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27Mhz FM Transmitter and receiver circuit.

Discussion in 'Radio and Wireless' started by Rakesh nampalli, Mar 14, 2019.

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  1. Rakesh nampalli

    Rakesh nampalli

    Mar 14, 2019
    Hello guys, I designed a 27Mhz Fm transmitter and receiver but not able to get constant frequency at transmitter, iam trying to keep crystal oscillator so can you suggest me proper circuit (or) any Integrated chips in market for 27Mhz Fm radios.

    Rakesh N
    [moderator edit: E-Mail removed. Please answer within this thread or use pm]
    Last edited by a moderator: Mar 14, 2019
  2. Audioguru


    Sep 24, 2016
    Toys used 27MHz for radio control and CB radio many years ago. FM was not used, instead AM or pulses were used.
    FM modulates the frequency so it would be difficult to modulate the frequency of a crystal oscillator.
  3. Rakesh nampalli

    Rakesh nampalli

    Mar 14, 2019
    Thank you for reply, iam generating 27mhz frequency but not constant frequency, trying make FM CB radio(27mhz)
  4. Audioguru


    Sep 24, 2016
    The MC2833 IC is obsolete and is not made anymore. I think CB radio is also obsolete.
  5. TCSC47


    Mar 7, 2016
    I assume that you have googled the subject. There are a few circuits out there but not many. There is a reason for this. CB radio was introduced in the 70's to give untrained people access to the sort of communication that radio amateurs have. But if any Tom, Dick, or Harry has access to DIY radio transmitters they could create havoc with the airwaves and the people and services who depend on them. Thus CB radio equipment was only allowed if it had been commercially manufactured and undergone a rigorous test scheme. If you are experimenting with radio transmission, it is important to keep the power output less than 100mW. If you are interested in the CB community (if it still exists?) buy a commercial set up.
  6. FuZZ1L0G1C


    Mar 25, 2014
    Available as a complete parts kit, RSE used to sell a 27MHz walkie-talkie, with components, PCB schematic and instructions.
    This had its antenna output attenuated, and warning re not to extend supplied antenna length.
    Forgot what the max output was - suppose <100mW.
  7. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    This thread is becoming aged, and @Rakesh nampalli is probably a drive-by poster who is no longer monitoring the thread. I will reply anyway because I have little else to do right now.

    AFAIK, 27 MHz was reserved for CB radios and was NEVER used for FM communications, narrow-band or conventional, although individual CB channels were (usually) crystal controlled, as @Rakesh nampalli appears to be trying to do.

    Back in the day (about the middle of the 20th century) crystal control was necessary to guarantee sufficient separation between CB channels to avoid interference among adjacent channels. This made the CB radios of early adopters very expensive because a pair of crystals (one for receive, one for transmit) was needed for each of the original CB channels. Later, a single crystal was used with frequency synthesizer circuitry to provide operation on every CB channel. Still later, some CB rigs incorporated single side-band or SSB modulation, which is a form of amplitude modulation (AM) exactly like the original AM CB standard, except the carrier frequency and one of the two (identical) side bands was suppressed during transmit. This was supposed to double the number of available channels, but I don't think it was ever very popular. And, as far as I know, there was never any FM modulation permitted in the 27 MHz CB radio band.

    Cell phones and portable GPS pretty much eliminated any requirement for CB by previous owners of CB radios... unless you happen to be stuck out in the boondocks with nary a cell phone tower visible anywhere. If that happens, you either need a (very expensive) Iridium satellite telephone, or a radio amateur license and transceiver rig with which to communicate.

    So, please, do tell us why you are trying to design and build a 27 MHz FM two-way (transceiver) radio? Unless you are very careful, whatever you put "on the air" at this frequency, and with that modulation scheme, will be illegal and make you subject to heavy fines and possible imprisonment after your transmissions are detected, located, and you are arrested. If your devices actually interfere with licensed radio services, it is almost certain you will be discovered and prosecuted. So, please tell us what you are trying to DO. Do not describe how you think you should do it. People here will give you advice about that. Just tell us what you are trying to DO.
  8. Martaine2005


    May 12, 2015
    Back in the early 80s, CB radio was very popular in the UK. We were only allowed 40 channels on FM. AM was illegal. The USA used AM.
    Every radio had a label (CB/27/81).
    Citizen band/27Mhz/1981.
    Of course, some people bought AM rigs, myself included and under the counter labels were bought too. When I transmitted on AM, all the TVs in the area went blank and my voice came through the TV speakers!. I didn’t know this for at least 2 months. Hopefully I kept the airwaves clean!.
    Anyway, just wanted to comment that in the UK, only FM was legal.

    EDIT:: Apparently, AM and SSB are now legal in the UK. Max 4 watts FM and AM or 12 watts on SSB.
    Might have to buy another ‘President’ homebase.

  9. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Wow! Thank you, Martin, for that. It does look like I need to get out more. I didn't realize that UK (maybe other parts of the former British Empire, like India perhaps?) used FM exclusively and AM was verboten. So maybe @Rakesh nampalli isn't wandering too far off the beaten path after all. Gotta wonder why anyone wants to re-invent this particular wheel at this time though...
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