G8MNY  > TECH     29.11.21 08:15l 103 Lines 4902 Bytes #193 (0) @ WW
BID : 54367_GB7CIP
Subj: FM Stereo QRM
Sent: 211129/0906Z @:GB7CIP.#32.GBR.EURO #:54367 [Caterham Surrey GBR]

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...

An X-Y plot of L & R audio from a stereo signal reveals quite a bit when a mono
source is being transmitted.
 旼컴컴컴컴컴  旼컴컴컴컴컴  旼컴컴컴컴컴  旼컴컴컴컴컴
          /          /굇/           /           //
        /           //           /           //  
      /            /            /           //    
    /            /           //          //      
  /            /           /굇/         //        
 읕컴컴컴컴컴  읕컴컴컴컴컴  읕컴컴컴컴컴  읕컴컴컴컴컴
  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

                    ___________________       ___________________
                   /   Rx IF window    \     /   Rx IF window    \

                   LSB   Deviation   USB     LSB   Deviation   USB
                   Mod  <-2x75kHz->  Mod     Mod  <-2x75kHz->  Mod
                 <57.5>旼컴컴컴컴컴<57.5> <57.5>旼컴컴컴컴컴<57.5>
                  _____ WANTED FM _____   _____ WANTED FM _____
                 탠컴컴컴컴컴탠컴컴컴컴컴 탠컴컴컴컴컴탠컴컴컴컴컴
                 < - - - - 265kHz- - - - > Ideal No Overlap adjacent ch  
                 |           |< - - - - 300kHz - - - - >
                 |           |   |Z|     |
                 |           |   |Z|     |
 Close spaced< - 200kHz - - >|< 100kHz>  |Overlapping Interferring Signals
       旼컴컴컴컴컴         |   旼컴컴컴컴컴
  _____쿌DJACENT FM_____   _____ 1/3 CH FM _____
 좔컴컴컴컴컴탠컴컴컴컴컴 좔컴컴컴컴컴탠컴컴컴컴컴

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!

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!)

                      __         __                               __
  0dB_ _____...---'''     P     ```---..._____   _____...---'''  
      납                   I                                    
      납                   L      Difference       Difference   
      납      L+R          O        Signal           Signal     
-20dB_납      MONO         T       DSB L-R          DSB L-R       RDS
      30Hz             15  19  23kHz            38kHz              53  57kHz

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.

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

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