Other Uses Of The Resonance Control Cell; Pole Frequency Control Voltage Rejection - Sequential Pro One Technical Manual

Control of the transconduc-
,
tance 1s accompl~shed with
a
current input. As the control in-
put
I S
a
low impedance summing
node
a t a
potential near ground,
the control current may be
derived from the resonance con-
zrol vol rage with an input re-
sistor, R R c , team~nated at pin 9.
This res~stor should be selected
so
that the maximum available
resonance control voltage pro-
duces the maximum desired
control current.
F~gwre
6
shows a graph of
the transconductance versus
control current. As can b e seen,
the slope of t h e curve becomes
more gradual as the control
current increases. This feature
allows the resonance to be con-
trolled with finer resolution as
the critical point o f osc~llat~on
i s approached.
The maximum control
current is therefore selected in
accordance with the amount of
control sensitivity which is
desired at the top of
the
control
range. The value of the input
resistor, RR
1,
is then selected
depending on where in the con-
trol scale osc~llation is desired
to
beg~n (when the control
voltage is 90% of the maxlrnurn
value, for ~nstance). The follow=
ing formula may be used:
where GrnOsc is the transcon-
ductance corresponding t o the
control current at which oscilla-
tion is desired
TO
beg~n; and
where AQSC i s the overall gain
from the resonance signal input
resistor, RRI,
TO
the filter output
required to sustain oscillation.
If the gain of stages 2, 3 and 4
are unity, then AOSC
=
1266 or
4 rn the case of the low pass
filter.
Wh~le/operatrng the filter in
t h e resqnant mode. care should
b e takgn not to 0verload the
input to the filter. If the si g naE
output of stage one is allowed to
become clipped, then not only
w1l1 the apparent resonance
of the signal
a t
the filter output
appear to be reduced, but the
D.C. level of the output signal
will shift,
When the resonance control
is advanced until sustained
osc~llatians are produced, ad-
vancing the resonance control
further will merely increase the
amplitude of the osc~llat~on. A
lesser effect is the shift of the
oscillation frequency. For
rnin~murn shift (typ~cally less
than
0.5%). the oscillation
amplitude should be kept below
the clipping level o f the first
stage output. Allowing the
osc~llatinn t o be clipped will
produce frequency sh~fts in
excess of 5%.
Other Uses of the
Resonance Control Cell
Other than controlling the
resonance, the variable trans-
conductance amplifier may be
used as an independent VCA
controlling the amplitude of the
input signal to the filter.
Or
the
cell may be set up
as
a sym-
metrical lirniter/clipper for
either preventing large dynamic
input
signafs
from overloading
the filter or for providing addi-
tional coloration
to
the input
signal.
Pole
Frequency Control
Voltage Rejection
The Q.C. voltage shift at the
filter output due to the fre-
quency control voltage
may
be
mlnirnized by adjusting the
current into the minus supply
pin, pin
73.
This i s accomplished
b y replacing the negative supply
current limiting resistor.
RE=,
L
T
F l C U R E 4 A L L PASS FILTER WlTH V C. RESORANCE
with a series resistor and trim
pot. The
fixed
resistor, RE, and
series trim pot, RT, should be
selected
so
that the current
into pin 13 may
be adjusted
from 5mA
trr
1 TmA. Or:
FIGURE 5. 5TP.f E V I I R I A I L E
FILTER WlTH V.C. "U"