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Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Thank you Reaching, that helps a lot. About tuning the secondary to the 4Mhz crystal, why not use a NE555 or TL494 and tune the primary to the secondary circuit so that you don't have to be perfectly at 4MHz?
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
a ne555 and a tl494 cant get higher than max 2mhz, and at these frequencies, the discharge makes still a lot of noise. 4mhz is totally silent. i used a crystal oscillator cause it was easier, but for sure you can use a 74hc4046 or some other high frequency pll to tune for the right frequency.
but at class e its difficult and makes not a lot of sense to tune the oscillator, cause you have an adjusted class e circuit which is only efficiently at a given fres, so i thought, why not tune the seondary to a given primary frequency with good adjusted and efficient class e circuit.
its really easy to tune the secondary and it makes a lot more sense, in case of simplicity and efficiency.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
I designed my 4MHz and 8MHz HF-SSTC units with the same viewpoint. I used a fixed crystal oscillator and set up all the gate-drive and matching networks to work correctly at the operating frequency. The only things that needed tuning were:
1. The Fres of the secondary 2. The coupling factor to get correct Class-E waveform with a soft-landing 3. The shunt cap across the MOSFET to set the power level.
The optimum shunt capacitor value actually varies depending on the supply voltage. This is one of the things that hampers efficiency and degrades linearity when high-level Amplitude Modulation is applied to a class E amplifier.
I also agree with Reaching's statements about tuning and layout becoming more critical with increasing frequency. The coil became a lot more sensitive to its surroundings at 8MHz than at 4MHz. And at 18MHz things get very finicky.
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Would this circuit work better in a Faraday cage to take away environmental EM interference then? In any case, I see your point about using the fixed frequency oscillator.
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Ok i tried to make the layout very low inductance and todays testing just led to a mosfet explosion and i am glad you told me that the shunt capacitor has to be of high quality. I dont have any pulse rated caps of that value so im probably going to end up sampling some cde caps for that.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
If I wanted to make the same coil without the audio modulation section and also change the oscillator frequency, would I have to change T1, C4, and C5? Also what does C10 do? If I were to use a different MOSFET would I have to use a different cap (C10)?
really late edit: HAHA. I laugh at myself now for asking these questions. It's a class E amp on the output so yeah, I would have to change stuff so everything switched at the right time.
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My 4Mhz amplitude modulated audio sstc
and i am glad you told me that the shunt capacitor has to be of high quality. I dont have any pulse rated caps of that value so im probably going to end up sampling some cde caps for that.
