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Registered Member #2115
Joined: Fri May 08 2009, 01:17PM
Location: Singapore
Posts: 46
Hi everyone,
As detailed on Richie's site and a few others, the final class E power amplifier has a relatively narrow bandwidth and thus frequency modulation is not recommended.
Instead, modulation of the high level +150V of the power amplifier itself is recommended.
However, the issue is, that in either case, modulation works by changing the size of the corona/streamers produced. And regardless of whichever modulation route you take -- whether you use FM or high level AM, you're still going to get a changing f0 due to different instantaneous levels of corona detuning.
Simply put: Assuming high level AM affects the coil's f0 as much as directly using FM (does it?), then why not just use FM directly?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well there is one important difference. Using high level AM, you can tune for efficient operation at high power. The circuit might go out of tune at lower powers, but these correspond to low DC bus voltages. On the other hand, with FM you will be driving the circuit out of tune at full DC bus voltage most of the time. The energy dissipated due to hard switching is proportional to the square of the voltage, so the second scenario has a lot more scope for blowing things up.
Also, if using AM, you're free to vary the drive frequency at the same time, to whatever gives the best switching conditions. I'm not sure this is very useful in practice though, as what really matters is the tuning of the Tesla resonator relative to the final amp's output network, and most people seem to use fixed-frequency drive for their Class-E coils anyway.
Registered Member #2115
Joined: Fri May 08 2009, 01:17PM
Location: Singapore
Posts: 46
Hi Steve Thanks for pointing that out,i had overlooked the fact that losses were =1/2 C V^2 f
Was thinking about some other low-level modulation methods which are more elegent than a high level AM/Class H modulator. Another drive method that allows modulation without touching the DC Bus is the double ended arrangement, using outphasing of one PA relative to the other to control power throughput (Max. power @ 180deg, minimum at 0 deg). (I think this is in Richie's site somewhere). In this case, the bus voltage would be constant at the full DC bus voltage. However, the frequency would still change with different depths of modulation and so losses will be as bad as using a FM, when using a PLL to track frequency/? Should we just scrap the PLL and use a fixed frequency drive?
Also with a double ended arrangement the load presented to each Class E amplifier is variable, is the final Class E power stag dependent on load impedance?
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Class E audio modulation--question
