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G8MNY > TECH 05.11.14 12:34l 98 Lines 4389 Bytes #999 (0) @ WW
BID : 14645_GB7CIP
Read: GUEST
Subj: FM Stereo QRM
Path: IW8PGT<IZ3LSV<I0OJJ<GB7CIP
Sent: 141105/1021Z @:GB7CIP.#32.GBR.EURO #:14645 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (New Sep 13)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Monitoring a community stereo radio station with a weak signal, I often Rx it
with QRM despite the capture effect of wideband FM (>20dB of quietening).
The QRM is mainly due to just off channel stations that are crammed into Band 2
but sometimes co-channel stations too. The effects are quite different...
SCOPE ANALYSIS
An X-Y plot of L & R audio from a stereo signal reveals quite a bit when a mono
source is being transmitted.
ÚÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄ¿
³ / ³ ³ /±±/³ ³ / ³ ³ /±/³
³ / ³ ³ /±/ ³ ³ / ³ ³ /±/ ³
³ / ³ ³ / ³ ³ / ³ ³ /±/ ³
³ / ³ ³ / ³ ³ /±/ ³ ³ /±/ ³
³ / ³ ³ / ³ ³/±±/ ³ ³/±/ ³
ÀÄÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÙ
Clean Mono HF QRM LF QRM Co-Ch QRM
19kHz filtered (depends on scope phase) (or noise)
Spectrum wise this is easy to understand, as FM demodulated QRM signals are at
there lowest frequency when the are instantaneously closest to the wanted FM
signal.
FULL & HALF ADJACENT CHANNEL QRM
___________________
/ Rx IF window \
LSB Deviation USB
Mod <-2x75kHz-> Mod
<57.5>ÚÄÄÄÄÄÄÄÄÄÄÄ¿<57.5>
_____³ WANTED FM ³_____
ÅÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÅ
< - - - - 265kHz- - - - >
< - 200kHz - - > < 100kHz>
ÚÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄ¿
_____³ADJACENT FM³_____ _____³ 1/2 CH FM ³_____
ÁÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÁ ÁÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÁ
With 200kHz spacing there is no overlap of the FM "carrier" signals just the
weaker sidebands. (instantaneous RF frequency)
But with 100kHz station spacing, adjacent channel band planning should always
make the sidebands too weak to cause much QRM. The QRM is due to the 2 deviated
FM carriers being so close & can actually overlap in frequency.
When this only causes Rx demodulated beats above 53kHz there is little effect
except on the RDS, beats below 15kHz cause QRM even on a mono Rx.
When these beats are in the range 23kHz-53kHz only, the stereo is damaged, QRM
is heard as a behind you noise (cancels out in mono), that worsens with either
station's modulation depth. E.g. the more modulation on the wanted station the
worse the noise. This goes against common sense, where more modulation usually
means better signal to noise!
RX TUNNING & FILTERS
Off tuning the Rx a few 10s of kHz away from the QRM, may have some effect on
reducing the QRM, but at the cost of some added Rx distortion. This is much the
same as using a narrow IF filter (Hifi ideally needs 280kHz, some car radios
use 180kHz filters for good /M reception, but at higher distortion!)
STEREO BASEBAND SPECTRUM
__ __ __
0dB_³ _____...---''' ³ P ³ ```---..._____ _____...---''' ³
³³ ³ I ³ ³ ³ ³
³³ ³ L ³ Difference ³ ³ Difference ³
³³ L+R ³ O ³ Signal ³ ³ Signal ³
-20dB_³³ MONO ³ T ³ DSB L-R ³ ³ DSB L-R ³ RDS
-30dB_³³ ³ ³ ³ ³ ³ ³
ÀÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÅÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÂÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÐÄÄ
30Hz 15 19 23kHz 38kHz 53 57kHz
CO-CHANNEL QRM
Little can be done to remove the spiky 2nd station noise other than the capture
effect, so improving the wanted signal a few dB over the QRM, will help quite a
bit more than the few dB.
CONCLUSION
Although Broadcast FM can give very good quality, often better than DAB, both
systems suffer from crowding. With DAB it is data rate quality designed in for
the number of stations per multiplex. With FM it limited by the clean QRM free
service area RF range, especially in crowded cities.
Also see my buls on "FM Stereo Radio Principles", "FM Deviation Calibration".
& "Band 2 6el Narrowband Yagi".
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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