High and low frequency signal generator circuit

In debugging and repairing household appliances, high and low frequency signals are often used. Figure 11-24 is a simple high and low frequency signal generator , which can output
1000Hz audio signal, 465kHz intermediate frequency signal and 525~1650kHz high frequency signal. It consists of three parts: audio oscillator , emitter output device and high-frequency oscillator.     (1) Audio oscillator transistor VT1 and other components form an audio oscillator. The audio signal is output from the collector and passes through GRi, Cz Rz,. A three-section phase-shifting network consisting of G and VT1 input resistors is fed back to the base of VT1. The result of the phase shift is that the signal sent back to the base is 180 degrees ahead of the signal at the collector. , forming positive feedback, producing audio oscillation, and outputting an audio signal approximately equal to 1000Hz, which is directly added to the base of transistor VT2.     (2) Emitter output device The emitter output device composed of , VT2, etc. is arranged between the audio oscillator and the high-frequency oscillator. It has high input impedance and low output impedance, which can solve the impedance matching problem of front and rear stages and prevent the influence between prefix and suffix. (3) High-frequency oscillator is a high-frequency oscillator     composed of transistor VT3, etc. , and adopts a transformer-coupled oscillation circuit. The oscillation frequency is determined by the primary side inductance L of the transformer. Determined by the loop capacitors G and C. The 1000Hz audio signal is input to the base of VT3 through VTZ, the high-frequency oscillation signal is amplitude-modulated, and then the amplitude-modulated high-frequency signal is output by G.     Hurricane and R in the circuit are the bias resistors of VT1 and VT2. The resistance between the collector and emitter of VT2 and Rs and R are the bias resistance of VT3. Ro also attenuates audio signals.     S] is the power switch, and S2 is the medium frequency and high frequency selection switch. Close S2. G and C8 are connected in parallel, and by adjusting C7, the oscillation frequency can continuously change between 450-550kHz, and an intermediate frequency of 465kHz can be obtained. Open S2, disconnect C8, adjust C7, and the oscillation frequency can continuously change between 525 and 1650kHz.