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White Noise Generator
As seen in Figure 3 , the white noise generator was built from two of the functional blocks within the PSoC chip. The first block is a 24-bit Pseudo Random Sequence generator which is clocked at 40Khz. Each clock also generates an interrupt where the lower six bits of data from the PRS is transferred to the second block, a 6-bit DAC. Since a 24-bit PRS can produce a total of 224-1 unique numbers before repeating, the sequence will run for (224-1)/40000 seconds or about seven minutes before repeating.

• Figure 3 - White Noise Generator Block Diagram

The noise generated by this system is not true gaussian white noise (GWN) because the amplitude of the signal is uniformly distributed within a finite range of 0 to 5 volts and the bandwidth is limited to 20Khz. In fact, if you look at the signal with an oscilloscope set to high sweep rate, you would see a waveform that looks like Figure 4 .

• Figure 4 - Magnified Noise Signal

If the sweep rate is reduced however, the signal on the oscilloscope will look like Figure 5 , which looks (and sounds) a lot like white noise.

• Figure 5 - Pseudo Random White Noise

Figure 6 shows a 4096 point FFT of the generated noise signal. Here, we can see that spectrally, our synthetic noise does look like GWN out to 20Khz.

• Figure 6 - Frequency Response of PRN White Noise Generator

Spectral Analyzer
Figure 7 shows the block diagram of the spectral analyzer circuit. The items outlined in black are hardware functions implemented within the PSoC chip while those in blue are outside the chip. Red functional blocks are implemented in software.

The process begins with the white noise for the system under test being detected by the microphone which is amplified by the MIC AMP (Block ACA00). This amplifier provides a gain of 16 and acts as a buffer. The amplified microphone signal is then routed outside the chip to port P03. This signal is then connected directly to input port P06 and to the input of a 2Khz low pass filter. The output of the LPF is fed into port P04. Pot inputs P04 and P06 are muxed into the Intermediate amplifier.

The 2Khz low pass filter is used for the lower four band pass filter combinations. The reason for this is because the two lowest frequency band pass filter configurations (110 and 220Hz) sample at 11788 and 23715Hz respectively. This translates into Nyquist frequencies of 5894 and 11858Hz. Using a 2Kz low pass filter blocks error causing high frequency signals from reaching the BPF for these lower frequencies. Although not needed for the 440Hz and 880Hz filters, the filter is left in place to simplify programming.

• Figure 7 - Block Diagram of Spectral Analyzer (Software Functions are Red and External Hardware is Blue)

The Intermediate Amp adds an additional gain of two and passes the signal on to the programmable bandpass filter. The BPF is a fourth order Butterworth design based on the values derived by the BPF4_design spreadsheet provided by Cypress Micro. Each of the eight filter combinations has a gain of 0dB and a Q of 1.414. The characteristics for each filter can be found in Table 1