Class-D AM-transmitter

AM for voice transmission may be a modulation of the past. But at least vintage radio hobbyists like me still use it.

Both class-D modes, voltage and current, VMCD and CMCD have interesting properties and at least CMCD is not widely used yet.

I wanted to learn more in form of real life test, to build a transmitter, where RF power stage is in current mode class D

and modulator in voltage mode class D. And here are the results.

The transmitter was built in modules because there were so many uncertainties. Testing and modifying was easier this way.

This experimental transmitter is for 80m AM only and by no means operators dream.

I think the attached block diagram and schematics tell how I did it.

Some comments on building this:

CMCD power amplifier was surprisingly easy to get working. Nothing critical in tuning or so. The loaded Q of the tank circuit is 4.

Lower Q shortens the voltage pulse and peak voltage increases.

The only difficulties with the VMCD modulator were with the switching diode in the output stage.

Normal silicon diode was too slow and died in overheat. Schottky diode solved that problem

PWM-modulator needed some trimming, amplitude, frequency and symmetry. Op-amps in the modeling library were different from real life products.

Peak level meter in the audio processor is a result of experiments. The fall time constant must be very large in order to get peak value from speech.

The 156mH coils used in the audio low-pass filter are Vogt 39mH common mode chokes, where the two windings are is series.

The VFO was from another earlier project.

Comments on the band have been encouraging. The audio quality is good and spectrum clean.

If you want to know more, drop me an email.

Pekka, OH1TV