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MFJ-1278B MULTI-MODE APPENDIX B MODEM CALIBRATION
Comprehensive Alignment Procedure
It is important that the tuning indicator alignment be optimized for the 300 baud 200 Hz shift
HF packet modem. It will indicate correctly for all other modes when aligned for the HF
packet modem.
As described elsewhere in this manual, the exact configuration of your radio's filters and
whether it has IF shift or not will determine the exact required center frequency for the
modem. 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 1/2 of the shift for the low tone and Fc PLUS 1/2 of
the shift for the high tone. For the 170 Hz shift 300 baud modem used for HF packet this
would be Fc + 100 Hz for the high tone and Fc-100 Hz for the low tone.
Set Modulator Tones Using Built-in Calibration Software
1. Make sure that the modem and TNC are both configured for 170 Hz shift 300 baud
operation. This is done by typing MODE HP <CR> from the cmd: command prompt
on the MFJ-1278B.
2. Place a push on jumper at JMP 4.
3. Place a push on jumper at JMP 9, pins 1 and 2. This connects the modulator square
wave output to the SIO chip so that the CPU can measure the tone frequency for you.
Also install a push on jumper at Pins 1 & 2 and 3 & 4 of JMP 16. The two jumpers
when installed properly will be positioned vertically. This would be if the leds on the
MFJ-1278B are facing you.
4. Type "CALSET n <CR>". Where n is a number determined by:
n = INT [ 525000 / F(low) ] + 1
This tells the CPU what tone frequency you are trying to achieve.
A table of CALSET numbers to use for the various "standard" modem tone frequencies will
be included at the end of this procedure.
5. Command the MFJ-1278B into calibrate mode by typing CALI followed by a <CR>".