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Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Another thread related to my Class E SSTC. Wow, you guys must be getting annoyed... I am having an extremely hard time scoping the gate and drain waveforms on my 4MHz SSTC. My scope is cranked up fully in timebase, manual timebase and multiply timebase by 10, but hey, thats OK. What is bothering me is that a huge amount of noise is fed into the scope, so if I wave my hands near it or change the settings, amplitude and waveform changes massively. The scope has a metal case and should be shielded by that, so I am wondering where the noise is comming in. Could it be due to improper grounding, so that the scopes ground leads get pumped full of RF energy? Are there clever ways around this?
This is a shot of the waveform I get before (lower trace) and after (upper trace) the gate driver. I think there is a phase shift which I dont like, and then the duty cycle is far from 50%. I wonder how I could improve that. The diodes I use to clamp the antenna are common 1n4148s, 3ns recovery and 4pF junction capacitance they are unlikely to mess up the antenna signal. I dont know about the input impedance of the 4422 gate driver, but I expect it to be rather high.
this is the gate and drain waveform, note how the drain voltage ramps up exponentially instead of doing damped oscillations. I blame this on a far too low Q, so that the circuit behaves as an RC instead of LCR circuit. This does not change even if I lower the coupling substanitally, I wonder why the quality of my TC is so bad?
Finally a picture of the overall setup. The FET sits on a big fan-cooled heatsink, which still gets too hot to touch after a few seconds of running. Breakout is just about visible. The circuit is kind of a mixture of Steves Class E SSTC and Richies HF-SSTC, I am using the antenna feedback directly into the 4421 from Steve, but I am using the "proper" Class E stage with the RF choke and DC blocking cap from Richie. The NE555 in the front is supposed to start of the oscillations, but it acutally doesn't.
PS: Hehe, my birthday present of about 100 beefy semiconductors and about one mile of fine magnet wire arrived today, lots more parts to blow!
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More Class E trouble
