CES CEM3394, Quick Start Manual

Page: 5 / 7

Page 5
VCF
Able to sweep over a minimum of 14 octaves, the filter is the classical wide range 4-pole low-pass
type designed for musical instruments.
The control scale has twice the sensitivity as that for the VCO: it is 3/8V per octave. The first 3
stages of the filter are unity gain transconductors while the last stage provides a gain of 57 (and
consequently has a transconductance of 1/57 of the first three). This requires three equal capacitor
values, and the fourth is 1/57
th
of the other value.
The frequency of the -12dB point on the cut-off slope is given by:
Pzcv = Gm/(2 x pi x Ceq) = (Iref)/(4 x pi x Ceq x VT)
where VT = KT/q = 26mV at 20 C. The reference current is set up internally an given a +3300 ppm
to compensate for the VT -3300ppm tempco. Thus this equation can be reduced to:
Pzcv = 4.3 x 10E-5/Ceq
Note that due to internal resistor tolerances, the 4.3E-5 term can vary 40% part-to-part.
Selection of the 4 filter capacitors is again determined by the maximum desired cut-off frequency
and optimum operating range of the OTAs. The range has been optimized for an operating range
of 5 umho to 5 mmho. Thus we can calculate C as follows:
C = 5E-3/(2 x pi x Fmax)
As an example, assume the highest cut-off frequency is 24Khz. C1-C3 becomes 33nF and C4 is
(33nF/57) = 580pF. The frequency at zero control volts is approximately 1300Hz; 24Khz will be
reached in 4.2 octaves, or at -1.6V. The filter can be opened up to >40Khz using these values, but
CV feedthrough becomes excessive.
The resonance VCA feedback circuit has been designed so that as the resonance is increased the
apparent loudness remains constant, providing a much richer resonant sound. Note, however, that
the peak-to-peak level of a pule/square wave actually doubles as resonance is increased, due to the
ringing on its fast edge transitions.
VCF MODULATION
The modulation VCA allows the VCO triangle wave to modulate the reference current of the VCF,
and hence the cut-off frequency. The Modulation Amount control voltage (Pin 5) controls the
‘depth’: at maximum setting the VCF is swept from a very low value to twice the unmodulated
frequency. Since the modulation is linear, the apparent filter frequency does not shift as
modulation is increased.
One application of this control is to set the filter into oscillation for obtaining linear audio FM of
one VCO by another (using the VCF as a VCO). However, an equally interesting application is to
audio FM the filter while it is filtering normally. The result is strong timbral effects, especially
with some resonance added.