MATLAB Examples

Modulate Two-Tone DC Signal Using IQ Modulator

Use the IQ Modulator block to Modulate a two-tone DC signal to RF level. Observe the impairments in the modulated output signal such as images due to gain imbalance, intermodulation distortion, and output third-order intercept (OIP3).

Contents

The two tones are at 10MHz and 15 MHz.The power of each tone is -30 dBm. The carrier frequency is 0 GHz.

IQ Modulator

The IQ modulator parameters are :

  • Available power gain: 10 dB
  • Local oscillator frequency: 2 GHz
  • I/Q gain mismatch: 0.1 dB
  • LO to RF Isolation: 90 dB
  • Noise Floor: -160 dBm/Hz
  • IP3: 10 dBm

Open the model.

open('model_IQmod')

Run the model and observe the spectrum analyzers.

Complex Output Power Density

In the complex output power density spectrum analyzer, you see the noise floor of the signal at -160 dBm/Hz.

Complex Output Spectrum Analyzer

In the complex output spectrum analyzer, you see the whole modulated signal including the imaginary parts. The output power level of the two tones (10 MHz and 15 MHz)is -20 dBm.

$$Output power level = Input power level + gain = -30dBm+10dB = -20dBm$$

The output third-order intercept( OIP3) is at 10 dBm. The spectrum analyzer measures this value.

Image Rejection Ratio

The images of the two tones are at -10 MHz and -15 MHz. The output power level of the two images are -67.8 dBm. Image rejection ratio is given by the formula:

$$IMRR = [(gain imbalance)^2+1-2*(gain imbalance)] / [(gain imbalance)^2+1+2*(gain imbalance)]$$

where,

$$Gain Imbalance = 10^ (0.1/20) = 1.0116$$

$$IMRR dB = 10*log10(3.3253e-05) = -44.78 dB$$

The image power level is given by the formula:

$$Image level = -23 dBm-44.78 dB = -67.78 dBm$$