Second intermediate frequency band variable frequency conversion circuit

Shown is a frequency conversion circuit with variable second intermediate frequency bandwidth . This circuit has a relatively simple structure and relatively good reliability. The first intermediate frequency signal sent from the tuner through the coaxial cable is frequency converted through the RF amplifier, electronically controlled attenuator, and image rejection filter. The electronically controlled attenuator is actually an automatic gain control amplifier that controls the attenuation of the circuit according to the amplitude of the demodulated signal. The converted intermediate frequency signal is amplified by an intermediate frequency amplifier. The intermediate frequency amplification circuit is equipped with a switch control circuit and two surface acoustic wave intermediate frequency filters. The bandwidths of the two SAWs are 27 MHz and 16 M}Iz respectively. The circuit is controlled so that the receiving state obtains a good IF carrier by changing the bandwidth, and also makes the demodulated signal have a good signal-to-noise ratio.