Class E AM Transmitter with Pulse Width Modulator


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Progress is slow, but I need to do *something* to keep my brain active. Let me blog the creation and evolution of a high-powered Class E AM transmitter with Pulse Width Modulation…

After many weeks, I have a modulated waveform I am reasonably happy with. Here are some pictures:

1.) Triangle wave modulation of a 400 Watt carrier (100V per division into a 50-Ohm load)

Triangle 400W

2.) Sine wave modulation of a 400 Watt carrier (100V per division)


3.) Sine wave modulation, with negative peak clipping engaged:

400W sine negative clipping

More to come — I’m pretty slow at just about anything right now, but I will update and give circuit diagrams for this transmitter. Here’s the RF Class D PA section:

Class D PAPower supply is 110V DC. Running at about 30V+ carrier (set via the Pulse Width Modulator), 150% positive modulation is possible with this transmitter. It is four IRFP460N FETS, driven by 2 x Tc4412. The transformers you see are for the driver supply only, the big HT transformer is out of shot.

The output binocular has 15mm copper pipe running up the inside to form the primary: nominal current at carrier is up to 10A @ 45V or so for 100% modulation. The nice thing about PWM is that you can use the pulse width as a carrier control — and unlike an analogue series modulator, there’s hardly any heat dissipated by the PSU. So you can run this tx from as little as 10Watts – with carefully controlled audio input controlling the PEP.

A crucial component is the Pulse-Width Modulation Output Filter: it has to pass the DC supply for the output anodes drains whilst removing any trace of the PWM switching frequency, otherwise there will be carriers every 150kHz (or whatever the frequency used) all over the band!

Fag papers for scale. This is the low-pass filter that goes between the modulator (switch mode class D) and the PA.
Using multiple cores to scale up the PCB supplied by Eric and designed by Dave, GW4GTE.

Output filter and cores may be available from Designed by Dave, GW4GTE and fabricated by Eric, GW3JJ. Note I have dramatically modified the Output Filter to stack multiple cores, changed the wire to a bigger gauge as I expect 400W carrier and the original board was designed to supply a 100W output stage.