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

Peter Hall, VK6HP

Hi everyone,

First, should say that I have now submitted a WSJT-X bug report to the developers' list and perhaps when the team has had a chance to consider it we may hear back from them.

Paul, on the matter of the eBay simple noise gate circuit, I can say that it does work, although it is not as sophisticated as Rik's digital implementation.  It's the same general concept but uses a JFET as an attenuator element in response to a control signal from an envelope detector.  There is no hysteresis but setting the threshold high enough works out well in practice.  I bought the blank PCB from the Polish seller simply because I was curious as to whether I could make it work at 475 kHz.  The actual circuit (MXR Noise Gate) has been around in one form or another for ages and is well-known in the music community.

You can find details of the PCB and a link to the schematic etc by searching for eBay item 254048537744.  I made two changes to allow good 475 kHz operation: the dual op-amp U1 is now an NE5532 (from my parts drawer) and the gain of the envelope detector is reduced (and its bandwidth increased) by changing R7 from 4k7 to 33 k ohm. For RF applications use a 51 ohm input termination and make R20 (the output resistor following the unity gain buffer) 51 ohm.  I outline the option as a starting point, not as a fully-tested solution.  It does drive my PA fine, and you can see the general action on the snapshot of the beginning of an FST4-60 transmission below,

73, Peter.

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