Balanced modem circuit diagram with amplification

Balanced modem circuit diagram

Figure 4 is a balanced modem with amplification function. In the process of radio transmission, the carrier is often suppressed and only the sideband component is transmitted. The transmission of the upper sideband and the lower sideband together is called double-sideband transmission, and the transmission of only one sideband component is called single-sideband transmission. When the sideband transmission mode is used, the received signal must restore its carrier to detect the low-frequency information. The modulator that can suppress the carrier is called a balanced modulator; the detector that can detect the low-frequency signal from the double-sideband or single-sideband signal is called a synchronous detector. If u1 in Figure 4 is a low-frequency modulation signal, u2 is a high-frequency carrier signal, and the amplitude of the carrier signal is large enough (can be used as a switching signal for switching channels), its output is a modulation signal with the carrier suppressed, that is, balanced amplitude modulation is achieved. If u1 in Figure 4 is the received band signal, and u2 is the carrier signal to be restored (reference signal), the output voltage contains the low-frequency modulation signal component, and the modulation signal can be obtained after filtering, thereby realizing synchronous detection.