Epson manual MFJ-1278B MULTI-MODE Radio Interfacing, Setting the Receiver Audio Input Level

MFJ-1278B MULTI-MODE RADIO INTERFACING

3.With the MFJ-1278B keying the transmitter and transmitting the higher of the two tones, adjust the transmit audio level as follows. With a small flat-tipped screwdriver, adjust trimpot located on the left side of them MFJ-1278B (R157 for radio 1 or R158 for radio 2) while you listen to the monitoring receiver. Adjusting the trimpot CW increases the output, while CCW decreases the output. Turn the adjustment on the trimpot clockwise (CW) until no increase in output level is heard at the monitoring receiver.

4.Rotate the adjustment on the trimpot counter-clockwise until the audio signal on the monitoring receiver decreases by half of the maximum level. This can be estimated by ear or accurately by measuring the output voltage at the transmit audio pin of the radio port with an oscilloscope or AC voltmeter.

5.Press the K key to return to receive mode and type Q to exit the calibration routine. Be sure to remove JMP4 if you placed it to defeat the watch-dog timer. You have now set your transmitter deviation to approximately the correct level.

If you notice a significant hum level in the monitored audio in Step 3, take measures to remove it. This may require shielded wire (recommended in any event) in your microphone audio circuit. The use of shielded cable is always necessary, in projects such as this. If your transmitter has an adjustable microphone gain control, try reducing the sensitivity of the transmitter microphone circuit and increasing the signal level from the MFJ-1278B to minimize hum or other noise problems.

Setting the Receiver Audio Input Level

The modem in your MFJ-1278B implements an advanced phase coherence type data carrier detection (DCD). There is a threshold control and a sensitive tuning indicator. Together they set the correct receive audio level for the modem, also 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 present 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).