Too often you see an overdriven signals on the digital sub-bands;
multiple audio sidebands on PSK, splatter from overdriven MFSK and
RTTY. There is absolutely no reason for a transceiver driven by
fldigi to exhibit this type of performance. You can set up your
computer / transceiver for good solid performance without excessive
drive.
The "Tune mode" provides a continuous single frequency audio signal at the exact frequency to which the waterfall cursor has
been set. The peak amplitude of this signal is the peak amplitude
of every modem signal generated by fldigi. None will exceed this
value, even the simultaneous multi-tone modes like Throb. Every
modern SSB transmitter uses some automatic level control ALC for
preventing overdrive for SSB voice. A little overdrive on a voice
channel can be tolerated to a degree. In fact, that is what an
analog RF compressor does, overdrive and then subsequent filtering.
But you absolutely cannot tolerate that with the digital modes.
Here is the way to set up your transceiver for a clean signal.
I recommend starting out with a dummy load, but an "off hour" for
a band might work just as well if you do not have a dummy load.
- Set your PCM level to about 80%
- Set your Transmit Level control for minimum output level.
- Enable the "Tune" mode in fldigi ... you do have CAT or PTT set up ...right?
- Make sure your transceiver's speech compression control is OFF
- Slowly bring up the Mixer audio out until your rig's ALC just
starts to function (a light blinking or a meter showing this condition).
- Reduce the Mixer audio output until the ALC is disabled.
- You are now transmitting at maximum output power without distortion.
- You can use any level below this and be assured that your output signal will be clean.
- You should always use the minimum power necessary to maintain
good comms, remember that even if you are clean a 100 W Psk signal will
be so strong among the QRP signals that it will overpower the AGC on many
receivers that are working another digital station within the same SSB
bandwidth that you are on. You will appreciate this the first
time that you are working a weak PSK DX station and someone blasts
through and captures your AGC.
You should try the the above adjustments at different audio
frequencies. Transceivers that achieve the SSB filtering with
crystal or mechanical filters will have a considerable amount of
variation across the passband of the filter. This will show up as
a varying amount of ALC that is dependent on the audio
frequency. Once you are comfortable with the process you can very
quickly repeat the "Tune" and set the power for the frequency to which
the waterfall is set.
Receive audio level should be adjusted so that the overload indicator
to the left of the AFC button does not illuminate red. When
observing the received signals on the oscilloscope view you should
expect that they do not exceed a peak-to-peak amplitude of 3/4 of the
full display height.