HP 4935A, Operating And Service Manual

Page: 47 / 350

Operation
Model
3-48 SINGLE FREQUENCY INTERFERENCE
frequency interference refers unwanted steady tones which may appear in lines.
bursts of low level tones which may occur from crosstalk of multifrequency signalling, for
do not fall in this category. Single frequency tones may interfere with certain data signals,
particularly narrowband signals which are multiplexed onto a voiceband channel.
usually detect this since
are easily noise is within limits, single frequency interference
is with setup shown in the received noise signal passes through
signal is applied to operator
may indicate a single
single frequency tone (or tones) of long duration is heard, single frequency interference may be
and should be measured. To determine the frequency and level of the interfering tone, a frequency
selective volt meter or spectrum analyzer must used. Single frequency interference during a C-message
filter occurs if the signal is or less below the noise limits.
3-52. IMPULSE NOISE
S-level Impulse Noise Mode measures one of the most important transient phenomena.
phenomena can cause data transmission errors and/or interruptions to datacom systems.
Impulse noise is that component of received noise signal which is much greater in amplitude
than normal peaks of the message circuit noise. It occurs as short duration spikes and/or bursts of
Studies by Hell Telephone laboratories have shown that impulse noise spikes have a duration of less
than millisecond, and that all significant effects of the noise spike disappear within four milliseconds.
Waveform in Figure illustrates a received holding tone (or test signal) that includes interfering
impulse spikes. The impulse noise measurement counts the number of noise spikes above a selected
threshold during a specified time period.
TRANSMITTED
HOLDING TONE
RECEIVED
HOLDING TONE
WITH IMPULSE
NOISE SPIKES
TIME
Figure 3-13. Impulse Noise Waveform Representation