[Lf] Poor waveform at output transmitter]

[Andre' Kesteloot]

Dave wrote:

> Wil / Dick / Tom and all.
>
>       As many amateurs seem to suffer from FET
>      blow-ups due to fatal voltage and/or current
>      peaks caused by proper TX loading only at the
>      fundamental frequency I wonder whether it would
>      be wise to insert a diplexer between TX output
>      and tuning/loading system; a combination of a
>      lowpass filter feeding the aerial circuit and a
>      highpass filter feeding a resistor that absorbs
>      the power in the harmonics. That might save
>      FET's.  It won't help of course when the loading
>      at thefundamental frequency is wrong, causing
>      FET overload.What do you think?
>
> I tried the diplexer idea and it didn't seem to have any
> advantage. As one of the country's greatest destroyers of
> fets I can make a few suggestions. Recently I have not had
> any blow-ups atall by following these rules.... (I'm
> touching wood!) .1. Always match the antenna to 50 Ohms
> and operate the TX via an SWR bridge so that you know the
> match is good before increasing power. The output filter
> is designed for 50 Ohms and will transform other
> impedances into possibly dangerous loads on the TX output
> stage. .2. Check the drain waveforms on the fets with a
> 'scope. They should be clean, usually looking like a
> square wave with a sine shaped peak. Lots of spikes or
> "fuzz" is bad news and can be cured by fitting a Zobel
> network between the two drains. i.e. across the primary of
> the output transformer. This network consists of a 10 to
> 27 Ohm large wirewound resistor in series with a 10nF
> pulse-rated C like the ones used in the LPF. Use 27Ohms
> with 75V rail or above and 10 Ohms with 30 - 40V
> rails. .3. Add a fast-acting protection circuit operating
> on overcurrent and bad SWR which cuts the drive, relays
> are too slow! See my circuit or G0MRF's for ideas. I am
> convinced that the trip has saved my 4 x IRFP450 rig on
> numerous occasions.If you use a 4013 as the phase-splitter
> / final divider stage, this can be forced to send both
> outputs high by putting a high on the set and reset pins.
> After an inverting driver stage this means both fets are
> turned off within a half-cycle. Good luck and may your
> fets last a long time! Dave G3YXM.
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