| |  | Author | Message | | | eric | | Posted: Mon Oct 09, 2006 11:55 pm Post subject: Programmable state-variable active filter | | | | Site Admin Joined: 01 Jan 1970 Posts: 311 | Design: H. Bonekamp
Dimensioning and testing active filters for audio and measurement
applications is not seriously good fun, in fact, many hobbyists seem
to avoid the subject altogether just because of the complexity of the
calculations involved. And indeed, there are so many parameters to
observe in active filter design (slope, phase response, high-pass, lowpass,
band-pass or stop pass, etc.) that only a select few are interested.
The project described here alleviates some of the above problems by
joining the forces of advanced hardware (a 4-channel digitally controlled
variable resistor chip) and some clever computer software (a
program written in C). The upshot is that you, the user, define the
filter you want, and the hardware/software combination does the rest.
The hardware consists of a basic active filter comprising four
opamps. The resistive elements in the filter (which determine the
filter response) are electronically controlled potentiometers contained
in IC1, a 4-channel ‘TRIMDAC’ type ADC8403. For example,
one such potentiometer (with a value of 10 kΩ typ.) is avaialble
on pins A2, W2 and B2 of the ADC8403, where W2 is the
wiper. By means of suitable control signals received from the PC’s
printer port, this pot can be programmed to take on 1 of 256 values,
i.e., the resolution is 8 bits. This particular pot controls the signal
level applied to opamp IC2a.
After a reset pulse on the RS terminal, the wipers of the four electronic
pots inside the ADC8403 are set to ‘mid travel’. The chip
reads its control information via 3 input terminals: CS (chip select),
SDI (serial data in) and SCLK (serial clock). Internally, the SDI
signal (data) is applied to a serial-to-parallel shift register. The data
consists of 10 bits. The first two bits select the desired DAC (1 of 4),
and the next 8 bits, the value for the relevant DAC. The SDO (serial
data out) pin allows two or more ADC8403’s to be cascaded.
The filter has one input and four outputs: HP (high-pass), LP (lowpass),
BP (band-pass) and BR (band-reject). The relation between
the filter type and the associated frequency band is summarized in
the Table. The highest and lowest frequencies may be lowered by
a factor
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