Posted by
wayne burdick on
Jun 07, 2018; 11:43pm
URL: http://elecraft.85.s1.nabble.com/Default-K3-transmit-delay-may-be-too-short-for-slow-QRO-amplifiers-tp7641779.html
It has come to our attention (thanks to N7YK and N6TV) that the default value of CONFIG:TX DLY, 8 ms, may actually be closer to 5 ms.
The associated T/R control firmware hasn’t changed in a long time, so hopefully there aren’t many users affected (or we would have heard about it). The shorter delay may be related to the improved KSYN3A synth, which switches faster than the original. We’re looking into this, and will update the displayed value to reflect the actual value.
Details:
When using a QRO amp with the K3, the KEY OUT line from the radio is used to key the amplifier, setting it up to receive RF. Since the K3 has very fast QSK, the amp’s T/R control circuitry has to be similarly fast.
Those of you who are using KPA500s and KPA1500s need not be concerned about the K3’s fast QSK speed, since our amps use PIN diode T/R switching. They can easily keep up with the K3 at any code speed and with the default setting of the CONFIG:TX DLY menu entry.
However, some other amps, especially older ones, use slow T/R relays. If the transceiver generates RF before the amp is ready, this might result in key clicks.
(In practice there’s a bit of a grace period because the RF keying envelope rises very slowly. It’s a "raised-cosine sigmoid," resulting in the transceiver itself having incredibly clean keying sidebands. See K9YC white paper on this topic.)
If you suspect your amp has a slow T/R relay, you should do a listening test, or better yet have someone else listen from a good distance. If you (or they) hear any evidence of clicks, increase the value of CONFIG:TX DLY.
Use the lowest setting that eliminates the clicks. CW keying at higher code speeds may be affected if the value is too high. Other modes not using fast QSK will generally be unaffected (voice and data).
An alternative to increasing TX DLY is to use a foot switch, as do many users of old amps regardless of transceiver type.
73,
Wayne
N6KR
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