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FM transmitter noise

spekis

 Hi. I'm an hobby diy enthusiast. I was trying to find a simple and stable FM transmitter circuit. Found one with MAX2606 voltage control oscilator.

image

 At first the circuit workd well so I went to the next step and connected the input signal from COOLAUDIO V2902 digital audio processor. And then everything got complicated.

image

 The signal started to distort and pop once I enabled the digital output. However the noise becomes quiter when i touch the signal components.

 So my question. Is this an shielding issue? Should i put the transmitter board in a metal box?

 Thank you in advance! 

  • 0

    do you have a schematic of the complete circuit? and photos of the built circuit? are you powering the circuit with a battery? is the audio source referenced to mains earth? this sounds like it could be shielding, grounding, or layout problems, or a combination of those

  • 0 in reply to Anthocyanina

    Hi Anthocyanina! Thank you for your fast reply. Unfortunately I can't provide aditional photos at the moment. I will return to the project next week.

     As for the power supply I'm feeding both circuits from the USB, tried with linux laptop and android phone. The results are compleatly identical. As the device is connected the transmitter works perfect. Only when the audio codec ic is activated the noise apears. So I belive there are no problems with power supply.

     Since its powered from battery it does not have mains earth.

     I will add some photos next week.

    Than you!

  • +1

    Other than shielding or grounding issues I would check signal levels. Distorted sound and "pop" also happen when the signal is constantly saturating the input of your transmitter.
    First, I'd use the analog outputs of the IC not the digital. I Googled the transmitter circuit that you are using and it looks to me like it expects an AC signal. The analog output voltage levels of this V2902 seem to have a 0.5V DC voltage, that you should get rid of with dc-blocking caps (as the picture shows).
    It would also be wise to add an op-amp or buffer between your audio IC and your transmitter.

    I'm definitely not an expert though, but that's what I'd check in that situation.

  • 0 in reply to battlecoder

    Hi battlecorder!

     Thank you for the hint. It saved my project. The IC was overloading the transmitter input. attenuating the output signal resolved the noise issue.

     Sorry for my extremely late response. Had to pause this project due to work.

  • 0 in reply to spekis

    Don't worry about the late reply, Happy to hear that what I said helped you fix the issue!