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AM or FM?

Discussion in 'Electronic Design' started by Herbert Blenner, Jul 18, 2003.

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  1. I have a written report on a spectral analysis of a radio reception. Does this
    report contain sufficient information to decide whether they used a AM or FM
    receiver?

    They reported:

    "On Channel-I spectra, several narrow-band high-energy tones are intermittently
    present for short durations. . . . Also at the end of each tone there is
    evident a sharp drop in total Channel-I energy. This indicates some kind of AGC
    (Automatic Gain Control) action."

    "These narrow-band tones are called heterodynes. They were generated when
    another transmitter came on the radio channel, while the transmitter with the
    stuck-open mike was transmitting. The difference in their carrier frequencies
    resulted in the heterodynes. If the second carrier is strong it should also
    activate the AGC action in the IF (Intermediate Frequency) stage of the radio
    receiver. . . ."

    "Most radio receivers have an AGC circuit at IF stage to maintain a steady IF
    signal level at the detector or discriminator. If there is a sudden increase
    in the RF signal (such as caused by switching on a strong carrier). AGC acts
    rapidly to reduce the IF amplifier gain to bring down the signal within
    acceptable limits. On the other hand if there is sudden decrease (such as
    caused by switching off a strong carrier) in the RF signal level, AGC acts
    more slowly to restore the IF amplifier gain. This is a typical characteristics
    [sic] of an AGC circuit: fast attenuation and slow recovery. The AGC action
    also affects the audio output level because of the drop in overall gain of the
    system. Therefore, when a heterodyne begins, we should expect a sudden drop in
    the recorded level of the signal picked up by the stuck-open mike. And after
    the heterodyne ends, we should expect a slow recovery in the audio signal to
    its original level. This phenomenon is indeed observed in Channel-I spectra."
     
  2. Almost certainly an AM receiver.


    A FM receiver tends to lock onto the
    strongest signal and ignore the weaker one, so heterodynes are nearly
    impossible.

    You didnt mention what kind of modulation the transmitters were using.
    If they were FM, then this report gathered with an AM receiver is still
    useful,
    but only if you keep in mind the mismatch of modes. For example, this AM
    receiver will helpfully point out interfering transmitters which might not
    have been
    noticed if they'd used a FM receiver. On the other hand, those interfering
    signals may be irrelevant if theyre somewhat weaker than the main signal,
    and listened to with FM receivers.
     
  3. James Meyer

    James Meyer Guest

    I do not think so.

    I also hope you did not have to pay anything for such an incomplete
    report.

    Jim
     
  4. Subject: Re: AM or FM?
    George, I appreciate your comments on this and the earlier AVC on FM thread.
    Since my interest is gathering reactions of technical people on these issues, I
    choose to keep my responses to a minimum in order to avoid biasing discussion.

    Regards, Herbert
     
  5. For FM, the gain of the system does not affect the audio output level
    (except in the extreme, by making it get lost amid static or other signals).
    Therefore, this is AM.
     
  6. Hi,
    This paragraph does not make much sense possibly because it has been edited or
    is out of context. In itself one would expect the removal of the tones to cause a
    drop in the total channel signal energy whatever the AGC did. I suspect though
    that the writer means a sharp drop from the Channel-I level before the arrival of
    the tones; without knowing all the facts it is difficult to judge though.
    With an FM receiver there is something called the "capture effect" where the
    strongest signal on a particular channel suppresses any weaker ones that may be
    present. However there is a narrow range of relative signal values where more
    than one can be discerned and, if they are not of exactly the same frequency,
    hetrodynes may be heard though this is not very common.
    Here the writer is almost certainly describing the AGC of an AM receiver with
    a 'fast attack' and 'slow decay' characteristic. Its commonest use is in the
    reception of SSB signals (single-sideband, suppressed carrier) where the absence
    of a carrier means that the AGC has to adjust to the peaks of the incoming signal
    but slowly decays between them to prevent pumping of the channel noise.
    Conventional AM (broadcast) receivers have an uncomplicated AGC system as the
    carrier is there all the time and may be used to adjust the IF gain without any
    fancy time constants.

    It is also possible that this analysis was done with some form of panoramic or
    surveillance receiver which would typically have an all mode capability. In that
    case, the type of receiver would depend on how it was actually used at the time.


    Cheers - Joe
     
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