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MFJ-1278B MULTI-MODE APPENDIX D HF RADIO
HF RADIO ALIGNMENT
If you have determined that it is necessary to realign the MFJ-1278B modem center
frequency to your radio filter center frequency, one of the following 2 methods should be
used to make the radio filter center frequency (Fc) determination. Both methods require
access to a frequency counter capable of measuring audio frequencies to a resolution of 1 Hz.
Almost any frequency counter should be capable of this.
FIRST METHOD: NOISE AVERAGE FREQUENCY
Method I requires an active noise source like a receiver noise bridge. There are several
suitable units available for this purpose.
1. Set the receiver to LSB mode with the 500 Hz. filter selected.
2. Connect the noise source to the receiver input. Make sure there is no antenna connected
to the system and that the receiver is tuned to a frequency which is free of coherent
internally generated signals (birdies).
3. Set the noise source output for a reading of approximately S-9 on the receiver S meter.
4. Connect the counter to the receiver audio output
5. Adjust the receiver output level for enough audio to reliably trigger the counter.
6. Make sure the radio's IF shift control, if one is present, is in its proper position (centered
or on detente).
7. Record the frequency indicated by the counter. The counter should indicate the average
frequency of the noise spectrum passed by the filter in the receiver and translated to
audio by the product detector. This will be the frequency used for the modem center
frequency (Fc).
NOTE: If the counter is a phase locked loop (PLL) based prescaling type, its PLL may not
lock properly to the noise signal. If this is the case, use method 2 below. If your
counter can resolve 1 Hz. with a counting gate time of less than 1 second, it is a PLL
prescaling counter.