Epson MFJ-1278B manual Important Note Harmonics Interference, Receiver Audio Setting

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.