Transposer
In broadcasting technology, a transposer is a device in the service area of a transmitter which rebroadcasts electromagnetic signals to the receivers which can’t properly receive the signals of the transmitter because of a physical obstruction like a hill. A high altitude transposer receives the signals of the transmitter and rebroadcast the signals to the area of poor reception. That’s why sometimes the transposer is also called relay transmitter or rebroadcast transmitter.[1] Since transposers are used to broadcast a shadow area their output powers are usuallt lower than that of transmitters. sTransposer
Pyhsical obstruction
Reception of RF signals is sensitive to the size of obstruction in the path between the transmitter and the receiver. Generally speaking if the size exceeds the wavelength the reception is interrupted. Since the wavelenth is inversely proportional to frequency, that means that higher frequency broadcasting is more sensitive to objects between the transmitter and receiver. Supposing the transmitter and the receiver are at the opposite sides of a hill, while MW radio signals can be received, UHF TV signals can’t be received at all. That’s why transposers are always employed for VHF and UHF receptions .
Transposer circuitry
The transmitters have the following stages:
The transposers have the following stages.
- RF input stages
- Input mixer
- IF stages
- Output mixer
- RF amplifiers.
It should be noted that the output stages of both devices are similar. But the input stages are quite different.
There is no AF or VF input to the transposer. The transposer receives input RF signals by means of an antenna just like a home receiver. Since received signal is already modulated there is no modulator. But instead of a modulatıor an input mixer is used to convert RF signal to IF signal. A second mixer (known as output mixer) convert the IF signal to output RF signal.
Relation between input and output RF signals.
The amplification of the input RF signal is automatically controlled usually by PIN diodes. If the output and ınput RF signals have the same frequency the output RF which feedbacks (thru output and input antennas) the input stage completely blocks out the transposer. So in transposers, output RF frequencies of input and output RF are always different.[2] Further more, an input filter which is a part of input stages and an output filter which is a part of the output amplifier isolate the input from the output.
See also