Two-Step Signal Processing
To review, the digital receiver chip performs two major signal processing operations controlled by two programmable parameters (Figure 17):
- Translation of the input signal down to DC is controlled by setting the tuning frequency of the local oscillator (LO).
- Low pass filtering bandwidth and output sampling rate are both controlled by setting the decimation factor.
Because everything inside the decimating low pass filter is performed with digital circuitry and DSP techniques, there are no undesirable effects normally associated with conventional analog filters. There are no initial component tolerance or temperature variations or aging characteristics. No calibration or preventive maintenance is required. This provides excellent channel-to-channel matching for applications where phase variation between channels is important, such as direction finding.
The FIR digital filters used have linear phase for well-behaved transient response. The filter bandwidth is programmable over a wide range (1000 to 1), with absolutely predictable and uniform response throughout.
Lastly, the signal is tailored precisely for DSP processing by preselecting only the signal of interest through bandlimiting and providing it to the DSP at the optimum sampling rate.
Returning to our overall digital receiver block diagram shown in Figure 18, our output signal is now translated, filtered, and bandlimited and is ready for further processing. Note that the output signal from the decimating low pass filter is still a sampled time signal which could represent any kind of modulated or unmodulated signal.
We could send this signal directly to a D/A converter, producing an analog waveform. For straight single-sideband frequency division multiplexed speech, for example, we could now connect the D/A output to a speaker and listen to the selected voice channel directly.
In many systems, further processing is required, as with modem demodulation for example. Since the output of the digital receiver is now at a much lower sampling rate than the original wideband input signal, this additional modem processing can now be readily handled by a DSP or an FPGA.
Virtually any form of demodulation can be implemented just by loading the DSP or FPGA with the appropriate algorithm (Figure 19). AM can be demodulated with an envelope detector, FM and PM can be demodulated using a phase or frequency discriminator algorithm.
The ability to quickly change the LO allows frequency-agile modulation schemes to be accommodated as well. Analysis functions include energy detection such as required by scanning receivers that may be implemented with an FFT, for example. Other analysis functions include cryptography, identification of transmitters based on transmission frequency, modulation schemes, and other signal characteristics.
