[Elecraft] K3 TX delay

Fri Feb 17 11:22:04 EST 2012

A very interesting discussion of the question "how fast is your QSK?". 
Another question is "how smooth is it?" -- by which I mean, do you hear 
any clicks or other audio artifacts when sending?

The K3 is better than many modern radios in these respects, but is far 
from perfect. The problems in both areas stem mostly from the use of 
common circuitry for transmit and receive and the need to 'turn them 
around' rapidly when sending CW.

Short of a full-duplex setup with the transmitter and receiver in 
different locations, the best way to achieve superior QSK is with a 
separate receiver and transmitter. Then the problem is greatly 
simplified into finding a way to switch the antenna quickly -- 
electronic t/r switches using vacuum tubes come to mind -- and a way to 
protect and mute the receiver without introducing artifacts.

I wonder if, for a CW operator, the benefit of a transceiver is 
overrated? Imagine a setup with a simple CW transmitter separate from a 
K3, which could be used as a receiver only. All that you would need to 
regain the convenience of a transceiver would be a button to match the 
transmitter to the receiver frequency, something which would be easy to 
do with a CAT program.

I may well try this with my K2 as transmitter.

The absolute best QSK I've ever had in my station was in the late 
1960's, a Hallicrafters HA-5 hetrodyne VFO followed by a Heath DX-60 and 
a single 813 amplifier. The receiver was a Drake R4B which needed one 
bypass capacitor added to the input of the first audio stage to mute in 
complete silence. T/R switching was handled by a simple tube switch in 
the plate circuit of the 813, which, like Bud's rigs, was class C.

