Ian White GM3SEK wrote:
> For semi-breakin, the PTT line only changes state at the beginning and
> end of the whole transmission - but those changes do need to be fast to
> avoid timing and arcing problems. The only difference for full QSK is
> that the PTT changes state much more often, between every dot and dash -
> what comes out of the transceiver's PTT jack is literally Morse code -
>
While the above statement is true in principle, it's not always
attainable in practice. Many popular transceivers, including the
Kenwood TS-940 and TS-950SDX which I have dug into quite a bit, energize
a separate "amp PTT" relay with heavier duty contacts whenever the
operator wishes to use the transceiver PTT to "key" the amplifier. (In
the TS-940, that relay's coil is automatically connected to its drive
circuit when the appropriate DIN plug is inserted in the REMOTE jack on
the rear panel. In the TS-950SDX, the user must manually throw a slide
switch on the rear panel of the transceiver to connect the relay coil.)
Instruction manuals for these rigs are very clear on one point: This
relay is NOT capable of, nor designed for, following the on/off cycles
of CW elements and should not be energized when the transceiver is in
the Full QSK mode. Further, the PTT relay is often objectionably noisy.
> and the changeovers in the amp need to be a little faster to avoid
> distorting the dots and dashes at high speed.
>
Not necessarily. In the 1970s I modified my first Heathit SB-220 to do
exactly as you suggest. I employed an external time-sequence keying
circuit to bias the amplifier on as soon as the key line was closed
while simultaneously delaying the key closure sent to the exciter. At
key up, the external box added a few milliseconds before letting the
amplfier bias return to cut-off value. (I used an electronic vacuum
tube TR switch to feed the receiver antenna input, so no PTT- or
amplifier-related relays needed to switch more than once an evening.)
But my amplifiers today simply add cut-off bias on CW by inserting a
50-watt 25-volt Zener (for 3-500Z tubes in grounded grid) in series with
existing operating bias circuitry. Using both an oscilloscope and
on-the-air listening tests by a distant critical observer, I have found
that I can have just as clean a signal with the ever-so-slightly cut-off
finals as with the complex time-sequence keying arrangement.
What my biasing circuit does NOT do, however, is take care of getting a
non-destructive signal to the RX Antenna for the operator. To do that,
one needs an electronic TR switch or a vacuum relay QSK box or a PIN
diode QSK box or a separate RX antenna (with overvoltage protection at
the receiver input jack). But PTT relays don't do that for you, either,
when running true QSK. Remember, the PTT relay in the transceiver is
usually used to drive a bigger changeover relay in the amplifier that
typically peforms both the biasing and antenna switching functions.
Both relays are far too slow and clunky for QSK, to say nothing of the
fact you now have multiple relay energization delays in cascade.
> If the transceiver outputs correctly-timed instructions on its PTT jack,
> and the amp follows those instructions quickly enough, support for full
> QSK is automatic and transparent to the user.
In my experience, those are very big "IF"s. While my TS-950SDX allows
as much as a 30 millisecond delay between key closure and RF output in
order to let amplifier changeover relays get properly actuated and
settled, I know of no normal amplifier relays that are capable of, or
designed for, switching on and off at the speeds required by individual
CW dot and dash elements. For the amplifier to do this requires special
QSK circuitry -- usually at much added expense -- and typically the QSK
circuitry either takes its information directly from the key line (i.e.,
the key line goes through the QSK circuit before it goes to the
transceiver, as in my LK-550) or from a small sample of RF at the
amplifier input. Neither of these approaches requires the use of the
transceiver PTT output.
A good rule of thumb is that PTT relays in transceivers and OPERATE /
STANDBY changeover relays in amplifiers should NEVER be used to follow
CW dots and dashes -- i.e., should never be turning on and off at the
information signaling rate during QSK or SEMI-QSK operation. In my
shack, I only use PTT for voice operation. PTT is a totally
non-essential control line for me when operating CW, so its timing is of
no consequence at all.
Many transceivers provide a truly electronic signal at the rear panel
for use with electronic amplifier switching schemes. In my Kenwoods,
its name is "TXB". During key down it supplies a small positive voltage
capable of driving a transistor or two. It follows dot and dash
elements quite nicely, so no additional relays are involved. But it is
not a "plug and play" solution to running QSK with an amplifier --
external circuitry must be added.
Bud, W2RU
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