1-9. Phase Jitter.
a. In measuring phase jitter, we are not concerned with long term (very
slow) changes in phase such as might be caused by the slow drift of a
carrier oscillator.
However, we are concerned with the maximum
instantaneous deviations from the average phase of the signal.
A common
cause of these deviations is the modulation of carrier supplies in
frequency, division multiplex systems by the power line frequency, or some
harmonic of the power line frequency. When the carrier supply frequency is
used to translate the signals to their appropriate position in the frequency
spectrum, the phase modulation on the carrier is transferred to the signal.
This results in the phase modulation of the signal at the rate equal to the
power supply frequency, or a multiple of that frequency.
b. Other causes of phase jitter, such as interfering tones on the
channel or variations in the media, may be present. All of these variations
might cause the instantaneous phase to deviate from the average.
For
example, an interfering tone 21.2 dB below the signal level will cause a
peak phase deviation of about 5 degrees, or a peak-to-peak deviation of 10
degrees.
c. Interfering signals such as tones or noise can cause phase jitter.
Normally, these causes should be found in other tests, and should be kept
within specified limits. It is important that interfering tone and noise be
within the acceptable limits before conducting phase jitter tests.
(1) Test arrangements.
Figure 1-17 shows the typical test
arrangement which is applicable to the test procedures described in this
test description that use the oscilloscope method.
Both the input and
output of the channel under test will be disconnected from the signal source
and terminal equipment.
The channel input at the sending station will be
terminated in a signal generator (A). The channel output at the receiving
station will be terminated in the vertical input of the oscilloscope (insure
the input impedance of the oscilloscope meets the output impedance of the
circuit under test) through a switch arrangement which will allow the
vertical input to the oscilloscope to be connected to either the output of
the signal generator (#2) or the output of the circuit under test.
32
Previous Page
