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MFJ-1278B MULTI-MODE RADIO INTERFACING
Receiver Audio Setting
The modem in your MFJ-1278B is equipped with an advanced phase coherence type data
carrier detection (DCD). A threshold control and a sensitive tuning indicator is also
provided. Together these can be used to set the correct receive audio level for the modem.
These can also be used to optimize the DCD characteristics for the various methods of
operation.
With your radio in the receive mode, open the squelch control so that a steady hiss is heard
on a speaker. Set the volume control to the minimum volume position. The tuning indicator
on the MFJ-1278B should drift off to one side of the display and become stationary. It may
drift enough to disappear off the end of the display. Slowly advance the audio output level
with the volume control until the tuning indicator "springs to life" and dances around a point
near the middle of the display. This is the absolute minimum audio level for marginal copy.
Continue to advance the volume control until there is approximately twice as much audio
present at the receiver output. This can be estimated by ear or measured with an oscilloscope
or AC voltmeter. This will be near the correct amount of audio for NBFM operation. Levels
higher than this will not degrade the modem performance as long as the receiver audio
amplifier is capable of producing the chosen output level without distortion (clipping).
While still listening to unsquelched receiver noise, rotate the DCD threshold control on the
front panel of your MFJ-1278B clockwise until the DCD LED just flickers with false data
carrier indications. This control should be set to produce approximately a 10% duty cycle of
false DCD activity when receiving unsquelched NBFM receiver noise.
The modem sensitivity to false DCD will be affected by the bandwidth of the audio coming
out of the receiver. some receivers produce wideband audio which will NOT produce any
false DCD activity regardless of the threshold control setting. In this case, set the DCD
threshold control to its maximum clockwise rotation. Modem performance will not be
affected.
IMPORTANT NOTE: Harmonics Interference
If you experience harmonic interference from the crystal oscillator in the MFJ-1278B, adjust
trimmer capacitor, C47. Trimmer capacitor C47 is located near the crystal. Slowly adjust
C47 to move the frequency of the harmonic.