MFJ-1270C/1274C MANUAL APPENDIX D: ALIGNMENT
Page 243
HF RADIO ALIGNMENT
If you have determined that it is necessary to realign the MFJ-TNC modem
center frequency to your radio filter center frequency, one of the following 2
methods should be used to make the radio filter center frequency (Fc)
determination. Both methods require access to a frequency counter capable of
measuring audio frequencies to a resolution of 1 Hz. Almost any frequency
counter should be capable of this.
First method: NOISE AVERAGE FREQUENCY
This method also requires an active noise source like a receiver noise bridge.
There are several suitable units available for this purpose.
1.
Set the receiver to LSB mode with the 500 Hz filter selected.
2. Connect the noise source to the receiver input. Make sure there is no
antenna connected to the system and that the receiver is tuned to a frequency
which is free of coherent internally generated signals (birdies).
3. Set the noise source output for a reading of approximately S-9 on the
receiver S meter.
4.
Connect the counter to the receiver audio output
5. Adjust the receiver output level for enough audio to reliably trigger the
counter.
6. Make sure the radio's IF shift control, if one is present, is in its proper
position (centered or on detente).
7. Record the frequency indicated by the counter. The counter should
indicate the average frequency of the noise spectrum passed by the filter in the
receiver and translated to audio by the product detector. This will be the
frequency used for the modem center frequency (Fc).
NOTE:
If the counter is a phase locked loop (PLL) based prescaling type, its
PLL may not lock properly to the noise signal. If this is the case, use method 2
below. If your counter can resolve 1 Hz with a counting gate time of less than 1
second, it is a PLL prescaling counter.
Second method: FILTER SKIRT AVERAGE FREQUENCY
1.
Set the receiver to LSB mode with the 500 Hz filter selected.
2. Make sure the radio's IF shift control, if one is present, is in its proper
position (centered or on detente).
3.
Using either a signal generator or a stable, relatively strong carrier from an
AM broadcast transmission (preferably ground wave signal), tune the receiver so
that the carrier falls near the center of the filter passband. Choose a signal level
near S-9 for this test. If your rig has a built in calibration oscillator, this is a
good source for this signal.
4. Slowly and carefully tune the receiver so that the tone frequency is
decreasing.
5.
Find the point where the signal is reduced by 1 S-UNIT from the peak
value reached near the center of the filter passband.
