Experimental AM Demodulation Circuit
Primarily designed for experimental purposes in AM demodulator, this circuit is more of a prototype than a practical solution. In our corresponding ‘AM Modulator,’ we discussed the challenges posed by mains hum, which can interfere with the AM mains intercom’s seamless operation. When the transmitter and receiver are not connected via the mains, the circuit performs adequately. The ‘receiver’ employed here is identical to the one featured in the ‘Mains Remote Switch’ and ‘Mains Remote Control Decoder,’ involving components C1 to C5, TR1, and L1.
Safety Considerations and Signal Amplification
To enhance safety, the capacitor connecting the small toroidal transformer to the mains has been split into two parts, although it’s essential to recognize that the entire circuit operates at mains potential. Caution must be exercised: refrain from soldering or taking measurements while the circuit is switched on. The input signal undergoes initial amplification to the appropriate level through a fast op-amp (AD827). P1 allows adjustment, providing this stage with a maximum gain of 20 dB. The demodulator itself is remarkably simple, consisting only of a diode, a capacitor, and a resistor (D2/R5/C9). Due to the RC time constant and the diode, the capacitor’s voltage tracks the envelope of the AM carrier wave. The circuit involving IC1b, D1, and R4 serves to linearize the characteristics of diode D2 slightly. However, if circuit simplicity is paramount, this part can be omitted.
Hum Removal and Interference Mitigation
A passive double-T filter follows this stage, aiming to eliminate significant hum from the signal. Tuned to 100 Hz, this filter addresses the strongest component of the interference. However, practical scenarios involve multiple harmonics of 50 Hz, making it challenging to suppress with a simple filter. Considering the power amplifier’s high sensitivity, the double-T filter is succeeded by a potential divider and first-order high-pass and low-pass filters (C14/R9/P2/C15). This configuration retains most of the speech signal while further reducing interference. The power amplifier employed is a TDA7052 (IC2) designed for 6 V operation but compatible with 12 V usage.
Careful attention is necessary to avoid excessive power consumption (a 16 Ω speaker, if available, is preferable). The amplification achieved is around 40 dB. Proper decoupling of IC2 from the supply is ensured through R10/C16/C17. Constructors must ensure that both the ground and the +12 V for the power amplifier are directly supplied from the regulator without being diverted to other parts of the circuit. Preset P2 is utilized for adjusting the volume level, although a genuine (logarithmic) potentiometer can be an alternative.
Standard Power Supply and Circuit Enhancements
The power supply follows a standard configuration: a bridge rectifier (B1) with suppression capacitors (C21 to 24), a smoothing capacitor (C20), a 7812 regulator (IC3), and a final decoupling capacitor. The circuit’s quality could be elevated by replacing the passive filter with higher-order active filters, possibly employing switched-capacitor types. However, this approach is suitable only for committed experimenters willing to venture into more intricate designs.