MFJ MFJ-8621, User Manual

MFJ-8621                                                                                             Data Radio 

6 

SELECTING BAUD RATE: 

Receiver Filter: 

The MFJ-8621 receiver features a special noise-reduction filter to enhance 1200-
baud operation.  When running at 1200-baud, this filter improves weak-signal 
performance.  To activate the 1200-baud noise-reduction filter, install a shorting plug 
at JMP-1 (this plug must always be removed when operating at 9600-baud). 
See Figure 3 on page 9 for placement of the jumpers on the pc board. 
 

Transmitting: 

The MFJ-8621 has two FM modulators selected by jumpers, JMP2 and JMP3.  One is 
a sensitive 

reactance modulator

 designed for 

mic-level

 AFSK signals.  Since most 

TNC's output mic-level signals at 1200 baud, the reactance modulator should be used 
at 1200 baud.  The other is a "true-FM" 

varactor modulator

 designed for 

line-level

FSK signals.  At 9600 baud, most TNC's have a line-level output and should be used 
with the varactor modulator.  If your TNC has a line-level output setting for 1200 baud 
TX-AFSK (usually set via a jumper inside the TNC), you may use the varactor modulator 
at both data rates.  
 

For 1200-baud AFSK operation,

 install JMP1 and set JMP-2 and JMP-3 in the 

reactance modulator positions (left-hand and center pins of each header). 
 

For 9600-baud FSK operation,

 remove JMP1 and set JMP-2 and JMP-3 in the 

varactor modulator positions (center and right-hand pins of the headers). 
 
See Figure 3 on page 9 for placement of the jumpers on the pc board. 
 

IMPORTANT NOTE: JMP1, JMP2 and JMP3 are set for 1200 baud at the factory.  

Always recalibrate the MFJ-8621 transmitter oscillator for frequency (L6) 
after switching modulators (see calibration, page 10). 

TRANSMITTER DEVIATION: 

Setting Deviation: 

The MFJ-8621 has no internal deviation control.  Deviation is adjusted via the AFSK 
output level control on your TNC (consult your TNC manual for specifics).  If you don't 
have access to a FM deviation meter,  you can set deviation by measuring the peak-to-
peak AFSK signal output from your TNC: 
 
With jumpers in the 

reactance modulator

 (1200-baud) jumper positions, a 

150-mV

 p-

p sine wave should produce approximately 3-KHz deviation.  With jumpers in the 

varactor modulator

 (9600-baud) jumper positions, a 

800-mV

 p-p sine wave should 

produce approximately 3-KHz deviation. 
 
Virtually any calibrated oscilloscope will allow you to make this measurement.  Connect 
your scope probe to PIN-1 on the 5-pin DIN jack at either the TNC or radio end (look 
for the TX-AFSK line if your TNC uses a different type of jack).  Use the CALIBRATE 
function on your TNC to generate a test-tone.