• FM Stereo QRM

    From G8MNY@21:5/101 to TECH on Sun Jul 21 10:32:46 2019
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    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.
    ┌───────────┐ ┌───────────┐ ┌───────────┐ ┌───────────┐
    │ / │ │ /▒▒/│ │ / │ │ /▒/│
    │ / │ │ /▒/ │ │ / │ │ /▒/ │
    │ / │ │ / │ │ / │ │ /▒/ │
    │ / │ │ / │ │ /▒/ │ │ /▒/ │
    │ / │ │ / │ │/▒▒/ │ │/▒/ │
    └───────────┘ └───────────┘ └───────────┘ └───────────┘
    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>┌───────────┐<57.5> <57.5>┌───────────┐<57.5>
    _____│ WANTED FM │_____ _____│ WANTED FM │_____
    ┼───────────┼───────────┼ ┼───────────┼───────────┼
    < - - - - 265kHz- - - - > Ideal No Overlap adjacent ch
    | |< - - - - 300kHz - - - - >
    | | |Z| |
    | | |Z| |
    Close spaced< - 200kHz - - >|< 100kHz> |Overlapping Interferring Signals
    ┌───────────┐ | ┌───────────┐
    _____│ADJACENT FM│_____ _____│ 1/3 CH FM │_____
    ┴───────────┼───────────┴ ┴───────────┼───────────┴

    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_│ _____...---''' │ P │ ```---..._____ _____...---''' │
    ││ │ I │ │ │ │
    ││ │ L │ Difference │ │ Difference │
    ││ L+R │ O │ Signal │ │ Signal │
    -20dB_││ MONO │ T │ DSB L-R │ │ DSB L-R │ RDS -30dB_││ │ │ │ │ │ │
    └┼────────────────┼───┼───┼────────────────┴┬┴────────────────┼───╨──
    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



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