MFJ MFJ-1270C, Системное руководство

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MFJ-1270C/1274C MANUAL ADVANCED OPERATION
Page 100
Radios which have provision for an auxiliary "narrow SSB" filter can have the
500 Hz bandwidth filter installed instead of the approximately 1.8 KHz
wide "narrow" SSB filter. This will allow direct selection of the narrow
filter for AFSK work when in SSB mode. If the filter is also desired for CW
operation and you don't want to invest in 2 identical filters, it is usually a
relatively simple modification to cause the radio to also select this filter when
in CW mode.
Some of the newer HF radios (and many older ones) allow selection of filter
independent of mode. For these radios it is a simple matter to select the
required mode and the optimum filter independently of one another.
If the HF radio is to be dedicated to packet use, the 500 Hz filter can be installed
in place of the SSB filter. This is possible on ANY radio designed for SSB and
CW use.
It is important to use lower sideband rather than upper sideband if you are
going to use a CW filter in the radio for packet operations. The reason for this
is simple. The filter designed for CW operation is usually centered
approximately 800 Hz away from the upper sideband suppressed carrier.
This is the carrier oscillator which is normally used as the BFO on CW. This
means that if the two carrier oscillators are 3 KHz apart, the CW filter center
will be near 2200 Hz below the lower sideband carrier. This is the reason 2220
Hz was selected for the center frequency of the modem.
If you are going to use a narrow filter, and serious HF operation is NOT
recommended without one, it will be necessary to take steps to be certain
that the filter passband is centered over the modem center frequency. If your
radio has IF shift, this is a simple matter. The tuning indicator and DCD
LED are used to give you an indication of filter centering. The following
discussion assumes that your radio has provision for IF shift.
To center your radio passband over the modem, use the following procedure:
1.
Set your radio for maximum RF gain.
2.
Set the radio for lower sideband operation.