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

Peter Hall, VK6HP

Hello everyone,

I've just joined this group and would be interested to know if anyone can shed some light on an issue I'm experiencing.  I do quite a bit of work on 630 m using a TS-890S exciter and a homebrew 300 W Class D amplifier.  The 0 dBm drive for the PA is taken from the Kenwood "DRV" output.  For those interested, the transmit antenna is a ZS6BKW HF doublet loaded as a Marconi T and the most usual receive antenna is a 1 m diameter active loop.  I'm running Windows 10 on a Microsoft SurfaceBook and a USB connection between the PC and the radio's internal audio interface ("sound card").  The software version is WSJT-X v2.3.0-rc2, although the -rc1 version behaves identically. I use PTT/CAT control but have experimented with VOX with no resolution of the central issue.

I've had no problems running any JT modes on any bands until I began trying out the FST4 and 4W modes on 630 m a few months ago.  Both WSPR and JT9 work very well on 630 m and the new modes also work, but with a small twist: the risetime of the FST4 envelope is so slow that it upsets the clocking arrangement in the push-pull PA.  This causes the PA protection to activate, with a consequent small loss of signal at the start of the transmission.  It's not a particularly serious problem, but is annoying.  I do have a hardware fix in the form of what is essentially a noise gate circuit between the radio and PA; the drive signal has to be above a set level (chosen to reliably clock the PA) before it is passed to the amplifier.  However, after some recent investigation I can quite clearly see a difference in the transmit delays and transceiver waveforms when doing nothing else but changing from e.g. WSPR to FST4W-120.

In brief, the WSPR envelope rise time is of order a few milliseconds and the delays are well-behaved, with a zero transmit delay actually corresponding to a few ms delay in seeing the TS-890S drive waveform.  With FST4W, the RF envelope rise time is very slow indeed, with many tens of ms observed.  The transceiver delay is also much longer -  typically more than 100 ms. 

I should add that I've successfully sent and received FST4W signals, including some 4-300 and 4-900 contacts.

I'm unable to spot any transceiver setting differences between old and new modes and have not yet experimented with other transceivers.  However one fellow member of the RSGB LF Group was kind enough to show that, using a simple sound card, he was unable to see any difference between the envelope characteristics of various modes.  It does look like a software issue to me and I'd appreciate comments from anyone running a similar setup, or with relevant suggestions.

73, Peter (VK6HP)

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