What Is a Local Oscillator?

The local oscillator is also called local oscillator (LOCAL OSCILLATOR), it is actually a self-excited sine wave oscillator. Its role is to generate a high-frequency equal-amplitude sine wave signal with a medium frequency (37MHz in China) higher than the received signal, and inject this oscillating signal into the mixer to obtain an intermediate-frequency television signal after mixing with the high-frequency television signal. . ^[1]

1. Stability of oscillation frequency.

2. The electromagnetic waves radiated outward should be as small as possible to prevent interference with the work of nearby TV sets.

3. The circuit structure is simple and reliable, easy to start, easy to change channels, stable work, and good output waveform.

4. A frequency fine-tuning mechanism (referring to a mechanically tuned VHF tuner) should be provided. The frequency fine-tuning adjustment range should be not less than ± 1.5MHz to ± 5MHz. ^[1]

The oscillation circuit used in the local oscillator is a three-point LC sine oscillator. Its basic types can be divided into two types: three-point inductor and three-point capacitor (Figure 2-10 (a) and (b)). In practical applications, in order to improve the frequency stability, a modified three-point capacitor (Figure 2-10 (c)) oscillator circuit is often used. This circuit is also called a Crapper circuit. ^[1]

Capacitive three-point LC sine wave oscillator is also called capacitively coupled oscillator or Kopitz oscillator. Figure 2-11a) shows a three-point oscillator circuit with common emitter capacitance.

Capacitive three-point oscillation circuit

In the figure, R _b1 , R _b2 , and Re are bias and current-stabilizing resistors. DC power is supplied to the collector of the oscillator through R _c . C _b is the DC blocking capacitor. C _e is the AC bypass capacitor of the emitter. L And C ₁ and C ₂ are oscillating circuits connected between the set and the base. When the power is turned on, the L, C ₁ and C ₂ circuits generate oscillation signals whose frequency depends on the LC value. In the actual circuit, because the capacitances of C _b and C _e are very large, the capacitive reactance is very small for high-frequency signals, which is equivalent to a short circuit, and the resistance values of R _b , R _c , and Re are the same as Compared with capacitive reactance, it is equivalent to an open circuit state for high frequency signals.

This can draw the equivalent circuit of Figure 2-11 (a) Figure 2-11 (b). In Figure 2-11 (b), U _sr is the input signal, U _sc is the output signal, and U _f is the feedback signal. When the power is turned on, the sinusoidal oscillation signal generated by the LC oscillation circuit is coupled between the base and the emitter of the transistor, and the amplified signal is output between the collector and the emitter. The working process of this circuit is: When the oscillation signal generated by the LC oscillation circuit causes the base voltage to drop, the collector voltage increases due to the inversion of the transistor. At this time, the polarities of the voltages on C ₁ and C ₂ are as follows: Shown in the figure. It is not difficult to see that the divided voltage on C ₂ causes the base voltage to fall further and the collector voltage to rise even more. On the contrary, when the oscillating signal raises the base voltage (when added to the positive half-cycle signal of the base), The rise causes the collector voltage to drop, and the voltage C2 feeds back to the base (in this case, it is upper negative and lower positive) causes the base voltage to increase further and the collector voltage to decrease further. This cycle continues so that the oscillations continue. That is, since the voltage applied to the bases C ₁ and C ₂ is a positive feedback voltage, which compensates the loss of the oscillation circuit, as long as the values of C ₁ and C ₂ are properly selected and the transistor has a certain amplification capability, the circuit will Can produce oscillations.

This circuit is like other oscillating circuits, and its oscillating frequency is also equal to the resonant frequency of LC in Europe, that is,

Where L is the inductance of the loop;

C is the series value of C1 and C2, that is,