[600MRG] Fwd: Class E Keying

[Jim Miller]

Hans

Class-E is inherently non-linear by design. Attempts to modulate the input
remove it from Class-E.

Although certainly not perfect RC shaping is better than no shaping at all.
More sophisticated PWM modulation can no doubt be accomplished to achieve
different PA waveforms.

PA modulation at a suitable frequency and properly filtered should not
produce undesirable sidebands.

73

jim ab3cv

On Fri, Feb 12, 2016 at 4:06 AM, Hans Summers <hans.summers at gmail.com>
wrote:

> Hi Frank (et al)
>
> This topic is of particular interest to me since I am experimenting with
> key-shaping of a CW PA on the workbench at the moment. Mine is broadband
> (all HF) and not Class-E but I think my comments apply equally. My
> questions/comments for discussion:
>
> 1) Why key the power supply?
>
> http://njdtechnologies.net/wp-content/uploads/2016/02/GW3EUP-keyed-pa.gif
>  and
> http://www.its.caltech.edu/~mmic/reshpubindex/papers/buckwalter.pdf
> I am not clear on what the point is, of keying the power supply. It appears
> to me, better to key-shape the amplitude of the PA input. This is at a much
> lower level, requiring smaller, lower power devices and simpler circuits.
> What am I missing? To be sure, the shape might not be ideal... but that
> brings me to my next point:
>
> 2) RC key-shaping isn't very good... is it?
>
> An ideal envelope shape is the raised-cosine. Now:
> http://www.its.caltech.edu/~mmic/reshpubindex/papers/buckwalter.pdf
> Look at figure 4. it's typical of what you get when you use a simple
> Resistor-Capacitor envelope shaper. The capacitor charge/discharge through
> the resistor is an exponential rise/decay. On key-down, the front of the RF
> envelope has a nice shape. But on key-up, the sudden exponential decay is
> far from ideal and will create a click. I'm sure the quoted 10ms decay is
> better than a hard on/off keyed envelope (rectangular) but still, it isn't
> ideal.
>
> 3) PWM sidebands?
>
> Finally on the subject of using PWM to shape the envelope, surely that has
> the potential to introduce sidebands on the output waveform, at the PWM
> frequency and its harmonics. Yes filtering will help but... will it be good
> enough?
>
> 4) My project:
>
> What I am exploring is adjustment of the bias on the driver stage. I am
> using an 8-bit Digital-to-Analogue Converter (actually a shift-register and
> R-2R network) to control the bias level to that driver stage. In my circuit
> It allows me to vary the final power output from zero to full power in 256
> steps. A capacitor smooths out the steps too. The DAC is programmed by a
> microcontroller to create the key shape. It is possible (and not very
> difficult) to quite closely approximate the ideal raised cosine keying
> envelope. I mean on both key-up and key-down. Yes, now you need a 74HC595,
> some resistors for the R-2R network, and a microcontroller - but all these
> things are very cheap (your project may already include a microcontroller),
> I bet they cost less than the lumpy MOSFET that's being proposed to ramp
> the PA Power - and the result is a real shaped envelope!
>
> My purpose is a future PA module QRP Labs kit, that can be used with the
> Ultimate3S QRSS/WSPR transmitter kit http://www.qrp-labs.com/ultimate3 and
> other future QRP Labs kits. One simple cheap IRF510, I have not tested it
> on 600m. The closest I tested it at is 160m. With 13.8V power supply I get
> 14W output at 75% efficiency.
>
> Comments very welcome... I feel sure I must be missing something(s) !
>
> 73 Hans G0UPL
> http://www.qrp-labs.com
> http://www.hanssummers.com
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