On 2/17/12 7:15 AM, W2RU - Bud Hippisley wrote:
> One thing I haven't seen mentioned in this discussion is the sending
> speed at which the benefits of QSK "disappear" in the K3.
>
> Speed in words per minute (wpm) is very roughly equal to the number
> of dashes in a 5-second interval.  Thus, at 25 wpm, there are 25
> dashes in 5 seconds, or 5 dashes per second.  But a dash consists of
> 4 "periods":  three consecutive "mark" periods, followed by a single
> "space" period. So...in one second there are 5 x 4, or 20 such
> periods in 25 wpm CW, and therefore a single period is theoretically
> 50 milliseconds (ms) long.  Of course, if keying "weight" is set
> above 50% (or 1.0-to-1.0 mark / space ratio for dots), the key-up
> period is LESS than 50 ms.  A similar calculation at, say, 35 wpm,
> gives us a key-up (or "space") period of about 35 ms.
>
> But to avoid generating excessive clicks, the leading and trailing
> edges of RF output during each mark element ramp up or down
> gradually, with the total interval at either end of the element
> having been measured at 4 ms (onset of RF output to 100%) in one
> specific "clickless" K3.  If we assume we have adjusted the weight
> using the 50% RF output level as the reference (an assumption that
> is, itself, worthy of a completely separate posting), there's another
> 4 ms (1/2 x 2 x 4) that we have to subtract from each space period.
>
> Of course, for QSK to truly be QSK, we have to be able to hear
> receiver audio output for at least part of the "space" period between
> two consecutive "mark" elements (either dot or dash).  The minimum K3
> TX Delay that can be set via the CONFIG menu is 8 ms.  It's my
> understanding that at least part of that delay is for "covering" the
> time it takes all the various synthesizer and DSP circuits in the K3
> to switch from RX to TX, especially since the RX and TX frequencies
> can be different when in QSK mode.  It's also my understanding that
> there's a similar delay at the END of a transmitted element, when all
> the circuits have to return to their RX states.  Because frequencies
> and other circuit parameters are actively changing during those two
> periods, it's unlikely that useful RX audio is available at any time
> during either 8 ms interval.
>
> Clearly, then, there is a MINIMUM total interval of (2 x 8) + 4, or
> 20, ms when no received signals can be heard by the K3 user between
> mark elements.  In fact, even at the 8-ms setting, it's very possible
> that MORE than 16 ms of each space period is "blanked".   I say that
> because a number of us have found that to get proper weight from the
> K3 in QSK mode, we have to artificially adjust the length of its
> "mark" periods (with, for instance, the "Compensation" parameter of a
> WinKey or other suitable outboard keyer) by adding 6 or 7 ms to each
> key-down interval.  (Note that this a fixed interval for a given K3
> regardless of sending speed, and is not "weight" in the conventional
> sense.  Good keyers, like the WinKey, allow weight, ratio,
> compensation, and other parameters to be independently adjusted.
>
> So now, with the K3 CONFIG menu set to 8 ms (its minimum value), we
> have 20 + 7, or 27, ms of each "space" when the receiver is blanked.
> Interestingly, that's just about the length of an entire space at 35
> wpm!  So, for all practical purposes, QSK is simply not possible with
> a K3 at CW speeds above 35 wpm.  Below that speed, the ability of QSK
> to help you detect the presence of the other station is statistical,
> depending on whether enough of the character elements sent by the
> other operator happen to coincide with some part of the narrow usable
> window within your own transmitted "space" intervals to cause you to
> conclude he's really sending again.
>
> But it gets even worse:  That same K3 above was measured as having 15
> ms total delay from initial closure of its KEY OUT line to the start
> of RF output when using the "new" QSK mode with TX Delay set to 8 ms,
> the minimum possible.  Further, there was an additional 15 ms delay
> between the cessation of RF output and the lifting (opening) of the
> KEY OUT line.  Since RX audio is muted the entire time the KEY OUT
> line is closed, then we have to add another 14 ms [2 x (15-8)] to the
> 27 ms; that gets us to 41 ms, the entire "space" interval between
> consecutive "mark" elements at 24 wpm or higher!
>
> Conclusion:  Modern transceivers filled with synthesizers and DSP
> circuitry are no match for properly designed analog receivers such as
> the old Hallicrafters and Collins units when it comes to QSK
> performance.  For the K3 and most other modern transceivers,  at
> common CW contest speeds "QSK" doesn't mean what it used to ("able to
> hear the other station between our own transmitted DOTS"); the best
> we can do above 25 wpm or so is to hear the other station between
> CHARACTERS.
>
> The K3's QRQ mode may be better in this respect than the figures
> discussed above but, since QRQ mode currently does not allow the use
> of RIT, XIT, or Splits of ANY delta-f, no matter how small, it's of
> limited interest to me since my primary interest in QSK is knowing
> when a DX station has come back to someone while I'm still sending.
>
> Finally, with respect to amplifier QSK:  I am aware of five ways of
> enjoying QSK capability when using a separate amplifier: 1.  PIN
> diode circuits (Alpha 87A, some Ameritron models, some outboard
> PIN-diode "boxes) triggered by the KEY OUT line from the K3 2.
> High-speed reed and vacuum relay combinations (current Alpha models,
> AG6K circuitry, etc.) triggered by the KEY OUT line from the K3 3.
> Electronic bias switching (EBS) in conjunction with a shunt-type
> vacuum tube TR Switch (E. F. Johnson, e.g.) and requiring NO trigger
> signal 4.  EBS in conjunction with a separate RX antenna (always with
> suitable RX front-end protection, of course) and requiring NO trigger
> signal 5.  Diplexing schemes which don't care if / when the
> transmitter is keyed
>
> Most important, the clunky, noisy, typically open-frame relays used
> for manual, PTT T/R switching in many amplifiers should NEVER be
> considered capable of being used in QSK mode!  In fact, many of these
> relays are sufficiently slow that they shouldn't even be used in VOX
> mode because they'll be "hot-switching" RF, even with the delays in
> the K3.
>
> PIN diodes and EBS can be VERY fast — with total switching times
> under 1 or 2 ms.  Inexpensive reed and vacuum relay circuits
> typically perform in 2 or 3 ms, thanks to the use of speed-up circuit
> techniques.  With respect to #3 above, the maximum voltage out of the
> RX ANT terminal of a Johnson TR Switch during transmitter key-down
> periods is high enough to trigger the internal RX front-end
> protection circuitry in the K3, so it is not a viable approach, even
> though I have successfully used Johnson TR Switches with Kenwood
> solid-state transceivers for over a quarter century.
>
> And FAR more than a "quarter century" ago I began my own ham "career"
> with a form of #4.  I used a separate receiving antenna and the EBS
> was very simple:  all my CW rigs and amplifiers back then were
> operated in Class C mode, and fully cut off during key-up intervals!
>
> Bud, W2RU
>
>
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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/