FFT Bit Templating – A Technique for Making Amplitude and Frequency Measurements of a BPSK Modulated Signal

Abstract

In many spacecraft receiver applications, the Fast Fourier Transform (FFT) provides a powerful tool for measuring the amplitude and frequency of an unmodulated RF signal. By increasing the FFT acquisition time, tiny signals can be coaxed from the noise and their frequency measured by determining which frequency bin the signal energy appears. The greater the acquisition time, the narrower the bin bandwidth and the more accurate the frequency measurement. In modern satellite operations it is often desirable for the receiver to measure the frequency of a carrier which is modulated with BPSK data. The presence of the BPSK data limits the FFT acquisition time since the signal may switch polarities a number of times while the FFT samples are being acquired. This polarity switching spreads the signal energy into multiple frequency bins making frequency measurement difficult or impossible. The Bit Templating Technique, used for the first time in the CMC Electronics Cincinnati TDRSS / BPSK Spacecraft Receiver, collects the modulated waveform energy back into a signal bin so that accurate amplitude and frequency information can be calculated.