Sampling Frequency and Spectral Regrowth

1. How is spectral regrowth measured?
2. What causes spectral regrowth?
3. How to reduce spectral leakage?
4. Why is leakage present in frequency domain measurements?

How is spectral regrowth measured?

To find the true spectral regrowth power, convert the measured spectral power levels to mW and subtract the spectrum analyzer noise floor from the measured DUT power. Reconvert to dBm to get the true spectral regrowth.

What causes spectral regrowth?

Because a transmitter's power amplifier gain is set to provide a specific average power, high peaks can cause the power amplifier to move toward saturation. This causes intermodulation distortion, which generates spectral regrowth.

How to reduce spectral leakage?

We have seen that spectral leakage is reduced by tapering the digital signal by a window function before the DFT takes place. A generalization of this technique is the short-time discrete Fourier transform (STDFT).

Why is leakage present in frequency domain measurements?

Spectral leakage due to FFT is caused by: mismatch between desired tone and chosen frequency resolution, time limiting an observation.