d. Performing the test, end-to-end with reference-return method.
In
the end-to-end method just discussed, the transmitting operator depends on
the receiving operator for accuracy of test results.
There is the
possibility of misunderstanding between individuals, even though an order
wire telephone connects them together. The end-to-end with the reference-
return method illustrated in B of figure 2-1 places all readings on the
single test set used by the transmitting operator; the receiving operator
merely stands by. As the auxiliary line returns the delayed signal from the
receiving to the transmitting test set for display, the delay in the
auxiliary line adds to the delay already in the signal.
However, the
auxiliary line delay remains a constant value because the carrier frequency
used for the return path remains a fixed value.
(1) Preparation.
The same general test setup is used in the
reference-return method as with the end-to-end method.
However, one
auxiliary line of good quality must be provided for reference return; this
is in addition to the order wire and the line under test.
Place the REF
RETURN switch to the REF RETURN position. Place the SWEEP/CARRIER/REFERENCE
switch to the REFERENCE position.
Adjust the REFERENCE FREQUENCY control
1,800 Hz on the readout for an all-wire circuit, or 2,000 Hz for a circuit
returning through a carrier channel. All other controls remain in the same
positions previously indicated in the end-to-end method.
(2) Calculations.
In the reference-return method, calculations are
similar to those in the end-to-end method.
However, the transmitting
technician taking the meter readings must be aware that the observed total
delays include auxiliary line delays. In a previous example of the end-to-
end method, final line delays were 2.025 and 1.975 ms, with reference delay
of 2,000 ms.
If the auxiliary line delay in the reference-return method
adds a constant value of 0.100 ms, the total delays will rise to 2.125 and
2.075 ms, with the reference delay rising to 2.100 ms.
However, the
envelope-delay distortion remains identical with that observed in the end-
to-end test.
Reference delay = 2.100 ms
Upper envelope delay distortion = 2.125 - 2.100 = +0.025 ms
Lower envelope delay distortion a 2.100 - 2.075 = -0.025 ms
e. Test equipment substitution.
Envelope Delay Test Set Model 340B
shown in figure 9 of appendix A, section V,
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