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MFJ-1278B MULTI-MODE APPENDIX D HF RADIO
SECOND METHOD: FILTER SKIRT AVERAGE FREQUENCY
1. Set the receiver to LSB mode with the 500 Hz filter selected.
2. Make sure the radio's IF shift control, if one is present, is in its proper position (centered
or on detente).
3. Using either a signal generator or a stable, relatively strong carrier from an AM
broadcast transmission (preferably ground wave signal), tune the receiver so that the
carrier falls near the center of the filter passband. Choose a signal level near S-9 for this
test. If your rig has a built in calibration oscillator, this is a good source for this signal.
4. Slowly and carefully tune the receiver so that the tone frequency is decreasing.
5. Find the point where the signal is reduced by 1 S-UNIT from the peak value reached
near the center of the filter passband.
6. Measure this tone frequency with the counter. Record this value as F(low).
7. Slowly and carefully tune the receiver so that the tone frequency is increasing.
8. Find the point where the signal is reduced by 1 S-UNIT from the peak value reached
near the center of the filter passband.
9. Measure this tone frequency with the counter. Record this value as F(high).
10. Calculate the required modem center frequency as:
Fc = [F(low)+F(high)]/2
Once the required center frequency has been determined, the modem calibration can be
carried out.
NOTE: It is essential that the modulator tones be properly aligned FIRST as they will be
used to align the demodulator center frequency.
Set the modulator tones to Fc MINUS 100 Hz for the low tone and Fc PLUS 100 Hz for the
high tone using the procedure given in the owners manual. Align the demodulator to the
modulator tones using the built in demodulator alignment routine invoked with the RXCAL
command as described in the Appendix B section of this instruction manual.