User's Manual

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manifests itself by the computer becoming sluggish or possibly "freezing"
entirely.
The first parameter, the
IF bandwidth filter
, is the length of the first filter in
the digital path, which is responsible for much of the receiver selectivity. The
factory default length is 63. Reducing this value will decrease the receiver
selectivity and make the receiver more prone to interference from nearby
strong signals. However, for slower computers it may be necessary to
compromise on this figure if the demodulator appears too slow (to the point
of the panel "freezing" or drop-outs of audio) because of insufficient CPU
resources.
The post-mixer filters, which appear in all demodulators, are all of the low-
pass type, their bandwidths being automatically related to the IF bandwidth
filter (whose center frequency is 12kHz). For AM, AMN, AMS, CW, DSB,
ISB, FM3, FM6 and FMN, the post-mixer filters bandwidth is half of the IF
bandwidth, while for LSB and USB both bandwidths are equal.
If you have a preference for particular IF bandwidths, you can associate
them with any of the ten preset buttons (located at the bottom of the
demodulator front panel) by double clicking the Bandwidth column of the IF
bandwidth presets table, and then typing in the numerical bandwidth value in
Hz. The buttons are numbered (1 to 10) from left to right. The factory default
values can be restored at any time, by pressing the
Restore default
settings
button.
For the AM, AMN, FM3, FM6 and FMN demodulators, a supplementary
low-pass filter was introduced in the audio output path, to reduce harmonic
distortions generated in the demodulation process. Its cut-off frequency (the
upper limit of the pass band) and length adjustments can also improve the
overall signal-to-noise ratio under bad receiving conditions.
Finally, a noise-derived squelch is implemented for frequency modulation
modes, FM3, FM6 and FMN. The energy of the noise present above the
useful voice baseband is used as a signal presence indicator. The
user can modify the length of the high-pass filter separating that noise,
squelch filter length
. The squelch control appears on the demodulator front
panel when any of the FM modes is selected. This makes it possible to
adjust the noise threshold level at which the squelch will mute the audio.
Finally, all demodulated audio output is scaled up by an
Audio gain
factor.
The scaled audio signal level is further adjusted with the Volume control in
the demodulator main panel. When the Audio AGC is active, the fixed Audio
gain is ignored. The attack and decay times for the three possible Audio