I bought Veronica 1W FM Transmitter online, hooking it up and tuning the radio beside me, I can hear the audio on multiple 'stations' or frequencies.

I've tried screwing the vc1 which the seller said changed the frequency, there are in total 4 pots.

Why is it sending to multiple frequencies, and how can I get it to send to only one frequency?

I think's it's this type: http://www.electroschematics.com/5/veronica-1w-fm-transmitter/

  • 3
    $\begingroup$ How far away are you placing your receiver from the transmitting antenna? Side by side may be saturating the receiver input. $\endgroup$
    – Juancho
    Aug 25 '16 at 22:02
  • $\begingroup$ Oh man. Googling Veronica 1W transmitters is a never-ending circle of people copying a common text without referring to the original source – including typos/orthographical errors. $\endgroup$ Aug 26 '16 at 8:33
  • $\begingroup$ Actually C12/C13 and L1 (together with T2/T3) is the oscilator, and will determine the frequency. -- this question is a bit off-topic, and would be better handled here $\endgroup$ Aug 26 '16 at 8:49
  • $\begingroup$ @Juancho, yeah the receiver is right next to a medium size antenna in the room $\endgroup$ Aug 26 '16 at 8:54
  • $\begingroup$ @Edwin van Mierlo, but is it possible that not tuning it correctly could make it send to multiple 'channels'? Or is it as Juancho said, the proximity of the receiver? $\endgroup$ Aug 26 '16 at 8:56

So you're clearly seeing signal where there shouldn't be any – this can have different reasons:

  1. Your transmitter actually transmits on frequencies it shouldn't
  2. Your receiver sees signal where there isn't actually signal

Both can again be caused by different things – the Veronica class of transmitters seem to be working with "kind of" two 180° offset intermediate frequencies of half the desired RF carrier. So your transmitter could actually just be leaking those on TX.

Also, your receiver might simply be picking up the strong signal although it's tune somewhere else completely, simply by the fact that the receiver's mixers never "calculate" the frequency difference you wish, but also other linear combinations of RF and IF – you can only counteract that by filtering, but filtering might simply not be sufficient if the input power is so large that things still make it through the filter in sufficient strength.

Then: every active component in both your receiver and your transmitter is potentially a mixer. Fundamental truth of RF design: every nonlinearity produces intermodulations (ie. sum and difference frequencies of the different signal frequencies present at the component). Most don't do that intentionally, and designers take care to eliminate the effects. However, especially amplifiers get the less linear the more you drive them with an overly strong signal – making them mix different frequencies present. So, suddenly, your receiver's (hypthetical, I don't know your receiver) IF amplifier becomes a mixer for the IF and the remnants of RF that passed through the "intentional" mixer, leading to surprising results.


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