Lesson 2/Learning Event 5
c. Telegraph Carrier Channel Signal Analysis.
One VF telegraph
channel connected back-to-back with a VF line is shown in C of figure 2-1.
The data analyzer generates the required digital signal which enters the
DC loop of the send channel.
The interconnecting VF line delivers the
modulated VF signal to the receive side of the channel under test. The DC
loop from the receive channel feeds a signal to the data analyzers for
analysis and meter reading.
The oscilloscope is used to examine the
waveshape of the received signal.
d. Analysis of Teletypewriter Dynamic Characteristics.
Each section
of the teletypewriter equipment is analyzed separately (D, figure 2-1).
(1) Printer. A dynamic test of a printer sometimes reveals unsuspected
operating characteristics. The test is performed by feeding a test signal
generated by the data transmitter to the printer through a DC loop. The
technician performing the test varies the amount of distortion on the
signal to find the teletypewriter range (the amount of distortion the
printer can tolerate before misprinting).
(2) Transmitter distributor.
The transmitter distributor of the
teletypewriter equipment generates a signal by tape, and the DC loop
delivers the signal to the distortion analyzer.
The operator can either
read the distortion indicated by the meter or observe the waveform on the
e. Recording Distortion Magnitude.
The magnitude of distortion is
indicated by a percent distortion meter on the data analyzer. The initial
meter reading is the total distortion including bias, characteristic, and
fortuitous distortion.
Bias and characteristic distortion give steady
readings because they are both forms of systematic distortion. Fortuitous
distortion will give changing readings since it occurs randomly.
(1) Total.
Total distortion is the initial reading that appears on the
meter.
(2) Bias. Bias distortion reading is obtained in a follow-up reading
by manipulating switches. Identification of the bias distortion as either
marking (M) or spacing (S) is achieved by indicating lights in conjunction
with a toggle switch.
(3) Difference.
The difference between total distortion and bias
distortion is the amount of other distortions, such as characteristic and
fortuitous.
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