FM receiver question.

Discussion in 'General Electronics Chat' started by kavkav, Jun 20, 2013.

  1. kavkav

    Thread Starter Member

    Jan 1, 2013
    I want to know if I understand frequency modulation correctly. Let me know if my example below is correct and if it is considered as frequency modulation.

    "If I have a signal of 5 MHz, I can give it a DC pulse of 1000 Hz. My receiver is tuned to pick up 5 MHz. Therefore my receiver is picking up a 5 MHz signal, 1000 times a second. The speaker of my receiver will give a 1000 Hz tone."

  2. vk6zgo

    Active Member

    Jul 21, 2012
    Firstly--No,that is not frequency modulation.

    Secondly,are you cutting off the 5MHz between pulses?

    If so,yes,it will sound like 1000hz,with a fair bit of noise in the background,on an AM receiver as the receiver will hear any noise present during the gaps.
    An FM receiver will not resolve the 1000Hz.

    If the pulse doesn't ever cut off the 5MHz,you are Amplitude Modulating the carrier,so an AM receiver will give a 1000Hz tone at the speaker.

    Again,an FM receiver will not resolve the 1000Hz.

    If,you could arrange for your 5MHz carrier to change to,say 5.05MHz on each pulse,& return to 5MHZ between pulses,you would have Frequency Modulation.
  3. kavkav

    Thread Starter Member

    Jan 1, 2013
    Ohhh Okay, that makes sense. Thanks!
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    No when you modualte an FM carrier it alters its frequency by the modulating audio signal, so if say your transmitting on a 1Mhz carrier and introduce an audio signal of 1Khz the fm carrier Deviates +/- producing Two sidebands at 1000 times as second ,

    the Max volume of the signal is +/- 75KHz for Streo commercial radio (150KHz total deviation ( Volume) alters the frequency by that amount, so your carrier is sweeping from 0.925Mhz to 1.075Mhz max Volume . and 0.9975Mhz to 1.0025Mhz minimum volume, They have a guard band of 25Khz for adjacent channel seperation to stop bleed through.

    So the speed that it sweeps from minimum to maximum frequency is set by the Modulating signal frequency, the amount of sweep distance is set by the Volume (Deviation) of the modulating signal.

    Most CB radios are limited to +/-2.5Khz deviation (Volume) with a guard band of 10Khz either side of the station to prevent bleed through.
    Last edited: Jun 20, 2013
  5. kavkav

    Thread Starter Member

    Jan 1, 2013
    Ya, I totally misunderstood this. I get now why FM can be so powerful since the signal can be weak but as long as the deviation is there, you can have a relatively loud volume. Thanks.