Circuit Conditioning Equipment

TM 11-5895-1012-10

or amplifier circuits to compensate for differences in

attenuation due to differing impedances at the various

frequencies being used. Adjustments are usually pro-

vided to accomplish the required equalization.

d. Envelope Delay Equalizers.

(1) There is still another way in which voice-grade

in electrical characteristics requiring

circuits can vary with frequency, and that is in the

conditioning equip

velocity of propagation. This variation does not usual-

distortion, envelope

ly affect speech transmission, but it can have an ad-

delay, signal level changes, and longitudinal balance.

verse affect on data transmission.

2-31. Circuit Conditioning Equipment

(2) In any circuit there is a finite time interval for

transmission from the sending end of the circuit to the

The following is a brief description of circuit condition

receiving end. This time is referred to as absolute de-

equipment used on the DCS and found in the Technical

lay and varies according to the facility used. This time

Control Facility.

interval will also vary considerably between various

a. Pads. Pads, which come in many forms and physi-

frequencies in the transmitted band. This means that

cal arrangements, are used to attenuate signals within

the shape of a signal wave at the receiving end can dif-

a circuit. Pads found in telecommunications normally

fer to an appreciable degree from that originally ap-

have the same input and output impedance; however,

plied at the transmitting end. This distortion is called

they can be designed to match impedance of different

envelope distortion; and it is measured in microsec-

values. The attenuation introduced by a pad is varia-

onds of envelope delay.

ble, most usually by using different strapping options

(3) Envelope delay distortion is the result of the

to adjust the amount of attenuation in steps of 0.5 dB.

deviation of the phase shift from a straight line pat-

b. Amplifiers. An amplifier is used to increase sig-

tern across the frequency band of interest. In carrier

nal strength without introducing appreciable distor-

systems, the variation in time and frequency is caused

tion and to compensate for attenuation of signals on vf

mainly by the channel filters. Often additional distor-

channels. Transistorized and vacuum tube amplifiers

tion will be found in the outermost channels of a car-

will be found in use on the DCS. Typical amplifiers

rier group. This effect will be more noticeable in some

used in circuit conditioning have an adjustable gain of

systems than in others and is caused by the filter cut-

from zero to 35 dB at 1000 Hz, stabilized within .2

off. Normally, it is good practice to avoid the lower

dB and with a frequency response from 300 to 3400

Hz. Input and output impedances are usually 600 ohms

and upper channels of a group or channels adjacent to

pilot tones when channels are being assigned for data

balanced.

service.

c. Amplitude Equalizers.

(4) Although delay distortion is found in cable and

(1) Decreased intelligibility in voice circuits can be

caused by the unequal attenuation of different fre-

channel terminals, it should be remembered that

equipment units, such as repeaters and coils, also

quencies. For example, as various frequencies pass

along a nonloaded cable, the high frequencies are at-

contribute to delay distortion. These units affect the

tenuated more than the lower ones. A l-mile section of

frequencies below 1000 Hz to a greater extent than

26-gauge high-capacity cable will have a loss of about

those above 1000 Hz. In calculating the overall

1.4 dB at 250 Hz; however, at 3000 Hz, the loss will be

envelope delay of a circuit, the delay inherent in each

4.5 dB, a difference of 3.1 dB per mile. Therefore, a 4-

equipment unit or line facility section, at specific fre-

mile section of 26-gauge nonloaded cable will result in

quencies, is summarized. When the delay of the

a 5.6 dB loss at 250 Hz and an 18.0 dB loss at 3000 Hz.

equalizer is added to that of the equipment and facil-

The 1000 Hz loss of the 4-mile loop would be 10.7 dB.

ities, the result should be within the limitations for the

The loss across the band, taken at these three fre-

circuit being implemented.

(5) For individual circuits which will be switched

quencies referenced to the 1000 Hz loss, would be 5.1

in tandem to form a completed connection, the delay

dB less loss at 250 Hz and 7.3 db more loss at 3000 Hz.

(2) The variation in loss across the band of in-

requirements are much more stringent than for the

terest can be compensated for by equalizing. The

overall connection. It is necessary, therefore, that each

equalizing process consists of inserting into the circuit

circuit, whether an interswitch circuit or access line,

additional loss which varies with frequency. Sufficient

fall within the delay distortion limits established

is added at each frequency to bring the response

under the transmission plan for the system involved.

across the band within a specified limit referenced to

This limit will be stated in the Circuit Layout Record

the loss at a specified frequency, usually 1 kHz.

(CLR) Card for the circuit.

(3) The amplitude equalizer is any combination of

(6) There are two general types of equalizers used

coils, capacitors, or resistors inserted in transmission

to compensate for the effects of envelope delay. One of

2-35