of Microprocessors

BASIC ANALOG MODULES 219
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200 n
Fig. 6--30. Voltage-controlled four-pole low-pass filter with corner peaking
impedance buffers are provided. The gain cells have a predistortion circuit
and therefore can accept input signals as large as 80 mV and still generate less
than 1% distortion. The four gain cells are fed equal control currents from a
built-in multioutput exponential converter. The integrating capacitors are
supplied by the user, however, and should be 1,000 pF or larger. One
limitation to keep in mind is that the buffer amplifiers are only capable of a
I-V peak output swing and can only supply 500 !LA of load current.
Figure 6-30 shows a voltage-controlled filter circuit using the 2040.
This is properly termed a "four-pole low-pass filter with corner peaking" and
is quite popular, although not as much so as the state-variable type described
earlier. The main difference is that the low-pass cutoff of 24 dB/octave is
much sharper than the state-variable cutoff of 12 dB/octave. A bandpass
function is obtained by feeding a portion of the filter's output back to its
input to create a resonance. Enough of this feedback will cause a pure sine
wave oscillation at the center frequency. Note that a true bandpass response
is not produced by the feedback because there is appreciable very-Iowfrequency
gain. This resonance technique is often called corner peaking to
distinguish it from true bandpass filtering, which has zero response at both
frequency extremes. At moderate to high Q settings, however, the audible
difference can be quite subtle.
The filter is really four identical single-pole low-pass sections in cascade
all tracking the same control voltage. Each low-pass section is functionally
equivalent to the VCF in Fig. 6-21D discussed earlier. When cascaded as
shown, each section contributes a cutoff slope of 6 dB/octave; thus, simultaneous
outputs of 6, 12, 18, and 24 dB/octave are available. When the