APPENDIX G

   USE OF TUNING INDICATOR TO CENTER FILTER WITH IF SHIFT

The  tuning  indicator can be used to aid in  centering  the 
radio  filter  passband over the modem Fc.   The  modem  and 
tuning indicator must first be properly aligned according to 
the procedure already presented.

The  Phase Locked Loop (PLL) in the demodulator attempts  to 
track  the  spectral components of the noise it sees on  its 
input.  If that noise has been bandlimited by a filter whose 
bandwidth is nearly the same as the PLL tracking range,  the 
loop is largely successful in tracking the noise.   In doing 
so,  the loop control voltage,  which is the signal used  by 
the  tuning  indicator,  takes on values which are  centered 
over  the value which would be required to lock the loop  to 
the  mean frequency of the noise coming through the  filter.  
Since the tuning indicator can't tell whether it was  signal 
or  noise  that  generated the  loop  control  voltages,  it 
gleefully  (and  correctly)  indicates the  frequency  error 
between  the  mean noise frequency and the  demodulator  Fc.  
When the indicator is centered, the error is zero.

  NOTE!  To make use of this property of the demodulator and 
         tuning  indicator,  there  must  be  ABSOLUTELY  NO 
         coherent  signals  in  the  filter  passband.   The 
         setting  will  not be correct unless  it  is  based 
         entirely   on  noise.    If  tuning  the   receiver 
         frequency   shows  up  as  motion  in  the   tuning 
         indicator  display,  there is a coherent signal  in 
         the passband (even if you can't hear it!).

To center the filter over the modem with the IF shift control:

1.  Find  a frequency which has plenty of noise but is  free 
    of  coherent signals.   Before I realized that the noise 
    bridge could be helpful for this,  I used the CW end  of 
    80  meters during the daytime hours when propagation  is 
    shut down.  If you have a noise bridge available, by all 
    means  use  it  as  a source of noise  for  making  this 
    adjustment.   Tune the receiver frequency back and forth 
    to  be  sure that there really are NO  coherent  signals 
    there.   If there are, the tuning indicator will respond 
    to receiver tuning.

2.  Use  the  IF Shift control to move the  filter  apparent 
    center  frequency  around.    Observe  that  the  tuning 
    indicator  follows  the movement of the  filtered  audio 
    spectrum.

  NOTE!  If  the tuning indicator stays all the way  to  one 
         side  or  fails to respond to the IF Shift  control 
         movement,  something  is wrong.  Either  the  modem 
         center  frequency  is  far outside  the  adjustment 
         range  of  the  IF Shift control or  there  is  not 
         enough  noise  coming through the  filter  for  the 
         demodulator to track.


    Set the IF Shift control so that the tuning indicator is 
    centered.   The  tuning indicator bar will appear  quite 
    jittery  when  responding to noise but this  is  normal.  
    Use the center of the jitter area for the adjustment.

3.  If  you are using a modem with a DCD threshold  control, 
    this adjustment can be made a little more precise.   Now 
    that  the tuning indicator is centered,  adjust the  DCD 
    threshold  control  so  that  the DCD  falsing  rate  is 
    approximately  20 percent.   Then SLOWLY  and  CAREFULLY 
    move  the  IF Shift control back and forth to  find  the 
    place  where DCD duty cycle is maximized.   This is  the 
    best position for the IF Shift control.

4.  To  keep  from having to repeat this exercise,  a  small 
    pencil mark can be placed on the radio panel to indicate 
    the correct setting of the IF Shift control for use with 
    the modem.

This  completes  the alignment of the IF Shift  control  for 
centering the radio's narrow filter over the modem's  center 
frequency.