[Elecraft] K3 Operator Error

[Kok Chen]

On Sep 2, 2009, at 9/2    9:26 AM, Kjeld Holm wrote:

> Just now I noticed that for the last second or so of the  
> transmission it
> seems like I have full output also on 80 meters. BUT it is not a  
> question of
> warming up as the full output is seen only for the last second  
> whether I am
> transmitting for two seconds or for about twenty seconds.

Is this happening to you only on PSK31, or is it happening also with  
RTTY?

This behavior is (full power at the end) is fully explained by how  
PSK31 works.

In addition to phase modulation, PSK31 also applies a cosinusoidal  
envelope to the signal whenever there is a phase transition between  
two signaling symbols.  Without this envelope shaping, a PSK signal  
would be quite wide.  This is why the transmitter chain must be  
absolutely linear -- it is to maintain this sinusoidal relationship.

This envelope modulation is the reason why a PSK31 signal looks like  
two carriers separated by 31.25 Hz on the waterfall when the op at the  
other end is not typing into the keyboard.  When the idle Varicode is  
sent, the signal looks like a two-tone DSB signal that modulated by a  
15.625 Hz tone.

As a result, the power output when PSK is idling is lower than when  
you type (the variation is even greater with with QPSK31 than BPSK31;  
this is quite obvious when you compare their modulation constellations).

The original specs for PSK31 calls for a (approximately 3/4 second)  
squelch tail at the end of transmission.  Most PSK31 modems obey this  
during transmission (whether they actually use it as a squelch or not  
during receive).  Unless the software actually drops the peak power by  
3 dB during this end-of-transmission squelch, you will see the output  
power rise during this period.

Since RTTY is a 100% duty cycle mode, it will not have this behavior  
even if you transmit a prolonged mark tone at the start or at the end  
of a transmission (many software transmits a slightly prolonged mark  
just before the RTTY diddle begins). The only time you should see RTTY  
change power when transmitting is if the RTTY tone pair is right at  
the edge of a filter passband so that one tone is more attenuate then  
the other one.)

If you are also seeing the more-power-at-the-end behavior in RTTY, you  
might want to check to make sure that your transmit tone pair (from  
software) is centered in your transmitter's passband.

73
Chen, W7AY