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Transcript
Well Controlled Audio Noise Source Dennis Seguine Member of Technical Staff Cypress Semiconductor A white noise source can be constructed from a zener diode or reverse-biased base-emitter junction, then amplified up to a useful range. These circuits work but they are temperature sensitive and not predictably calibrated. Another solution is to use a pseudo-random sequence (PRS) generator, which when filtered yields a noise characteristic that is flat in frequency, Gaussian in amplitude distribution, and as stable in amplitude as the reference provided. The noise source, including PRS, filter and driver can be implemented entirely within a PSoC1 device as shown in Figure 1. The PRS is a modular linear feedback shift register (LFSR) with the output fed back to specific taps XORed with the data stream. The tap values are available from a variety of sources, including the PSoC device's own design software. The PRS in the PSoC is a modular design, available in bit lengths from 2 to 32. The bitstream output is a digital signal which is converted to an analog noise with a filter. A sync pulse is available triggering once per sequence as shown in Figure 2. The bitstream output of the PRS repeats every 2^n-1 clock pulses. For a 1.0 MHz clock, a 16-bit PRS repeats its pattern every 65 msec. If the system is slow and repeating patterns might interfere, just use a longer sequence. A 32-bit sequence yields a repeat pattern at 1.19 hours. The bitstream digital output can connect directly to the input of an on-chip switched-capacitor low-pass filter. This yields a signal that is proportional to the supply voltage. A supply independent source can be generated by setting the filter input to a reference, then connecting the PRS bitstream to the filter's modulator input. This multiplies the reference fed to the filter by +1 or -1 and gives an amplitude stable source. The analog noise output and the bit-stream are sync'd as shown in waveforms in Figure 2. The bitstream amplitude is constant and doesn't look very Gaussian because it's always driven to the rails. Making the signal Gaussian is a matter of limiting the bandwidth and keeping the output from hitting the rails. This can be accomplished by setting the filter corner frequency at less than 5% of the PRS clock rate. This implementation is an white noise generator with a bandwidth of 20 kHz, sampled at 1.0 MHz. The noise spectra are shown in Figure 3. The PRS noise (red trace) has a sin(x)/x shape with the first null at the sample rate. The filtered output noise (blue trace) is as flat in the band as the 20 kHz filter allows. The finished noise source has an observed noise output of about 1.5 Vp-p and measured amplitude of 275 mVRMS. Small clock spurs occur at some sub-multiples of the clock rate but they are easily filtered out with an external single-pole low-pass R-C filter. Higher corner frequencies can be realized by reducing the over-sample ratio and increasing the clock rates. The practical corner frequency limits for a PSoC lowpass filter are 300 Hz to 120 kHz. Figure 1 4.0 MHz Pseudo-random Noise Generator Block Diagram ÷4 1.0 MHz Vref Figure 2 Sync 16-bit PRS PRS BitSream 20 kHz Low-pass Filter Pseudo-random Noise Generator Waveforms Sync PRS Bitstream Out Analog Noise Out Pseudo-random Noise 20 kHz Figure 3 Pseudo-random Noise Generator Specta dBV in 100 Hz band 0 -10 -20 20 kHz Filtered 16-bit PRS at 1.0 MHz -30 -40 -50 -60 -70 -80 -90 1 10 100 kHz 1000 10000