locked Re: FST4 modes with TS-890S - slow RF envelope rise problem #txaudio


N1BUG
 

Hello Rik, Chris, Peter,

The different rise times depending on T/R period might explain why some appear to be OK for me while others do not. Very strange things can happen when you start putting some FET drivers into undefined states. I say appear because surely all of them are transitioning through an undefined state for some period of time, it's just that I don't see the symptoms with all T/R periods.

Has anyone checked to see if the envelope tapers slowly at the end of a transmission? I would guess it does. I have lost many FETs in the past due to slow envelope decay putting the driver into undefined states. It has not been happening much since I upgraded to higher voltage and current rated FETs but it is still bad for them and perhaps other components also.

We should be protecting our PAs from slow envelope rise and slow decay times. I guess a properly designed circuit should do both easily enough. I don't know how to design such a circuit but I it seems I better start working on it.

73,
Paul N1BUG

On 1/4/21 9:54 AM, Rik Strobbe wrote:
​Hello Chris, Paul, Peter,
I'm afraid I made a mistake in my earlier measurement: instead of waiting for the next half hour for FST4-1800 to start I just pressed "tune". Here I measured 0 rise time.
Today I had a look at the rise times for the real transmission start (rather than waiting 30 minutes I changed to PC clock) :
FST4(W)-1800 = 2.5s
FST4(W)-900= 1.2s
FST4(W)-300= 0.4s
FST4(W)-120= 0.15s
FST4(W)-60= 0.075s
Rise times seems to be identical for FST4 and FST4W.
I assume that these slow rise times are meant to avoid excessive bandwidth at the start (and end?) of a transmission. But these can indeed cause problems with class D amplifiers as used by the majority on 630m and 2200m.
Since I did blow the final power MOSFETS a few times in the past due to the slow rise times in other software and therefore added a electronic "hysteresis switch" between TX and PA to avoid these kind of problems. So I didn't notice the slow rise time when using FST4.
But the better solution would be an option in wsjt-x to set the rise time to 0 (as it is with JT9).
73, Rik  ON7YD - OR7T
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*Van:* main@WSJTX.groups.io <main@WSJTX.groups.io> namens Peter Hall, VK6HP <p.hall@...>
*Verzonden:* maandag 4 januari 2021 15:30
*Aan:* main@WSJTX.groups.io
*Onderwerp:* Re: [WSJTX] FST4 modes with TS-890S - slow RF envelope rise problem
Chris, Paul
Thank you for your comments, which I think indicate there is an issue to be at least understood.  Chris, your waveforms look much like mine from the TS-890S.  I don't know if you had a chance to check out the URL to the data provided by Rik, ON7YD, in the earlier RSGB-LF group discussion which I initiated.  Rik seems to have a simpler soundcard (rather than an internal transceiver interface) and does not see the effect I described.  However, with several transceiver-based systems showing the effect, it would be useful to have a discussion with the WSJT-X developers.  Being new to this group, I'm not entirely sure whether issues get picked up as a matter of course from these forums or whether a more direct report is required.  Perhaps Paul might know the answer to that question.
My "noise-gate" is a hardware solution and, in retrospect, it's not such a bad thing to have a circuit which only clocks the amplifier when the drive is above a certain level.  However, it'd obviously be useful to correct any software issue, if only to allow users of very simple non-linear PAs to use them reliably with the new modes.
Paul, I've never had any issues with WSPR or JT9 either, and it sounds as though your PA is also unhappy about a very slow rising clock amplitude ambling through a transition point at the beginning of the transmission.  I can't explain your difference between the long and short duration transitions, though; all my transmissions in the new modes hiccup at the start but I've had good contacts using transmit durations from 15 sec to 900 sec.  and local test decodes at up to 1800 sec.
73, Peter.

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