by KM5TZ/6 » Mon Jan 10, 2005 3:58 pm
Bill,
Yes it looks like the (9)610 is the problem. The gate drive waveform looks very messy on 40m but does look good on 80 and 160m.
I evaluated a couple of different 4420's. The Micrel parts are crap - good only to a few MHz above which they exhibit internal timing problem. The Maxim parts work very well - they cost a little more (~$4.50 each from Digi-Key) but are good up to 10MHz. I just got some parts from Intersil which also look good on paper but not sure I'll spend much time on them since I already bought more Maxim parts.
Since the driver FET's are not modulated, 11N90's are overkill for the Driver. Gate capacitance is what kills you on 40m so I selected a couple of lower Gate C FET's (11N40's) that I plan to drive with 3 parallel Maxim 4420's per FET. A quick breadboard test revealed very nice gate drive waveform on 40m. Each 4420 is dissipating just below its max 1W limit at this frequency.
David.