W2RU - Bud Hippisley wrote:
>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.
We're certainly a long way from all transceivers and all amps being
automatically plug-and-play to give full QSK; but many of the problems
can be overcome by simple modifications at the interface.
>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.
Use of a slow mechanical relay in the PTT output will make it impossible
for a transceiver to control an amp in full QSK mode. I'd regard that as
a very good reason to modify the transceiver, to give it a fast
transistor-switched output like many others have.
No doubt the reason for having the relay is to protect the transceiver
from amplifiers that require a high-voltage or negative PTT. Again, that
is a very good reason to modify the amp. Most modern amps seem to have
something like a +12V PTT interface, which will switch when shorted to
ground and draws no more than a few milliamps.
Those are simple enough modifications, which clear away some of the
basic obstacles to QSK. Then you have to look at timing and speed of
response.
>> 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.
We're agreed on that. The relay in the transceiver can be eliminated
completely, and then you need to speed up the changeovers in the amp.
>> 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 --
As others have said, vacuum relays are plenty fast enough, and
inexpensive if you do it yourself.
>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.
>
Both of those approaches are workarounds. The first one is a workaround
for transceivers that are totally incapable of controlling an amp for
full QSK, so you plug the key into the amplifier and let the amp control
the transceiver instead.
The second approach, sensing RF at the amplifier input, looks rather
unpleasant (if I'm understanding it correctly). If RF is already present
at the amplifier input, it's too late for the amp to change over without
hot-switching. In some amps, the presence of RF at the input will
actually *prevent* it from switching - and rightly so.
>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.
>
As explained above, I wouldn't agree with that as a generalization. If
the PTT timing is 'broken' and will not support CW changeovers without
hot-switching, how does it prevent hot-switching on voice modes?
>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.
Absolutely... but if some transceivers can support QSK via the PTT line,
and some amps can accept that control, then maybe it's time to modify
those that won't.
Interface levels can always be fixed, on both sides, and unnecessary
timing delays from mechanical relay interfaces can be eliminated.
Transceiver timings cannot always be modified, especially if incorrect
timing for amplifier control is programmed into the control logic. But
almost all amplifiers can be speeded up to support full QSK, at
relatively low cost.
--
73 from Ian GM3SEK
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