A TV Tuner is a completely sealed and isolated unit which receives UHF/VHF signals from the antenna. It consists of RF amplifier, local oscillator and mixer stages. It is also called the front end of the TV receiver.
The following are the functions of TV Tuner to be performed:
1. It selects the channel to be received by switching pre-tuned circuits in the RF stage and the oscillator.
2. It matches the impedance of the line at its input and amplifies it to maintain a good signal-to-noise ratio. The gain is controlled by AGC voltage, to suit the input signal strength.
3. It converts the RF into IF by mixing it with the local oscillator frequency, to feed into the video IF amplifier.
4. It blocks the interfering antenna pick-up signals in the IF range and prevents them from entering the receiver and mix up with the local oscillator and hence to the IF amplifier.
5. It isolates the local oscillator signals from the antena, due to the RF amplifier acting as a buffer, preventing radiation and interference to other receivers.
6. It rejects the image frequencies by means of the RF selective circuits.
Depending upon the tuning range required, the tuner is designed as a single channel, multichannel VHF or UHF tuner. The VHF tuner may be a step-switch type tuner, using preset tuned circuits on a turret or a rotary switch. The UHF tuner uses continuous tuning usually employing transmission line or strip-line tuned circuits.
Factors affecting Tuner Design:
The following are the factors that affect the design of a TV Tuner:
1. Choice of IF and Local Oscillator Frequencies: The following are the merits of choosing intermediate frequencies (IF) close to 40 MHz are
a. High image rejection ratio.
b. Reduced local oscillator radiation.
c. Ease of detection.
d. Good selectivity at the IF stages.
The local oscillator frequency is kept higher than the channel carrier frequency since this results in a relatively narrow oscillator frequency range. In the 625 – B monochrome system, the picture IF = 38.9 MHz and sound IF = 33.4 MHz.
2. Need for an RF Amplifier Stage: At very high frequencies, the problems of image signals, radiation from the local oscillator through the antenna circuit and conversion loss at the mixer are such that a stage of amplification prior to the mixer is desirable, That is, an RF amplifier stage is desirable. Also, this stage feeds enough RF signal into the mixer for a clean picture without snow.
3. Coupling Networks: Parallel tuned networks are used to accomplish the desired selectivity in RF and IF sections of the receiver. For minimum power loss, the coupling network should match the output impedance of one stage to the input impedance of the following stage.