AM Synchronous Demodulator

The advantage of synchronous reception of AM signals is widely known. It includes high selectivity, linearity of detection and a low noise level at the output compared to other ways of reception. The circuit diagram is shown in figure 1. The operation of the circuit is based on method of direct locking of the local oscillator frequency by the inpu

t signal. This method has been tested by the author in the medium wave receiver as described in [1], and showed good results. The proposed detector consists of a synchronized oscillator based on the transistors VT1 and VT2, a switching mixer based on the transistor VT3 and an AF filter L2C3C4. Synchronized oscillator is made on the basis of the oscillator circuit (Fig. 2), proposed by E. Savitsky [2]. The frequency tuning elements C2, R3 and the FET transistor VT1 has been added to this oscillator circuit. When the input signal is applied to the gate of the transistor, the input signal mixing with the signal of the local oscillator in the transistor`s channel. The component of the base current of the transistor VT2 modulates the signal of the local oscillator frequency, and when the difference frequency becomes equal to the lock range, then the local oscillator switches from the beat mode to the sync mode. This establishes the equality of the input signal frequency and the local oscillator frequency, and the phase difference between voltage of the input signal and the local oscillator † becomes equal to 0 ° or 180 °. The equality of the frequencies and phases is maintained by the DC component of the base current of VT2, the DC component is proportional to the sin †. As you can see, the device works in the same way as a system PLL, the difference is only in the phase shift, for a PLL system the phase shift is 90 ° in sync...

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