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Transmit Delay Determination for Non Flex Exciters with Flex Amp and Flex Tuner

The requirement when using a non Flex exciter with a Flex Tuner or Flex Amp is that the transmit delay (TD) must be 20 ms or greater. Admittedly a recent post in this discussion group included a correspondence from Flex indicating that transmit delay is more critical in CW operation than in USB operation and mentioned that newer Yaesu trancievers will meet the Flex TD requirement. TD settings are normally adjustable for CW ops (QSK) on newer Yaseus--for example, on the 991A, it is adjusable from 15ms to 30ms in 5ms increments. But the fact remains that TD is not adjustable for USB or for other modes. And for the owner who would like to know what these actual TDs are (even though Flex said the figures were acceptable), a difficulty arises. They are not in the Yaesu Manual nor are they are discoverable via an internet search. When I mentioned this to a friend, he suggested that I take a look at the ARRL test data for the radio as published in QST. Since they seem to test a plethora of parameters, there's a good chance the USB TD might have been measured also. And as it turns out, it definitely was (in addition to the TDs for a couple of other modes also). The 991A TD in USB is 31ms. Out of curiousity, I thought I would take a look at the TD for another popular radio--the ICOM 7300. In the case of the ICOM, the ARRL test lab measured its USB TD at 15ms. Too close for comfort? For any Icom users, I suppose to be on the safe side, asking FLEX about it would not hurt.


  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin

    @Michael Hyman

    Let me comment.

    Yes, TX delay is available for all modes for FlexRadios thanks to the onboard sequencer. Generally, we suggest starting at 20-30ms for OEM amplifiers and see if you get a hot switch over.

    In the timing chart below, you can see that TX_DELAY is added on to the longest of the 5 other delays. I had to hide one paragraph.

    You can try a test by setting your TX_Delay to 1000ms and see what happens. It is pretty interesting.

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