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    FM Transmitter, receiving on two frequencies

    Hi.
    I have a problem with my FM transmitter. It's simple transmitter using two transistors. The problem is I receive signal from two frequencies. For instance, I tuned it on 114Mhz and I receive on 114Mhz and on 93Mhz. The two signals have the same strength. Is there any solution to avoid receiving on 93Mhz, and why it occurs?

    •   Alt30th October 2013, 20:50

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    Re: FM Transmitter, receiving on two frequencies

    114MHz is in the aircraft radio band. Keep away from it.

    I think you have a cheap FM radio that is overloaded by the very strong nearby signal.

    Many cheap FM radios like my Sony Walkman are fairly sensitive but are easily overloaded. They have a "local/distant" switch that attenuates the signal in the local position. In the distant position a strong signal appears all across the dial.


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    Re: FM Transmitter, receiving on two frequencies

    This receiver consume 6 mA at 5V, so it has short range, I use it to monitor my garage and I don't want to my neighbor be able to hear it in standard radio. My car radio don't receive it at 93Mhz. I tried shorter antenna and added resistor to decrease range but the both signals has the same strength. Can I do something with that?



    •   Alt31st October 2013, 02:13

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    Re: FM Transmitter, receiving on two frequencies

    Some radio scanners use a 21.4MHz (2 x the normal 10.7MHz) IF frequency. 114-93MHz = 21MHz. So it appears you are picking up some image frequency on 93MHz.

    What receiver are you using?
    Last edited by E-design; 31st October 2013 at 03:56.


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    •   Alt31st October 2013, 03:51

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    Re: FM Transmitter, receiving on two frequencies

    I'm using normal radio with stretched coil inside it in tank circuit. It can receive up to 123.6Mhz.



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    Re: FM Transmitter, receiving on two frequencies

    It is not the problem of transmitter. Receiver catches two frequencies. 10.7MHz is IF (intermediate frequency) in them so it rceives 10.7 upper and lower frequencies to local oscillator. That is it rceives two frequencies 21.4MHz apart.


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    Re: FM Transmitter, receiving on two frequencies

    Quote Originally Posted by Mat1603 View Post
    I'm using normal radio.
    I think the details are missing.
    I can buy a "normal radio" at The dollar Store. Its battery and earphones are worth more that the cost of only one dollar so the 'radio" is free. Its performance is horrible and is sound is even worse.

    A cheap clock radio is also a "normal radio" like my cheap Sony Walkman FM stereo radio. The clock radio has poor sensitivity so it does not get overloaded. The Sony Walkman has a local-distant switch.

    My hom stereo and car radio are also ``normal radios`. `The re extremely sensitive but strong local stations (or my fM transmitter up close) do not cause overload.



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    Re: FM Transmitter, receiving on two frequencies

    Thank you for guide me to topic about IF. In my case the key is MIRR (image response rejection ratio) so i think I can't do nothing with that, it's depending on receiver. I'm new in radios, I feel more confident with programing microcontrollers, I tried generate carry wave by microcontroller and mixing voice but it isn't working very well.



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    Re: FM Transmitter, receiving on two frequencies

    A high quality FM radio has excellent image frequency rejection.
    A cheap radio is a cheap radio and has poor performance.



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