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  Channel Tuning

Introduction to Channel Tuning

There is a unique adjustment for every channel. Resonance is acquired with coil's distributed capacitance, stray and trimmer capacitors. The oscillator coil is generally constructed with an aluminium or brass screw as the core for fine tuning. Inductance reduces with aluminium or brass core due to eddy currents in the metal. The screw is turned through a plastic gear wheel that is engaged while the fine tuning control is pushed-in against the holdings springs. So, the frequency variation acquired is sufficient to tune in the next channel station.

A number of receivers use automatic frequency running (AFT) in which an automatic frequency control is applied to the local oscillator in the tuner.

VARIOUS SECTIONS OF A VHF TUNER

As displayed in diagram the tuner contains the following sub-sections

WORKING OF THE TUNER

The Tuner must comprise input impedance equivalent to the characteristic impedance of the antenna feeder for highest signal transfer to the tuner and to prevent reflections on the line. The balun matches the double wire ribbon feeder impedance of 300 ohms to the 75 ohm impedance of RF amplifier. It contains bifilar windings' two 150 ohm (Z0) quarter-wave sections on a ferrite core. As displayed in diagram, at one end the two quarter-wave lines are connected in parallel to provide 75 ohm impedance and at another end they are linked in series to give an impedance equal to 300 ohms. If a coaxial cable is employed to connect the antenna to RF amplifier a similar parallel arrangement can be employed on both sides of the balun to give the match. Either side of the balun can be employed for input with the reverse side for output.

The RF amplifier is prepares to give sufficient gain to weak - signals. It is necessary to keep a high signal-to-noise ratio at the mixer that produces lot of noise. The equal noise voltage at the input of the RF amplifier sets a limit to the least signal strength which should be received to prevent excessive snow effects. As described before RF amplifier besides giving gain to the selected channel signals

a. Stops radiation from the local oscillator and

b. Rejects image frequency signals.

The local oscillator produces an un modulated sinusoidal (Carrier Wave) voltage which is heterodyned in the mixer along with the incoming RF signal to influence frequency it translation to the IF band The local oscillator frequency is changed when a dissimilar channel is chosen to keep difference equal to the intermediate frequency among the tuned channel carrier frequency and the local oscillator frequency. The fine tuning control permits the oscillator frequency to be changed over a narrow range.

The task of this circuit is to convert the incoming RF signal frequencies from dissimilar channels into a common IF pass-band of the receiver. This is acquired in the mixer through heterodyning or beating the local oscillator frequency with the signal acquired from the RF amplifier. The mixer stage that combined with the local oscillator may be referred like a frequency converter.

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