Spectrum Software
Industrial Strength Simulation




Spectrum Software has released Micro-Cap 11, the eleventh generation of our SPICE circuit simulator.

For users of previous Micro-Cap versions, check out the new features available in the latest version. For those of you who are new to Micro-Cap, take our features tour to see what Micro-Cap has to offer.




QAM Modulator Macro


The analog QAM (Quadrature Amplitude Modulation) scheme uses amplitude modulation in order to convey the signal information of two separate analog signals. The term quadrature refers to two periodic waveforms whose phase difference is one fourth of their period. The two carrier waves used within the QAM modulation are sinusoidal and out of phase by 90 degrees which make them quadrature carriers.

The output of the QAM modulator is in the form:

Out(t) = In1(t) * Cos(2*PI*F0*T) + In2(t) * Sin(2*PI*F0*T)

where In1(t) and In2(t) are the two signals to be modulated and F0 is the carrier frequency. This equation can be easily modelled within Micro-Cap. The QAM modulator can be simulated through the macro circuit shown below.

QAM modulator macro circuit

The QAM modulator macro circuit has one parameter. The F0 parameter defines the carrier frequency for the quadrature carriers. The two input nodes for the macro are InCos and InSin. The InCos input is for the signal that is to be modulated by the cosine carrier. The InSin input is for the signal that is to be modulated by the sine carrier. The InCos input is fed into the X2 Mul component which multiplies the input signal by the cosine carrier wave. The cosine carrier wave is produced by the E1 NFV source whose VALUE attribute is defined as:


The InSin input is fed into the X3 Mul component which multiplies the input signal by the sine carrier wave. The sine carrier wave is produced by the E2 NFV source whose VALUE attribute is defined as:


The output of both Mul components are then summed through the X1 Sum component which produces the final modulated signal.

An example circuit using the QAM modulator macro appears below. The inputs to the QAM modulator consist of a pair of sine signals. The V1 voltage source produces a .75V, 20kHz sine wave, and the V2 voltage source produces a .6V, 50kHz sine wave. The QAM modulator macro has its F0 parameter defined as 1Meg in order to produce 1MHz carrier signals.

QAM modulator test circuit

The output of the QAM modulator is then demodulated through the use of a product detector demodulator. This demodulator multiplies the modulated signal separately by cosine and sine signals that are identical to those used for the carrier waveforms. Multiplying the modulated signal by a cosine signal produces the waveform:

In1(t) * Cos(2*PI*F0*T) * Cos(2*PI*F0*T) + In2(t) * Sin(2*PI*F0*T) * Cos(2*PI*F0*T)

Using the standard trigonometric identities where:

Cos(2*PI*F0*T) * Cos(2*PI*F0*T) = .5 * (1 + Cos(4*PI*F0*T))
Sin(2*PI*F0*T) * Cos(2*PI*F0*T) = .5 * Sin(4*PI*F0*T)

The equation can be rewritten as:

.5 * In1(t) + .5 * In1(t)*Cos(4*PI*F0*T) + .5 * In2(t) * Sin(4*PI*F0*T)

Removing the higher frequency terms would leave just the .5*In1(t) signal. The modulated signal that is multiplied by the sine signal would operate in a similar manner.

A low pass Butterworth filter is used to remove the higher frequencies from each of the demodulated signals. This filter was created through the Passive Filter Designer in Micro-Cap. It is designed to have a passband gain of 0dB and a passband frequency of 100kHz.

Finally, an Amp component is used at each output to restore the signal back to its original magnitude. To compensate for the attenuation caused by the demodulator and the resistances at the input and output of the filters, the Gain parameters for the Amp components are set to 4. The resulting transient analysis is shown below.

QAM modulation/demodulation transient analysis

The top plot displays the modulated signal at the output of the QAM modulator. The middle and bottom plots display the original input signals to the QAM modulators and their corresponding outputs from the demodulation circuitry.

A delay has been introduced due to a phase shift from the low pass filter, but other than that the input signals have been replicated nicely at the output of the demodulator.

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