The basic block diagram of the tuning circuit

In satellite TV receivers, the low-noise amplifier and downconverter are usually collectively referred to as tuners, also known as BS converters.
    Since the electrical signal received by the antenna is very weak, the performance of the tuner greatly affects the overall indicators of the receiving system. Since
the low-noise characteristics of microwave field-effect transistors have reached a level that can only be achieved by low-noise parametric amplifiers at room temperature, they
have been widely used in low-noise amplifiers for satellite receivers.
    Figure 6-35 shows the basic block diagram of a satellite receiver tuner. Among them, the input matching network is designed based on the condition that the first-stage amplifier can obtain
the minimum noise figure. The number of stages or amplification distribution of the low-noise broadband amplifier and the pre-IF amplifier can be different, but
the total gain of the two should be within 60 dB or so to meet the sensitivity requirements of the receiver indoor unit.
    Both the bandpass filter and the frequency converter use microstrip circuits. The first local oscillator signal source mostly uses a dielectric frequency stabilization circuit. The system can
achieve high frequency stability without constant temperature. The frequency converter mainly uses a balanced mirror. Image suppression frequency conversion circuit.
    1. Low-noise amplifier
    As can be seen from the block diagram shown in Figure 6-35, the microwave signal received by the satellite receiving antenna is first amplified by the low-noise amplifier and then
mixed.
    Low-noise amplifiers used in satellite television receiving systems are required to have high gain in addition to low-noise characteristics. Since
the matching degree of the amplifier input terminal will affect the noise performance of the amplifier, and the matching degree of the output terminal will affect the gain of the amplifier, the input and
output matching network is an important part of the low-noise amplifier .