Sunday, 3 December 2017

VHF Tuner, RF Tuner Block Diagram

The block schematic of a TV tuner is shown below. At the antenna input terminals, the tuner must have input impedance equal to the characteristic impedance of the aerial feeder so that the matching provides a maximum power transfer and a voids reflections on the line the standard impedance is 300 Ω, corresponding to the twin wire ribbon feeder commonly used. The balun matches this impedance to the 75 Ω input impedance of the RF amplifier. It consists of a ferrite core upon which are four tightly coupled and evenly spaced bifilal windings of a couple of tums each, in the form of two quarter wave lines each of 150 Ω, that provide by series connection a 300 Ω balanced impedance on one side and a parallel connected 75 Ω impedance unbalanced, on the other side, by grounding of one terminal.

Block diagram of RF Tuner
A pair two small capacitors 470 pF each are included in each lead to prevent or block the dc path from chassis to antenna and prevent damage due to lightning the 2 Meg shunting resistors discharge any static accumulated charge on these capacitors.

IF Trap & HP Filter

Unwanted spurious signals in the IF range of 33 to 40 MHz are blocked by the IF trap and high pass (HP) filter as the rejection of these by the low QRF circuits, may not be adequate due to broad bandwidth particularly on the lowest channel number 2 viz, 47-54 MHz. It is very difficult to reject them once they reach the mixer and IF stages. The trap usually consists of an HP filter with pass beyond 40 MHz in the form.
Tuner input Balun & Filters
RF Amplifier:

This provides a gain to the input signal ensuring better signal-to-noise ratio, isolation to the local oscillator radiation and better image selectivity. Its primary function is to provide adequate gain (20 dB) to weak signals maintaining a good signal-to-noise (S/N) ratio at the mixer. The mixer generates more noise because of its heterodyne function.

The equivalent noise voltage at the input of the RF amplifier sets a limit to the minimum signal that can be received. The noise voltage is visible on the screen of the picture as shown background of black and white randomly moving spots. The noise voltage is typically around 10 µV. The RF stage is best suited for AGC because the signal level is small and the gain control is most effective here producing minimum distortion. Application of AGC to RF stage is usually delayed suitably in order to maintain good S/N ratio at weak signals.

The stage provides isolation by acting as a buffer between the local oscillator and antenna terminals to minimize radiation from the local oscillator. These irradiations can be a source of considerable interference to neighbouring receivers showing as diagonal line patterns on the screen. The field strength should be less than 100 µV/m at 100 ft in the VHF band or as per regulations prevailing. A judicious placement of the coupling coils windows, use of RF chokes, feed-through capacitors in the tuner supply lines, viz, Vcc, B+, filament supply wires and proper tuner case shielding, are all important to bring down the radiation below specified limits. Separate chassis ground return is generally often employed for tuner circuits. Feed through capacitor construction shown in figure, provides very efficient by passing and decoupling because of its coaxial construction. Reception of image frequency corresponding to (fs  + 2IF) which also produces the IF at the mixer, should also be adequately suppressed by the RF tuned circuits.

RF Response:

The RF amplifier has a pass band broad enough to pass the channel selected and also allow for the variation in the local oscillator fine tuning and variation due to the AGC voltage that should not affect the RF gain. Double tuned filters with a suitable dip between peaks can provide broad bandwidth of over 11 MHz and also good transient response. Below figure shows a typical response with a dip of 1dB between the peaks.

Mixer:

The mixer produces the IF signal by heterodyning the RF signal with the local oscillator frequency. As there are two carriers in the RF signal, viz, picture carrier and sound carrier, two Ifs are produced, the picture IF equal to 38.9 MHz and the sound IF equals to 33.4 MHz. The local oscillator frequency is higher than the RF carriers so that for channel 4 for example, the Low frequency is (62.25 + 38.9)101.15 MHz; and for channels , it is (176.25 + 38.9) 215.15 MHz.

Local Oscillator:

This provides the local oscillator frequency which should be essentially stable free from drifts due to temperature aging of components or small changes in the supply voltages, etc. It should have minimum harmonic content. The oscillator frequency control is basically the fine tuning control of the receiver. This may be manual varactor tuned or automatic fine tuning using varactor diode bias control from the frequency discriminator at the IF.

Feed Through Capacitor