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AM transmitter modulation circuit design question

AM transmitter modulation circuit design question

AM transmitter modulation circuit design question

I'm currently in the process of building an AM radio transmitter, and it's working, but not great. When modulating the signal, a section of the negative part of the wave is being cut off. Does anyone know what might cause this to happen? Here's an image pulled from the oscilloscope, with the input signal (yellow) and the modulation (blue):

RE: AM transmitter modulation circuit design question

What sort of modulator circuit are you using?


The most straight-forward concept is 'Plate Modulation'. This is where the high power audio signal is imposed on the 'Plate' (or solid state equivalent) voltage in a final power amplifier stage.

You might simply be over-modulating, and it just so happens that you're running out on the negative side before the positive. In other words, if it's 'Plate Modulation', your plate voltage is going negative.

RE: AM transmitter modulation circuit design question

The most frequent cause of THAT picture is your amplifier is operating too close to its cut-off and is not amplifying the lower part as much as the upper part. That's why it is very flat and 'quiet' down there. You need to move it's operating point up and possibly not go for as much gain or raise it's bias voltage.

It would be very useful to show us the schematic. Use the picture with the circle in it.

Keith Cress
kcress - http://www.flaminsystems.com

RE: AM transmitter modulation circuit design question

Increase the negative supply voltage.

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