Impedance Matching In Telecom Network

Impedance Matching In Telecom Network

In telecom networks, such as attenuators, equalizers and amplifiers etc., impedance matching is of paramount importance to affect the transfer of maximum energy from one telecom network into another. Impedance Matching In Telecom Network

Correct impedance matching also reduces the impairment to the transmission of telecom signals. For example in the case of impedance mismatch between the two telecom networks, say a cable pair and an amplifier input terminals, reflection of the signal takes place and the reflected signal travels back to the talking subscriber. It amounts to echo and is extremely annoying to the talker and can interfere with his normal process of speech. Impedance Matching In Telecom Network

Impedance matching

Since it can be designed to have any desired impedance values looking in either direction, a simple resistance pad can be used to match any two resistive-impedance. It, however introduces high attenuation lossImpedance Matching In Telecom Network

Impedance may also be matched by a T or X pads made up entirely of inductors and capacitors. Though such a pad would introduce relatively little loss, its effectiveness as an impedance matching device would be limited to the frequency for which the reactance values have been determined. Repeating coils and transformers come under this category.

The reading of power level at a point in a circuit can be obtained in two ways.

(a) Level or through measurement,

(b) Transmission terminating or loss measurement.

Level or through measurement

A level measurement is obtained by tapping a high-impedance voltmeter across the circuit, as in Fig 1.

Decibel meter

The high impedance of the meter is essential to ensure that it will not disturb the circuit under test. (Ex. a 5000 ohms meter will introduce a shunt loss of 0.5 dB). If the impedance of the circuit under test is 600 ohms, the meter will give correct reading in dBs.

Any variation in circuit impedance from 600 ohms will destroy the accuracy of the measurement. In case, however the impedance is known, a correction factor may be applied. Impedance Matching In Telecom Network

In transmission circuits, sometimes the impedance encountered is 75 ohms or 150 ohms instead of the standard 600 ohms impedance. Correction factors for that impedance are calculated below:

E = voltage at a point in a circuit.

150 = impedance at the said point. Power indicated on dB meter = E2 / 600

Actual Power = E2 / 150

Error = 10 log10 150/600 = 6 dB

i.e. the meter will read 6 dB. Which implies that 6 dB is to be added to the meter reading to arrive at the correct power level. Similarly, the correction factor for level measurement at a point of 75 ohms impedance is -9 dB.

Transmission, terminating or loss measurement

A transmission measurement is made by terminating the circuit in a 600 ohms resistance and measuring the voltage across it using the meter. Refer fig.

Transmission, terminating or loss measurement

Many decibel meters having both facilities, a high impedance meter being used, the 600 ohms resistance for TRANS measurement being brought into circuit by operation of a switch and failure to cut off the 600 ohms when making a LEVEL measurement will give a reading that is 3.52 dB low. The meter shows E/3 where as voltage reading should be E / 2.

Discrepancy of reading = 20 log 2 / 3

= – 3. 52 dB.

Reflection loss

The phenomenon of reflection is very common in our every day life. When we look into a mirror or hear an echo, we know that these are due to reflection of light and sound wave respectively. From these, it can be deduced that any wave suffers reflection whenever there is an abrupt change of medium through which the wave is propagating.

In the case when electromagnetic energy being propagated over a transmission line, the wave motion is guided, between the two wires constituting the line and is called a “ guided “ wave or a “ travelling “ wave. The transmission line may further be connected to equipment or to another transmission line having different electrical characteristics, thus causing change of medium and hence reflection of energy takes place. The electrical characteristic that causes reflection is the impedance of medium. Any variation in impedance will cause reflection. Similarly impedance mismatch in networks (active or passive) causes reflection. Impedance Matching In Telecom Network

One direct consequence of reflection is that the amount of power transferred to the load differs from that when matching conditions exist. All theoretical treatments of networks and transmission lines are based on the condition of perfect matching, which can only be approximated in practical applications. Hence the difference between the matched and non-matched condition is expressed as a “ Reflection loss “

Return loss

Return loss is defined as:

20 log 1 / reflection co-efficient

in which, Reflection co-efficient = reflected signal / incident signal at the point of impedance discontinuity or impedance mismatch.

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