APPENDIX A

USE OF OSCILLOSCOPE FOR 50 PERCENT DUTY CYCLE DETERMINATION


The  oscilloscope can be used to indicate a 50 percent  duty 
cycle on square wave data in two different ways.   The first 
is  based  on timing each half cycle of the square wave  and 
comparing  the  time durations.   This  method  requires  an 
oscilloscope  that can be reliably triggered on the incoming 
data.  It also requires that the sweep speed can be operated 
in uncalibrated mode with continuous adjustment.

The second method uses the vertical input coupling capacitor 
to  integrate the incoming square wave so  the  oscilloscope 
can  display the DC offset.   This method has no  triggering 
requirements  but the oscilloscope must be able to free  run 
in  the  absence of triggering.   This method also  requires 
that  the  vertical  position  and  the  vertical  gain   be 
adjustable, and that the input can be AC coupled.

Both  methods  use  the  logic level data  output  from  the 
demodulator.   This  means that the input impedance of  just 
about  any  oscilloscope  is plenty  high  enough  for  this 
application.



First method:

            TIME BASED DUTY CYCLE DETERMINATION


1.  Connect  the  'scope  vertical input to the end  of  R68 
    which  connects to Q13's collector as described  in  the 
    demodulator alignment procedure.

2.  Do  steps  4  through  7 in  the  demodulator  alignment 
    procedure so that there is a square wave data signal  to 
    observe on the 'scope.

3.  Set  the 'scope to trigger on the positive going edge of 
    the square wave.

4.  Adjust  the sweep rate so that exactly 1 complete  cycle 
    of  the square wave is displayed.   Use  the  horizontal 
    position  control  to exactly align the  positive  going 
    trigger  point edge of the signal with the left edge  of 
    the  graticle  on the 'scope face.   Use the sweep  rate 
    controls  to exactly align the next positive going  edge 
    of  the signal with the right hand edge of the  graticle 
    area.   There is some jitter present in the data at this 
    point  and it will appear as though there are  really  2 
    transitions on the second positive going edge very close 
    together.  Use the midpoint of the 2 edges for alignment 
    to  the  right hand side of the graticle.   It  will  be 
    necessary  to  use the continuously variable sweep  rate 
    capability  of  the 'scope for this  adjustment  as  the 
    calibrated  steps  are  unlikely to result in  an  exact 
    alignment.

5.  Now  adjust the demodulator center frequency  using  the 
    appropriate  variable resistor for the demodulator being 
    aligned.   Observe  that the NEGATIVE going edge of  the 
    signal in the central graticle area moves in relation to 
    the 2 POSITIVE going edges at the graticle extremes.

6.  Set  the  demodulator  center  frequency  so  that   the 
    NEGATIVE  going edge occurs exactly on the main vertical 
    graticle line in the exact center of the graticle.  This 
    assures  equal time for the data when it is in the  high 
    state as when it is in the low state.   Again,  use  the 
    center of the jitter area.

End of method 1

 
 
Second method:

      DC OFFSET DETERMINATION OF 50 PERCENT DUTY CYCLE


  NOTE! This  is the PREFERRED method as it does not require 
        estimation of jitter areas. 


1.  Connect  the  'scope vertical input to the  end  of  R68 
    which  connects  to Q13's collector as described in  the 
    demodulator alignment procedure.

2.  Do  steps  4  through 7  in  the  demodulator  alignment 
    procedure  so that there is a square wave data signal to 
    observe on the 'scope.

3.  Set the 'scope timebase to free run.

4.  Adjust  the  vertical gain so that there are  exactly  6 
    major  graticle  divisions (usually CM)  between  the  2 
    horizontal lines displayed on the 'scope face.

5.  Ground  the 'scope vertical input either by  temporarily 
    removing  the probe from the test point and attaching it 
    to  ground  or  by using the switch  provided  for  this 
    purpose  on the 'scope vertical amplifier.   Adjust  the 
    vertical  position  so that the horizontal  trace  falls 
    EXACTLY on the middle horizontal graticle line.
     
6.  Make  sure  the  'scope vertical input  is  set  for  AC 
    coupling during the rest of this procedure.

7.  Reattach the probe to the test point if necessary.

8.  Now  adjust  the demodulator center frequency using  the 
    appropriate variable resistor for the demodulator  being 
    aligned.   Observe  that the 2 horizontal lines move  in 
    the   vertical  plane  in  relation  to  the  horizontal 
    graticle lines.

9.  Set the demodulator center frequency so that both  lines 
    are  exactly the same distance from the middle hoizontal 
    graticle  line.    This  should  place  one  of  them  3 
    divisions above it and the other 3 divisions below it.

End of method 2