[Elecraft] K3 PSK31 with Digipan

[Richard Ferch]

Users of narrow-band digital modes such as PSK31 need to remember the
difference between transmit and receive in the way we perceive dynamic
range.

On transmit, a 10 dB difference is a factor of 10 in power. If your power
meter only goes up to 5W when you were expecting 50W, you will likely
perceive this as a serious issue. Even a 3 dB difference shows up as a
factor of 2 in transmitted power.

On receive, a difference of 10 dB is only a couple of S-units and well
within normal signal variability on HF. Looking at a normal waterfall, a 10
dB variation across the receive bandpass is barely noticeable as a slight
change in colour or intensity of the background noise.

Even within the transmit filter bandpass, relatively small variations in
response will be more easily detectable on TX than RX. K8ZOA's plot of the
2.7 kHz filter response shows a ripple of around 2 dB, which is probably
next to undetectable on RX, but is easily noticeable on TX as a change in
transmitted power.

With wide DSP bandwidth settings, the K3 can receive signals at frequencies
where the transmitted power level through the transmit filter would be very
low.

The simplest way to avoid this problem is not to use DSP width settings that
are wider than the transmit filter bandwidth. That way you won't receive
normal signals at frequencies where your transmitted power would be at QRPp
levels. You should also avoid using the shoulders of the filter response,
i.e. the last 200 Hz or so at either edge of the 2.7 kHz filter bandwidth.

Alternatively, you can use a wide DSP bandwidth for scanning the band, but
once you find a signal you want to work, move your dial so that the desired
signal falls within the optimum range.

Many PSK31 programs have an "align" or "optimize" macro capability, which
allows you to center a signal in the waterfall with a single mouse click. It
does so by retuning the radio so the chosen signal ends up on the designated
frequency (e.g. 1500 Hz). Unfortunately, AFAIK DigiPan does not have this
capability.

On a related point, in order to suppress audio harmonics I also like to try
to avoid transmitting digital modes on low audio frequencies. This is not a
significant problem on voice modes, but audio harmonics on PSK31 show up on
the air as spurious transmitted signals. Using a wide filter on transmit
could make it harder to avoid transmitting harmonics of your audio signal,
so personally I would not want to use wideband TX in DATA A.

73,
Rich VE3KI


Steve N9SZ wrote:
> 
> But in Data A mode, one can expand the receive width out to 4 kHz using
the
> 2.7 kHz filter. If you attempt to transmit below 200 or above 2800 Hz,
power
> output will drop to zero.
> 
> The user needs to be aware when clicking on the waterfall as one can
receive
> signals on frequencies in which they cannot transmit.
> 
> Steve N9SZ
> 
> 
> 
> Lyle Johnson wrote:
> > 
>  > > Just getting my new K3 working on PSK31 and noticed that it seems to
be
>  > > important to limit the range of the transmit frequency...
> > 
> > I just swept my K3 in DATA A mode.using the LINE IN jack for sourcing
> > the Tx audio.  It is flat from below 200 Hz to higher than 2800 Hz
> > using a 2.,8 kHz IF filter.
> > 
> > Be sure you don't have TxEQ set to restrict the transmit bandwidth.
> > TxEQ is active for *all* Tx audio paths in all modes.  For DATA A or
> > DATA AFSK, it should be flat.
> > 
> > 73,
> > 
> > Lyle KK7P
> > 
> >