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Tuesday, 26 February 2019

Microwave Link Repeater

The microwave repeaters act as a link between two long distance communication terminals. The different subsystems of the repeater station (Block diagram of Microwave Link Repeater) are shown in below Figure.

Block Diagram of Microwave Repeater Station
Here the signal will be received and retransmitted in the desired direction by the repeater.

The Antenna: Two antennas are provided, one is meant for east direction and other is meant for west direction. The antennas are parabolic reflectors. Hoghorn antennas are referred to obtain broader bandwidth along with low noise level.

The Circulator: In microwave repeater the signal received from one direction will be amplified and then transmitted to the other direction. Here same antenna will be used for receiving a signal from one direction as well as to transmit the signals received from the other direction. So there should be some circuitry that prevents the power being transmitted from the front end of mixer stage.

Here this purpose is accomplished by the use of circulator. That is the circulator connects the antenna to both the receiver and transmitter sections without causing any interference in these two sections due to the other. Along with this circulator, some receiver protection circuits are also used for protecting the receiver.

The Mixer Stage: The frequencies of the order of 3 GHz and 6 GHz are used for transmission. So the amplification at these frequencies is difficult in repeaters. The signal was down converted to around 70 MHz. Amplification can now be achieved very easily at low frequencies. In order to get this low frequency signal the incoming and local oscillator signals are mixed in the mixer stage. The output IF signal is now passed through a band pass filter whose centre frequency is tuned to 70 MHz and which has 12 MHz bandwidth. Thus channelization is achieved in microwave repeater circuits.

IF Amplifier: It is a low noise amplifier. It provides required gain by adequate number of I.F. amplifier. IF amplifier receives AGC signals and hence maintains gain within the limits. The active device used is a low noise broad band transistor.

Amplitude Limiter: This stage avoids the unwanted amplitude by the amplitude modulation and thus prevents the noise amplification.

Transmitter Mixer: Transmission is done at high frequencies. This mixer stage converts the frequency of the amplified signal to the required transmitting frequency. The output is applied to a transmitting band pass filter. This stage allows required signal to next stage of power amplification.

Power Amplifier: The power amplification depends on bandwidth of the link. For low power amplification usually reflex klystron oscillator itself is sufficient. For high power amplification either push-pull disk seal triode type or TWT power amplifiers are used. Travelling wave tube amplifiers provide high power amplification at high frequencies than the semiconductor devices. That is why normally TWT amplifiers are used.

Microwave Source: The source that generates the microwave frequency is a VHF transistor crystal oscillator. The oscillator output frequency is multiplied using a varactor multiplier. The signal from this source reaches power splitter after multiplication.

Power Splitter: This stage splits the power and diverts the 75% of power to the transmitter mixer and the remaining is to mixer stage. The mixer is also supplied to shift oscillator.

Mixer and Shift Oscillator: The output of the mixer is passed through a band pass filter and is given to a receiver mixer. The function of this circuit is to provide the receiver mixer with a frequency which is higher than the incoming signal by 70MHz. Then only the IF of 70MHz is obtained. The band pass filter removes unwanted frequencies form output of balanced mixer.

The different blocks of Microwave Link Repeater block diagram shown in the above figure are for west reception direction and transmission to east direction. Similarly the identical blocks are required for west transmission and east reception.

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