Download Well Controlled Audio Noise Source

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