Epson Multi-mode Data Controller MFJ-1278B - Manual (Page 162)

MFJ-1278B MULTI-MODE ADVANCED OPERATION
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.
3.
Make sure the MFJ-1278B is in HF Packet mode and that the HF Packet modem has
been selected.
4.
Set the audio output level so that the tuning indicator is responding to the background
noise coming through the filter. This can be verified by observing whether or not the
tuning indicator "follows" the noise as the IF shift control is rotated.
5.
Tune to a frequency which has ABSOLUTELY NO signals in the passband. It is very
important that only noise is coming through the filter. If you have access to a RX noise
bridge, an alternative is to use the bridge as a noise source for the receiver. No antenna
is required in this case.
6.
Adjust the IF shift control on the radio so that the tuning indicator on the MFJ-1278B is
centered in its display area.
7.
Adjust the DCD threshold control on the front panel of the MFJ-1278B so that the DCD
LED is flickering with false DCD indications. Make the adjustment so that the DCD
duty cycle is more than 10 but less than 50 percent.