Model 8903B Service
of Attenuator
1
and Amplifier
1
of the Output Amplifierfloltrneter Assembly (A4) and the Notch Amplifier. The
total
gain is either
5.5
or 55 depending on the setting of Attenuator
1.
The input
(VNOTCH)
can be considered
as having components in phase with the input to the Notch
Filter
and in quadrature (that is,
90"
out-of-phase)
with
it.
The waveform for
VNOTCH
in Figure 8B-2 is shown
as
being totally in phase with the input.
The in-phase component represents a balance error in the Notch Filter, that is, the notch does not completely
null out the signal. The quadrature component represents a tuning error. When perfectly tuned, any output from
an imperfectly balanced Notch Filter will be in phase with the input.
To sense the in-phase component, a current, produced by the input signal (VNOTCH), is chopped in phase with
the input
to
the Notch Filter. Chopper
Si
is
driven by the bandpass output of the Notch Filter which is in phase
with the fundamental component of the input
to
the Notch Filter. If
VNOTCH
is in phase with
Si
(as
in this
example), the chopped current (Ili) will be a half-wave-rectified signal. Current I2 is proportional to the input
voltage but half the level of the chopped current.
Ili
and
I2
are summed together to produce a current
(I3i)
whose
average (dc) level represents the level error
of
the in-phase component of the signal. For this example, the result
is
a full-wave-rectified signal
(I3i).
ISi
is
integrated (that is, amplified and filtered). The integrated current controls
the gain of the Balance Multiplier which balances the Notch Filter.
Sensing the quadrature component is similar.
A
current, produced by the input signal
(VNOTCH),
is
chopped in
quadrature with the input to the Notch Filter. Chopper
S,
is driven by the low-pass output of the Notch Filter
which is
90"
out of phase with the fundamental
of
the input
to
the Notch Filter.
If
VNOTCH
is in quadrature
with
S,
(as
in this example), the chopped current
(I1,)
will consist only of the half cycles going from the positive
to
the negative peak. (The dc component of this signal
is
zero.) Current I2 is
as
before.
11,
and I2 are summed
together
to
produce a current
(I3,
)
whose average (dc) level represents the level error of the quadrature component
of the signal. For this example, the result has a dc value of zero.
Is,
is
integrated and controls the gain of the
Tune Multiplier which tunes the Notch Filter.
More commonly,
VNOTCH
will have both quadrature and in-phase components. The tune and balance detectors
isolate the respective components and drive the respective multiplier with the error signal. The resistors through
which
Ili,
I,,,
and
I2
flow can be switched to change the magnitude of the current by a factor of ten. This
compensates for the gain change in Attenuator
1
of assembly A4.
In summary, the Tune and Balance Multipliers can be visualized
as
variable-gain amplifiers with ac inputs from
the bandpass and high-pass outputs of the Notch Filter respectively. (The bandpass output
is
in phase with the
fundamental of the input
to
the Notch Filter; the high-pass output
is
in quadrature.) The dc inputs (from the
Tune and Balance Integrators) control the gain of the multipliers, including the sense of the gain (that
is,
inverting
or
non-inverting). The output currents of the multipliers are summed together (their outputs are in parallel) and
the composite
signal
is
summed with the output
from
Integrator
1
in Sum Amplifier
2
to
create a feedback loop
that fine adjusts the tuning and balance of the Notch Filter.
Output Amplifier/Voltmeter
(A4)
The signal from the Notch Filter is further processed before being detected by the voltmeter or sent the the
rear-panel MONITOR output. The signal
is
amplified by Amplifier
1
(gain=5), the Low-Pass Filters (gain=2),
Amplifier
3
(gain=lO), and Amplifier 4 (gain=lO). The maximum gain
is
thus
1000
(or
60
dB).
Attenuators
1,
2,
and 3 each have a selectable attenuation of
0
or 20
dl3.
The overall gain
is
set by the Controller
so
that the
level sensed by the Output RMS/Average/Quasi-peak Detector is
3V
or
less. Switching of the attenuators can be
controlled by Special hnction
3
and is summarized in Table
8B-3.
Selection
of
the active low-pass filters
is
from the front panel
or
HP-IB. When no filtering
is
selected, the 750
kHz filter
is
active. The other
two
filters are
80
and
30
kHz.
The Buffer amplifier has
0
dB
gain. The High-Pass Filters following the Buffer are either 0.3 Hz in ac level
or
13
Hz
in the other measurement modes. The 13
Hz
filter is
an
active filter. Amplifier 4 drives the MONITOR output,
the Output RMS/Average/Quasi-peak Detector, and the Counter Input Schmitt Trigger (see Service Sheet
BD4).
The Output RMS/Average/Quasi-peak Detector converts its input into a dc voltage corresponding
to
the
rms
level of the signal. The detector can also be configured to produce the absolute-average
or
quasi-peak value
of
the signal using Special Function
5.
The output
is
measured by the
DC
Voltmeter for computing measurement
rev.20JUN87
8B-3
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