Re: LF: Signal bandwidth in Op32 LF

[Andy Talbot <andy.g4jnt@gmail.com>]

Partly, perhaps, but I suspect your dead FETs could be due to antenna reflections of wideband transients of a hard switched waveform.  A sharp rise (or fall) time hitting a high Q antenna will cause it to ring.  The resulting oscillations will most likely be out of phase with voltages present in the PA and could be causing over-voltage.   The correct way to do on-off on a really high power switching PA is to key the PA itself - ideally with a ramped waveform

WSPR uses phase continuous small frequency transistions, so the Hi-Q antenna sees nothing to make it 'ring'

In the early days of 73kHz when people used  'Coherent', that was hard switched 10 Bit-per-second  PSK some users reported damaged PAs.  My loading coil certainly made  clicking noises when the phase shifted, although the modified 100 Watts MOSFET audio amp  (a linear PA) survived.

Andy

www.g4jnt.com

On Sat, 29 Sep 2018 at 11:09, Chris Wilson <dead.fets@gmail.com> wrote:

Hello Andy,

Could  this  in any way supply a reason for me seeing the odd dead FET
when  running  OPERA  and  never  when  running WSPR? I always had the
feeling  that the fact OPERA stops and starts so many times in 32 mode
that  it  was  a  cause  of  an occasional glitch that blew the FETS in my Class D
amp... To be honest it's the main reason I rarely TX with OPERA on LF.
The  amp  is  permanently  powered,  the exciter, normally my U3S then
stops  and  starts whatever mode is selected. With OPERA I think a FET
will usually go at the START of one of the  TX cycles, not at the end.

Thanks. 2E0ILY

Saturday, September 29, 2018, 9:42:10 AM, you wrote:

> If you stop and start a divider, the phase jumps about randomly. 
> Each time you start the divider, its output signal starts up in a
> differnet phase to what it would have been if left running


> To achieve phase coherent, you must keep the divider running and gate the OUTPUT separately


> This issue is a problem with beacons in general that employ FSK
> keying for their CW ident.   If they adopted on-off keying, then
> long term integration cold take place though the keying cycle.  But
> unfortunately, IARU (or whatever organisation that tries to mandate
> these things), appear to require FSK keying.  SO most beacon keepers
> comply.   (I don't, the Bell Hill microwave beacons, GB3SCx all use on-off keying)
> Andy
> www.g4jnt.com





--
Best regards,
 Chris                            mailto:dead.fets@gmail.com