HP 4935A, Operating And Service Manual

Page: 48 / 350

Operation
Model
3-55.
NOISE-TO-GROUND MEASUREMENT
The uses noise-to-ground to measure the longitudinal noise present on a telephone circuit,
with reference to ground. The transmitting provides a quiet termination at one end of the voice
channel. and the receiving provides a frequency weighted filter and detector at the other end. The
basic measurement technique used for the noise-to-ground measurement is very similar to the message
circuit noise measurement; the main difference lies in the use of a ground reference. Figure illustrates
this
Noise-to-ground measurements are usually made for troubleshooting purposes; to measure the
magnitude of longitudinal signals, which may indicate the susceptibility of a cable pair to electrical coupling
from external sources. These measurements are also made to coordinate new installations with power
companies, to minimize power line coupling.
The relative line balance of an end loop can be calculated by subtracting the measured
ground value from the measured message circuit noise (N.,) value. This calculation is only valid if the
measurements are made on a twisted pair and it is assumed that the message circuit noise is caused by
longitudinal noise converted to message circuit noise by line imbalance. It is recommended that both
message circuit noise and noise-to-ground be measured with the 3 flat weighted filter to include the
effects power line related noise.
3-59. PEAK-TO-AVERAGE RATIO MEASUREMENT
The uses the peak-to-average ratio (P/AR) to measure the channel dispersion (spreading in
time of signal amplitude) due to transmission imperfections. As the P/AR signal traverses a dispersive
medium, the peak-to-average ratio will deteriorate. Then by measuring the peak-to-average ratio at the
receiving end, a simple measure of dispersion is obtained. P/AR
is measured as the ratio of the peak to
wave rectified average values of a specially processed test signal transmitted over a line. The test signal has
a peak-to-average ratio and a spectral content that approximates a data signal. Figure illustrates the
frequency spectrum of the transmitted P/AR test signal, and Figure 3-16 illustrates the signal envelope.