MEASUREMENTS

Differential Phase
Select DOUBLE in the 1781R DIFF PHASE mode to make this
measurement. First look at the vectorscope screen and use the
phase shifter to set the signal vector to the reference 9 o'clock
phase position. Neither vectorscope nor waveform monitor gain
is critical in this mode (see Note 18), but setting the vector to
the graticule circle is a good starting point. Now refer to the
waveform monitor (right-hand) screen, and use the phase
shifter to overlay the blanking level portions of the two
waveforms. Press REFerence SET to set the phase readout to
0.00 degree, as shown in Figure 68.
The next step is to use the phase shifter to overlay the point in
the R-Y waveforms that deviates most from blanking level. As
shown in Figure 69, the phase readout now indicates the
amount of differential phase distortion. In this example the
phase error is all in one direction so peak and peak-to-peak
results will be the same. If the signal has both positive and
negative phase errors (the R-Y signal extends both positive and
negative from blanking), repeat the process for the largest
positive and largest negative signal excursions.
The double trace technique is similar if you are using a 521A
Vectorscope. Start by setting the "calibrated phase" dial to
zero. Use theA or B0 goniometer to null the blanking level,
and then use the calibrated phase shifter to null the largest
excursion. The number above the calibrated phase dial will
now give the amount of differential phase distortion.
VM700A Automatic Measurement
To make an automatic measurement of differential phase with
the VM700A, select DGDP in the MEASURE mode. Both differential
phase and differential gain are shown on the same
display; the lower graph is differential phase. Measurement
results are also available in the AUTO mode.
Figure 68. The 1781R double trace DIFF PHASE
display; phase readout zeroed.
Figure 69. The double trace DIFF PHASE display
showing results.
• NOTES
18. 1781R Waveform & Vector Gains
In the single trace mode, the vector gain must be set so the signal
vector extends to the graticule circle. The waveform gain must
be in the calibrated (1 volt full scale) position. The graticule is
calibrated to one degree per division only under these conditions.
With the double mode display, however, you can introduce more
gain for greater resolution. Additional vectorscope gain and/or
waveform vertical gain can be selected without affecting the results.
19. Noise Reduction Filter
A digital recursive filter is available in the 1781R to facilitate differential
phase and gain measurements in the presence of noise.
Select the NOISE REDUCTION ON touchscreen selection in
the DIFF PHASE or DIFF GAIN menu to enable this filter. The
filter removes about 15 dB of noise from the signal without any
loss of bandwidth or horizontal resolution. This mode is
particularly useful for VTR and transmitter measurements.
Figure 70. The VM700A DGDP display.
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