Coherent Detection of DSB-SC AM Signals Using AD633 Multiplier IC

Double Sideband-Suppressed Carrier (DSB-SC) Amplitude Modulation (AM) is a key technique in communication systems, often used for efficient signal transmission. Demodulating a DSB-SC AM signal can be challenging, but one effective method is coherent detection, also known as synchronous detection. In this method, the DSB-SC signal is multiplied by a locally generated carrier signal that is synchronous in both frequency and phase with the carrier used at the transmitter. In this article, we explore how to demodulate a DSB-SC signal using the AD633 Analog Multiplier IC.

What is Coherent Detection of DSB-SC AM Signals?

Coherent detection works by multiplying the received DSB-SC signal with a local carrier signal. The multiplication is performed by a product modulator, which ensures the frequency and phase of the local oscillator are synchronized with those of the transmitter’s carrier. This method enables the extraction of the original message signal from the DSB-SC modulated carrier.

Components and Circuit Overview

In a typical coherent detection setup, the following components are involved:

• AM Modulator: Generates the DSB-SC signal using the AD633 multiplier IC.
• BJT Amplifier: Amplifies the DSB-SC signal before transmission.
• Product Modulator: The AD633 IC is used to multiply the received DSB-SC signal with the locally generated carrier signal at the receiver.
• Low Pass Filter: The output from the product modulator is passed through a second-order RC low-pass filter to remove high-frequency components and extract the message signal.

DSB-SC AM Signal Generation (Transmitter Side)

The AM modulator circuit generates a DSB-SC signal using the AD633 multiplier IC. The following is the circuit diagram.

The message signal (frequency: 1kHz, amplitude: 1.2V) and carrier signal (frequency: 120kHz, amplitude: 1.2V) are applied to the X1 and Y1 pins of the AD633 IC. The X2, Y2, and Z pins are grounded. The equation governing the output W of the AD633 multiplier IC is:

Since $Z = 0$, we have:

• $X1 = A_m \cos(w_m t)$ (Message signal)
• $Y1 = A_c \cos(w_c t)$ (Carrier signal)

Thus, the resulting DSB-SC signal at the transmitter is given by:

where:

• $A_m$ is the amplitude of the message signal.
• $A_c$ is the amplitude of the carrier signal.
• $w_m$ and $w_c$ are the angular frequencies of the message and carrier signals, respectively.

This DSB-SC signal is then amplified using a BJT amplifier and transmitted over the communication channel.

Coherent Detection at the Receiver

At the receiver end, the received DSB-SC signal is multiplied by a local carrier signal generated by a local oscillator. The AD633 IC is again used to perform the multiplication, acting as the product modulator. The circuit diagram for the receiver, including the product modulator and low-pass filter, is shown below:

• The received DSB-SC signal is applied to the X1 pin.
• The locally generated carrier signal is applied to the Y1 pin.
• The X2, Y2, and Z pins are grounded.

The output of the AD633 product modulator is then passed through a second-order low-pass filter.

Mathematical Derivation of the Demodulation Process

The received DSB-SC AM signal is given by:

The local carrier signal is represented as:

Where $\phi$ is the phase difference between the local carrier signal and the transmitter’s carrier signal.

The output from the product modulator is the product of these two signals:

Expanding this equation:

This simplifies to:

Thus, the output consists of:

• The message signal $\cos(w_m t)$ multiplied by the scaling factor $\frac{A_m A_c}{20} \cos(\phi)$,
• High-frequency components at 2w_c and sinusoidal terms at 2w_c.

The high-frequency components are removed by the low-pass filter, leaving only the message signal.

Demodulated Output

The output after filtering is:

This indicates that the demodulated signal is proportional to the message signal. The amplitude of the demodulated signal depends on the phase difference $\phi$. The demodulated signal reaches its maximum amplitude when $\phi = 0$ and is minimized when $\phi = \pm \frac{\pi}{2}$. This phase-dependent attenuation is known as the quadrature null effect.

Phase Sensitivity and Practical Considerations

• When $\varphi =0$, the demodulated signal is undistorted.
• When $\phi = \pm \frac{\pi}{2}$, the signal is attenuated to zero.

In practical systems, noise in the communication channel can cause random variations in the phase $\phi$, leading to varying levels of attenuation in the demodulated signal.

Low-Pass Filter Design

To remove the high-frequency components and extract the message signal, a low-pass filter is used. In this circuit implementation, a 750 Ohm resistor and a 10nF capacitor form the RC low-pass filter. With these values, the cutoff frequency is approximately 22kHz, suitable for audio signals. Two RC filters are cascaded to improve the performance of the demodulation.

Conclusion

In this article, we explored the process of coherent detection for DSB-SC AM signals using the AD633 multiplier IC. Coherent detection is an effective method for recovering the original message signal from a modulated carrier, with the demodulated signal’s amplitude being dependent on the phase alignment between the transmitted and locally generated carriers. By understanding this method and the role of the AD633 IC in performing multiplication, engineers can design efficient communication systems that utilize DSB-SC AM modulation.

For further learning on similar topics, refer to these related resources:

• Standard AM with AD633 Analog Multiplier IC
• Demodulation of AM signals in Matlab
• Frequency Modulation using 555 Timer

By following these tutorials, you can gain a deeper understanding of modulation techniques and their practical applications in modern communication systems.