Filter
Top Products
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).