عمل المذبذب المحلي .. كتاب التلفزيون الملون والعادي

E-8 Work of the local oscillator:

This oscillator generates a frequency greater than the frequency of the RF signal coming from the antenna by the amount of intermediate frequency IF - and super heterodyne collision is performed in it, as in a radio device.

It is important that the frequency of the oscillator be fixed and stable and that there is no deviation from its correct frequency due to the thermal effect, or a change in the values ​​of the circuit elements or in the value of the DC voltage or the load.
A slight shift in the oscillator frequency of a monochrome TV can be tolerated. As for color TV, it can cause color loss. For this reason, there is the Automatic Fine Tuning (AFT) circuit, which is used to maintain the stability of the oscillator frequency, and since the frequency of the oscillator changes with the change of the tuned station, the automatic fine tuning circuit makes the oscillator at the correct required frequency. Therefore, the frequency of the oscillator is higher by (36-40 MHz) than the frequency of the received signal .
The use of the transistor in the local oscillator circuit: The Colpitts oscillator transistor circuit shown in a transistor TV set is shown in Figure (5-24), so transistor 2 is an amplifier, and the base is biased by the voltage obtained in the voltage divider 1 and R2, so it gives a positive voltage The base of the transistor type NPN to secure the position of the transistor in the connected state - as for the file L
It is the collector load - which allows the oscillations to reach the desired amplitude or amplitude. As for setting the frequency by L-C-C, one of these capacitors is often variable in capacity to be used to control the fine tuning of the device. It is noted that the capacitor C is connected directly between the collector and the radiator, and this capacitor usually has one shape, and the capacitor C is necessary to prevent a short circuit in the passage of current from the collector to the base through a file, as it is used in providing

The required positive feedback signal from the collector to the base circuit.

The transistor is connected in the local oscillator circuit by the common base method
The transistor oscillator in the UHF field: We distinguish in the figure that the oscillator in the UHF field is of the type (Colypts) - and it is tuned with a capacitive diode (CR16). Base 2 is forward biased by R7 and Ras to enable the oscillator to boot and start - the Laz coil is the tuning coil which is a copper coil on a board circuit.
: typical transistor voter

Figure (5-26) shows a transistor voter for the VHF domain - the antenna has an input
Separate for VHF field and UHF field