[UK-CONTEST] Key clix

[Ian White GM3SEK]

G3SJJ wrote:
>Several people, including myself, commented on key clix in AFS. I have
>been observing this problem around the bands for sometime now. It is a
>difficult one since simple key click filters are really things of the
>past and for the most part we have to rely on manufacturers, which
>hasn't proved very successful.
>

There seem to be several layers of problems:

1. Generations of bad information about what we're aiming for.

The "classic" RC-shaped keying envelope that you see in all the 
handbooks is NOT GOOD! Key-down is nice and soft, but when you release 
the key it drops very sharply, causing a nasty click.

What we're actually aiming for is an S-shaped rise and fall, with NO 
SUDDEN CORNERS ANYWHERE. For a full discussion of this, see 'In 
Practice' for April 2005.

W8JI has some .wav files that show the differences very clearly:
http://w8ji.com/keyclicks.htm

Listen first to the recording of W1AW:
http://w8ji.com/WAVE/No_clicks_mono.wav
The keying sounds very "crisp", but as soon as Tom tunes away, the 
signal is completely gone.

2. Clicky transceivers. These range from old rigs that just bang a bunch 
of relays over, to more modern rigs like the FT-1K that simply got the 
design wrong.

Listen to W8JI's recordings of unmodified FT-1000s - all models have 
nasty clicks over several kHz. An unmodified FT-1K has a lot in common 
with an unmodified dog!


Adding an amplifier can add another layer of key-click problems. These 
have nothing to do with the higher power as such. It is perfectly 
possible to have a signal that's both loud and click-free.

3. The problems are in the timing between the amplifier and the 
transceiver. Some transceivers have a long delay between key-down and 
the "amplifier PTT" control signal appearing at the rear panel. This 
doesn't give the amplifier enough time to get ready for the RF, so the 
result is "hot switching" in the amplifier, and key clicks.

4. Amplifiers that are very slow to change over, so they can't keep up 
with the PTT signal from the transceiver. These are usually the ones 
with open-frame power relays, which are very slow to change over, 
causing large RF arcs which are clearly visible - and clearly audible 
too. Fortunately the owners usually don't attempt full QSK because of 
the clatter, so the amplifier-generated clicks only occur once per 
transmission.

With semi-breakin, the amplifier click problem is limited to once per 
transmission (or every time the key-operated "VOX" kicks in)... but with 
full QSK it happens with every dot and dash.



-- 

73 from Ian GM3SEK