Curious questions About RF circuits?

Discussion in 'Wireless & RF Design' started by Mathematics!, Apr 30, 2009.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    So for a transmitter. We have mic/input , tunning/tank/modulation circuit stage , antenna stage. Maybe we need some amplification stage as well.
    But for transmitters I would think it is just gettting a carry wave and modulating it into space.

    For a reciever we have antenna stage , tuning stage , demodulation stage , amplification stage and then to the speaker or output port.


    What I want to know is what would happen if you transmitted on the FM station but used amplitude modulation while transmitting. (i.e modulated a carrier wave FM frequency with AM)
    Would you still hear it ? How would it sound?

    As Well what happens if you used frequency modulation and transmitted on the AM stations? (i.e modulated a carrier wave AM frequency with FM)
    Would this spam the AM stations?

    In both the reciever and the transmitter circuits is the tunning stages the same. Like if you made a transmitter could you use the same tuning circuit that you used in the transmitter for the tunning circuit of the reciever?

    Also is their anything more to the demodulation stage of AM then it being just a diode? It seems as though this is all you ever need to cut the negative part away and just use the positive part .
  2. deepak007

    Active Member

    Sep 30, 2007
    hello again!

    the circuit you are trying to build produces both AM and FM. you are tuning it on 88 TO 108Mhz FM band. right? but,if you use a 88-108Mhz AM receiver,you can hear the signal on that too! got it?
    but,what happens here is, you are using a FM receiver to receive the signal, in FM receivers the AM is, you won't get any disturbance. open your radio and look for a CAN type transformer, take a screw driver and tune it a little bit,until you hear the noise increase.then you can hear the AM SIGNAL as well.
  3. bertus


    Apr 5, 2008
  4. leftyretro

    Active Member

    Nov 25, 2008
    A typical receiver designed for FM demodulation would not extract any modulation information from a AM transmission on the same channel frequency. The IF stages in a typical FM receiver are driven to full saturation that eliminate any AM content.

    However, interestingly a AM receiver can sometimes demodulate a FM signal by slightly offset tuning and using slope detection. It's not optimum but I use to be able to hear 2 meter ham FM stations on a old 2 meter AM rig I had in the 60s, sounded fine no distortion at all. This was narrow band FM being sent so I don't think it would work as well with wide band broadcast type FM.

  5. PRS

    Well-Known Member

    Aug 24, 2008
    I like the idea about using slope detection. But why not just make a circuit that switches between FM and AM detection? We do that for SSB upper and lower.
  6. radiohead

    Distinguished Member

    May 28, 2009
    There are some schematics out there that will enable you to transmit on both AM and FM simultaneously. I haven't built one, but I've seen the schematic. One more thing is that some small/micro transmitters might have spurious emenations (harmonics) that may unintentionally bleed over to the AM band.
    Last edited: Jun 2, 2009

    AAC Fanatic!

    May 26, 2009
    I have a SSB transceiver (which is like AM but with the lower portion cutt off), and for the band 20-meters, you use USB (upper side band). I once put LSB (lower side band) on and listened and it sounds very peculiar. Nothing intelligeable but you can still detect some sort of signal.