(2) Frequency distortion. Frequency distortion is always a possibility
in circuits containing reactive elements (capacitance, inductance).
Distortions result when transitional frequencies are displaced in
phase. Further, the higher order harmonics in the transitions are
attenuated more than the lower order harmonics, giving the
demodulated pulse a modified waveshape compared with the original.
(3) Phase delay or envelope delay. Phase, or envelope, delay is by far
the most troublesome characteristic in a circuit intended to convey
digital signals. Theoretically, all component frequencies that give
a pulsed digital signal its characteristic waveshape are present in
the transmitted signal in correct amplitude, frequency, and phase.
When the digital signal modulates a carrier for transmission over a
circuit, all the component frequencies should retain their
amplitude, frequency, and phase relationships.
However, reactive
elements within the circuits cause frequency distortion as well as
phase distortion of the high-order harmonics in the signal
transitions.
Nonlinear phase shift of these frequency components
delays recognition time of the demodulated data pulses. As long as
the phase shift is directly proportional to the change in frequency,
the delay of the demodulated data pulses remains constant.
Nonlinearity of phase shift (envelope delay) with respect to change
of frequency is corrected by an equalizer designed to make all
envelope delays equal to the longest (maximum) delay of any received
signal element.
There is no way to shorten delays that already
exist in the envelopes of the received signal waveforms.
(4) Pulse delay, or delay distortion.
Envelope delay of the circuit
affects pulse recognition delay in demodulators.
When properly
equalized, the circuit has equal-length delays.
Under this
condition, the demodulated digital pulses are all delayed an equal
amount. Since all pulses are similarly affected, delay distortion
of pulses in the signal is minimized.
Envelope delay and pulse
delay are therefore related functions. However, each of these two
items is measured in a different manner.
Since envelope delay
varies with frequency in an unpredictable manner in an uncorrected
circuit, the effect of delay is measured at selected frequencies.
On the other hand, pulse delay is measured in percentage of pulse-
length variation. However, the technician can quickly identify the
presence of envelope delay by observing the waveshape of demodulated
pulses, and measuring their lengths. When received pulses exhibit
their original waveforms, along with their normal lengths, the
technician can be assured of two characteristics: envelope delay of
the circuit is proportional to frequency, and all pulses in the
signal are delayed an equal length of time. A properly equalized
(corrected) circuit produces these desirable qualities.
(5) Summation.
In summery, envelope delay and delay distortion are
related terms, although measured by different instruments. Further,
by making all envelope-delay values uniform by the equalizing
process all digital pulses are uniformly delayed.
The primary
purpose of equalizing envelope delay of the voice circuit is to
minimize the effect of delay distortion on received dc digital
signal.
45
Previous Page
