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TNC2MDM1.TXT
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1988-07-15
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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