| |
G8MNY > TECH 07.12.19 12:01l 103 Lines 4902 Bytes #999 (0) @ WW
BID : 34699_GB7CIP
Read: GUEST
Subj: FM Stereo QRM
Path: IZ3LSV<IQ2LB<IW2OHX<IR2UBX<IW0QNL<JH4XSY<JE7YGF<GB7CIP
Sent: 191207/1035Z @:GB7CIP.#32.GBR.EURO #:34699 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Feb 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Monitoring a community stereo radio station with a weak signal here, 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 (1/3 of a channel) 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 \ / Rx IF window \
LSB Deviation USB LSB Deviation USB
Mod <-2x75kHz-> Mod Mod <-2x75kHz-> Mod
<57.5>ÚÄÄÄÄÄÄÄÄÄÄÄ¿<57.5> <57.5>ÚÄÄÄÄÄÄÄÄÄÄÄ¿<57.5>
_____³ WANTED FM ³_____ _____³ WANTED FM ³_____
ÅÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÅ ÅÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÅ
< - - - - 265kHz- - - - > Ideal No Overlap adjacent ch
| |< - - - - 300kHz - - - - >
| | |Z| |
| | |Z| |
Close spaced< - 200kHz - - >|< 100kHz> |Overlapping Interferring Signals
ÚÄÄÄÄÄÄÄÄÄÄÄ¿ | ÚÄÄÄÄÄÄÄÄÄÄÄ¿
_____³ADJACENT FM³_____ _____³ 1/3 CH FM ³_____
ÁÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÁ ÁÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÁ
With 200kHz close spacing there is no overlap of the FM "carrier" signals just
the weaker sidebands. (instantaneous RF frequency)
But at 100kHz spacing full power instantanious full power carrier signals
actually overlap |Z| the other channel. Propper 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.
When this only causes Rx demodulated beats above 53kHz, there is little effect
except on the RDS, carrier beats lower than 15kHz even mono signal is dammaged.
When these beats are in the range 23kHz-53kHz only, the stereo is damaged, QRM
is heard as a noise behind you (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 FM
capture effect, so improving the wanted RF signal over the QRM, will help quite
a lot more at AF than the few of actual RF dB improvement.
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 over QRM, 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
Read previous mail | Read next mail
| |
