: DL1LCA GUEST DL8ZX
Subj: Reducing Electronic RF QRM
Path: DB0FHN<DB0MRW<OK0PKL<OK0PPL<DB0RES<ON0AR<GB7CIP
Sent: 060706/0728Z @:GB7CIP.#32.GBR.EU #:37231 [Caterham] $:37231_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Mar 05)
(8 Bit ASCII Graphics use code page 437 or 850)
There are 3 principles to removing QRM & 1 not.
1/ Stop the RF being generated at the source.
2/ Stop Source RF from radiating.
3/ Stop Rx system seeing the RF QRM.
4/ Living with it.
1/ In electronic systems the use of radio frequencies is common, but the need
for wideband signals is less common.
Lets look at a logic gate that runs at 1MHz but uses a fast 40MHz totem pole
output stage. This produces a high current power rail pulse @ > 120MHz on
the switching edges.
Gate ³ ³ High bandwidth
Current ÄÄÄÁÄÄÄÄÄÄÁÄÄÄ High QRM
+5V ÄÄÂÄÄÄÄ
³_
______ ³ `\ ______
/ 1MHz \_____ ÄÄ´GateÃÄÄ ³ ³____
Low Bandwidth ³__./ 40MHz edges
low level QRM ³ Medium Bandwidth
0V ÄÄÁÄÄÄ Medium level QRM
The output fast edge shape of often wanted, but power rail noise is not.
Filtering is normally proved on the power rails to provide am on PCB path
for these high pulse currents..
PCB ANT
\³/ \³/
+5V ÄÄÂÄÄÄÄÄÂÄÄÄ 5V ÄÄÂÄÁÄÄÄÂÄÄÁÄÄ
³ ³_ But due to ) ³_
=== ³ `\ PCB track 1uH ) ³ `\
10n ³ ÄÄ´GateÃÄÄ lengths this ) Ä´GateÃÄÄ
³ ³__./ often looks 10n ³ ³__./
³ ³ like this... === ³ \³/
0VÄÄÁÄÄÄÄÄÁÄÄÄ 0v ÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄ
One cure is to put a C directly across any noisy ICs, these can be found by
scopeing the power rails on a PCB looking for the worst RF noise sources.
Then putting a Cs (surface mount?) across the power pins without loads of
track en-route.... Cs of 10n-10u can make difference to power rail noise
radiation.
PCB ANT
\³/ \³/ Much less
5V ÄÄÂÄÁÄÄÄÂÄÄÁÄÄ RF Noise
) ÚÄÄ´_ on PCB
1uH ) ===³ `\
) ³ ´GateÃÄÄ
10n ³ ³ ³__./
=== ÀÄÄ´ \³/
0V ÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄ
It is not usually possible to add L to the power rails to help the
decoupling.
Data bus radiation is more difficult to cure as there are more lines & the
RF power level is normally quite low from any particular line.
But filtering of a R or LC is sometimes effective (eg on a PC printer port),
but both type must consider the timing delay & loss & any undamped resonance
on the data line.
__ QRM __ Much less __ Very little
³ `\ \³/ ³ `\ \³/ ³ `\ \³/
ÄÄ´GateÃÄÄÁÄ Ä´GateÃÄ100ÄÂÁÄ Ä´GateÃÄ((()ÄÂÁÄ> Z load
³__./ ___ 1MHz ³__./ | stray ³__./ L ³
³ ³ ³___³ ³ === C ³ 1uH === 1n
ÄÄÄÁÄÄÄÄÄÄ ÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄ ÄÁÄÄÄÄÄÄÄÄÄÄÁÄ
Noise Source With R C the RF With L & C the
dB -6dB/Octave for Harmonics drop Harmonics drop
³ ³ | odd harmonics @ 12dB/O after @ 18dB/O after
³ ³ ³ ³ | -3dB point the damped
³ ³ ³ ³ ³ ³ | where Xc=R resonant Freq.
ÀÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁ
0 1 2 3 4 5 6 7 8 9 11 MHz
The actual radiation from PCB track is proportional to the frequency in
addition to any resonant track. So a 1MHz square wave may be very weak at
11MHz as the PCB track length is very small compared to a 1/4 wave, but
radiate quite loudly at 50 to 144MHz where the track becomes a good aerial.
These principles also apply to SMPSUs as well as TV/Monitor Line stages,
just that it is very difficult to apply in those high power cases.
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 echonomy 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 the QRM source off. (TV / PC / RF lamp)
b) Prove path of QRM to Rx, 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
Read previous mail | Read next mail
| |
