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MFJ-1278B MULTI-MODE APPENDIX B MODEM CALIBRATION
CW MODEM TUNE-UP PROCEDURE
The CW modem in the MFJ-1278B is fundamentally different from the other modems. It is
NOT an AFSK modem. It sends and receives only 1 tone. For this reason, it requires a
different alignment procedure than the others.
NOTE: If the Modulator and Demodulator are not EXACTLY on the same frequency for
BOTH stations involved in VHF FM MCW operations, the CW receive
functionality becomes degraded if not eliminated.
NOTE: If the DEMODULATOR is not EXACTLY centered in the receiver's CW filter
passband, CW receive performance through a narrow filter will be degraded. Also,
the demodulator will likely not be receiving on the transmitted signal frequency.
This note is for the HF direct keyed radio case (NOT MCW via SSB transmitter
microphone audio which is discouraged).
Since MCW operation is primarily intended to provide a means to transmit code practice and
not for CW reception, the demodulator center frequency may be set to a different frequency
than the modulator without penalty in most cases. Typically, the modulator will be set to 700
Hz, and the demodulator will be set to whatever frequency is dictated by the HF radio with
which it is to be used.
The CW demodulator can be used without a narrow filter in the radio. It will of course be
subject to more noise and QRM in this case. The tuning range for the demodulator will be
VERY restricted even though the wide filter is used in the radio since the demodulator itself
acts as a very narrow filter.
An oscilloscope will be required to adjust the demodulator to a 50 percent duty cycle square
wave output. The main difference when aligning the CW modem is that there will be only 1
continuous tone presented to the demodulator for this adjustment.
Tuning the CW modem is basically a 4 step procedure. The steps should be followed in the
order presented. The steps (in broad outline) are:
1. Determine the required demodulator center frequency (Fc) for the radio you intend
using in the direct keyed CW mode.
2. Set the CW modulator tone to this frequency.
3. Set the demodulator center frequency using the modulator tone as reference.
4.
Set the modulator tone to 700Hz.