CHAPTER 5 FUNCTIONING OF EQUIPMENT - TM-11-6625-922-150026

TM 11-6625-922-15

CHAPTER 5

FUNCTIONING OF EQUIPMENT

this measurement. The oscilloscope measurement can

5-1. Transmission Distortion

be read directly in terms of time, or in the case of a sine

wave, time can be computed by a knowledge of phase

Types of Distortion.

Any communications

shift and frequency (fig. 5-1). The time delay is

medium (wire, radio, microwave, coaxial cable, etc) will

proportional to the phase shift divided by the frequency.

degrade a signal passed through it because of at least

three significant distortions.

b. In actual practice, for perfect data transmission it

is only necessary that all frequencies in the information

(1) Amplitude variation with frequency. Typical

band be delayed by the same amount of time. No

circuits invariably have a high-frequency cutoff, and

arbitrary amount of time delay has to be realized. The

may have a low-frequency limitation.

constant time delay associated with this ideal

transmission system will cause only a time delay in the

(2) Phase distortion.

If the transmission

reception of the information, but no distortion.

medium has a phase shift characteristic that is not linear

(that is, if phase shift is not directly proportional to signal

c. The requirement for a constant delay for all

frequency), an effect known as delay distortion occurs.

frequencies in a band can be interpreted as requiring a

constant differential phase shift versus frequency.

(3) Nonlinear amplitude response. The system

may have a response which is not linear with level,

d. The phase characteristics outside the frequency

leading to harmonic and intermodulation distortion.

band may have shape without affecting the information

However, this topic is not covered in the manual.

transmission.

b. Distortion Measurement. The delay distortion of

5-3. Delay Measurement

a transmission system is more difficult to measure than

amplitude response. If delay distortion is present,

Measurement of transmission delay is most

certain frequencies in a signal band will be delayed

conveniently accomplished by the scheme shown in

more than others. The effect of this differential delay on

figure 5-2. A carrier frequency oscillator with an output

a system depends on the information characteristics.

frequency fc in the desired frequency range is

For instance, moderate amounts of delay distortion have

amplitude-modulated with a relatively low frequency, fo.

comparatively small effects upon voice transmission,

This modulated signal is passed through the network

since the ear is not particularly sensitive to phase

under measurement (e.g. transmission line), and the

variations. On the other hand, differential delay on a

output is demodulated to recover the modulating signal

channel used to transmit complex information, such as

f1o. The recovered modulating signal, f1o, is then

radar, video, or television, can result in a large

compared in phase with the original modulating

degradation of the received signal. Delay distortion on a

frequency, f1o. The phase meter is calibrated to read in

high-speed data link can result in a prohibitive error rate,

milliseconds the amount of time delay incurred by the

or a completely unusable channel.

modulating waveform as it passed through the network

under test. This delay of the modulating frequency, f ,

5-2. Concept of Envelope Delay

corresponds to the delay incurred by the carrier

frequency fc, which is of prime interest.

a. The simplest form of time delay is the absolute

transmission delay of a signal traveling from one point in

5-4. Block Diagram Analysis

a circuit or system to another. When the signal is a

(fig. 5-3)

single pulse or a burst of pulses, the absolute time delay

can be determined. A dual trace oscilloscope with a

a. Carrier Frequency Generation. The carrier

common synchronization trigger could be used to make

frequency generator in the TS-2669/GCM uses a

5-1