Model 89038
PRINCIPLES
OF
OPERATION---SERVICE SHEET
4
Service
SERVICE SHEET 4---A3 Notch Filter Assembly (Tune and Balance Circuits)
PRINCIPLES
OF
OPERATION
General
This portion
of
the Notch
Filter
Assembly
(A3)
contains the notch balance and tune circuitry that improves
the depth and tuning accuracy of the notch.
Chopper Circuits
The Tune Comparator and Chopper are driven by the signal from Integrator
2
(see Service Sheet
3).
This signal
is in quadrature with the fhdamental of the input
to
the Notch Filter. The Balance Comparator and Chopper
are driven by the signal from Integrator
1.
This signal is in phase with the fundamental of the input
to
the
Notch Filter. The Tune and Balance Comparators drive the chopper FETs (44 and Q5)
so
that they switch
at the zero crossings of the comparator inputs. When the outputs of the comparators go high
(for
example,
when their open-collector outputs shut
off),
the FETs switch on (short circuit) because the gates are grounded
through R46
or
R48. When the comparators go low, putting a large negative voltage at the FET gates, the FETs
switch
off.
The
current from the Tune
(or
Balance) Chopper and the Inverting Amplifier are summed together at the
inverting
(-)
input of the Tune (or Balance) Integrator. The chopper current is weighted by a factor of two
because the path resistance
from
the chopper is about half of the path resistance from the Inverting Amplifier.
When Gain Switch
42
is
ofT,
the Tune Chopper’s path resistance is R49 in series with R54; the Inverting
Amplifier’s path resistance is R6O. When
42
is switched on, the current in both paths increases
by
about a
factor of ten. R55 is now in parallel with R54, and R58 with R60. This increases the Tune Integrator gain
by
a factor of ten and compensates for a
20
dB reduction in attenuation when Attenuator
1
is switched out (see
Service Sheet 5). The action of the Balance Chopper
is
similar to the Tune Chopper.
Integrators and Multipliers
The
sum
current from the Tune Chopper and Inverting Amplifier is integrated by the Tune Integrator. The
dc
value of the sum current represents the tuning error
of
the Notch Filter. The integrator amplifies and filters the
error current and drives the Tune Multiplier. The integration capacitors are switched for different frequency
ranges
to
compensate for the resultant gain change with frequency. When the Notch Filter range is changed,
FET Q1 is momentarily switched on
to
discharge the integration capacitor and speed up the tuning correction.
Operation of the Balance Integrator is similar
to
the Tune Integrator. The tune and balance offset adjustments
(R62 and R63) compensate for dc offsets in the filter to maximize the depth of the notch.
The Tune Multiplier receives a dc control input (the
Y
input) from the Tune Integrator and an ac input (the
X
input) from Integrator
1
(see Service Sheet 3). The sensitivity of the
X
input is
set
by R77. the
Y
input
by
R81,
and the overall multiplier by R75 and R79. The output
of
the Tune Multiplier is a current which is summed
directly with the current from the Balance Multiplier and applied
to
Sum
Amplifier
2
(see Service Sheet 3). The
dc level into the Tune Multiplier is also measured by the DC Voltmeter
to
give the Controller an indication of
whether
or
not the Notch Filter is properly tuned.
8B-35
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