Circuit diagram of synchronous detector composed of MC1496
MC1496 / MC1496B / MC1496P pdf datasheet
The demodulation process of the amplitude modulation signal is called detection. Common methods include envelope detection and synchronous detection. Since the envelope of the carrier amplitude modulation signal directly reflects the changing law of the modulation signal, the method of diode envelope detection can be used for demodulation. However, the envelope of the double-sideband or single-sideband amplitude-modulated signal that suppresses the carrier cannot directly reflect the changing law of the modulated signal, so the envelope detection cannot be used for demodulation, and the synchronous detection method must be used.
Synchronous detection is divided into superimposed synchronous detection and product synchronous detection. It is very convenient to use the multiplication principle of the analog multiplier to realize synchronous detection. Its working principle is as follows: input an amplitude modulation signal at one input end of the multiplier such as a double-sideband signal that suppresses the carrier wave us (t) = usmcoswctcosΩt, and another The input synchronization signal (ie carrier signal) uc (t) = ucmcoswc (t) at the end, multiplied by the multiplier, the output signal can be obtained by formula (7-9)
U0 (t) is
In the above formula, the first item is the DC component, the second item is the required low-frequency modulation signal component, and the last three items are the high-frequency component. The DC component and the high-frequency component can be filtered using a DC blocking capacitor and a low-pass filter Thus, the demodulation of the carrier amplitude modulation signal is realized. The synchronous detection demodulator circuit composed of MC1496 analog multiplier is shown in the figure
7-5. Among them, the ux terminal inputs the synchronization signal or the carrier signal UC, and the uy terminal inputs the modulated signal US. The output end is connected with a low-pass filter composed of R11, C6, and C7 and a DC blocking capacitor C8, so this circuit can demodulate both the AM signal with carrier amplitude modulation and the carrier suppression amplitude modulation signal.
The demodulation operation of the circuit is as follows: first, the static operating point C of the measurement circuit should be basically the same as the static operating point of the circuit in Figure 7-3, and then the carrier signal UC is input from the ux terminal, fC = 20.945MHz, UCP-P = 100mV . shilling
uy = 0, adjust the balance potentiometer RP so that the output U0 = 0 is the balanced state. Then input the modulated signal US with carrier from the uy terminal, its fC = 20.945MHz, fn = 1kHz, USP-P = 200mV, Modulation degree m = 100%, at this time, the output U0 (t) of the multiplier is output through the low-pass filter U'0 (t), and the waveform of the output UΩ (t) after the DC blocking capacitor C8 is shown in Figure 7 respectively. -6 (a) shown. Adjusting the potentiometer RP can increase the amplitude of the output waveform U0 (t) and reduce the waveform distortion.
If US is a modulated signal that suppresses the carrier, the output waveform Un (t) after MC1496 synchronous detection is shown in Figure 7-6 (b).
(a) Carrier signal demodulation (b) Carrier signal demodulation Figure 7-6 Demodulator output waveform
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