Model 8903B
PRINCIPLES OF OPERATION---SERVICE SHEET
7
Service
SERVICE SHEET 7---A4 Output Amplifier/Voltmeter Assembly (Ripple Filters, Voltmeter
Input Selector, and Voltage-to-Time Converter)
PRINCIPLES
OF
OPERATION
General
This portion of the Output Amplifier/Voltmeter (A4) contains the Voltmeter Input Selector, the Voltage-to-
Time Converter, and several input ripple filters.
Ripple Filters
Several of the dc inputs to the Voltage-to-Time Converter are filtered to remove ripple and noise. This gives
a
smoother-reading display. The line from the Differential-to-Single-Ended Amplifier (see Service Sheet l), from
which dc
level
is read in the dc
level
measurement mode, is filtered by the passive filter consisting
of
R118, C65,
and C66. CR23 and CR24 are needed to prevent reverse voltages from damaging the electrolytic capacitors,
C65 and C66, since the input voltage can be
of
either polarity. Passive filter R116 and C67 heavily filters the
line from the Output RMS/Average Detector in the SINAD measurement.
The Ripple Filter, consisting
of
UlOC and associated components, provides a complex impedance across the
line from the Output RMS/Average Detector and gives two poles
of
high-frequency rolloff. The output
is
buffered by U
1.
A similar Ripple Filter (U 10D and associated components) filters the line from the Input RMS
or
Average Detector. It is
a
more complex filter, having an extra real
pole.
The output is buffered by UIOA.
Diodes CR17 through CR22 protect the inputs to the Voltmeter Input Selector against high or reverse voltages.
Voltmeter Input Selector
Multiplexer U18 forms an eight-pole, single-throw switch. The multiplexer is always enabled by the high on the
EN input. The Detector Select Switches (U9B and U21C) selects between the Input Range Detector and the
Input RMS/Average Detector.
To
change a switch setting, the Controller issues the appropriate code
to
the
Instrument
BUS.
The code is decoded and latched on the Latch Assembly (see Service Sheet
12)
which puts the
appropriate logic level on lines VO SEL
0,
1,
and 2. For a discussion
of
Instrument BUS control, see Service
Sheet BD4.
Voltage-to-Time Converter
The dc voltage at the DC Voltmeter input is converted to a pulse, with a duration linearly related
to
the
magnitude of the voltage, by the Voltage-to-Time Converter. The pulse length is then measured by the Counter
(see Service Sheet
14).
The
converter consists of the Comparator, Ramp Generator, and Reference.
The Reference supplies a voltage
of
controlled temperature co-efficient
to
the input to the Ramp Generator.
The basic reference is the temperature-stable, reference diode VR4. The reference is fed from the current
source Q 10, which itself is temperature stable because its base-emitter junction and its reference, VR3, have
similar thermal behavior. The negative reference supplies current to the inverting
(-)
input of U15 through
R125 and R127. CR25, R123, and R124 add a slight temperature coefficient
to
the current to cancel the effect
of the temperature coefficient of C70. The DC Voltmeter sensitivity is adjusted by means of R125.
U15 (with C70) integrates the negative input current to produce an increasing ramp. The ramp is generated
only when Q9 is
off
(for example, when RAMP GATE
(L)
is low). This is initiated by the Controller. When
Q9
is
on, the voltage across VR5 appears across C70. This clamps the output
of
U15 at approximately -6.2V.
The current
for
Q9 is supplied
by
current source
412.
When Q9 goes
off,
the ramp begins, starting at -6.2V.
The ramp begins when the RAMP
GATE
(L)
line goes
low.
The output
of
comparator U20 at this time is high
because the voltage at its non-inverting
(+)
input is higher than the inverting
(-)
input. The output of U20
is inverted
by
FET Q8. The Counter now begins clocking the duration
of
the ramp. When the ramp reaches
the voltage at the non-inverting input, the output of U20 goes low and (via
QS)
inhibits the clocking
of
the
Counter. R131 and R136 add a small amount
of
hysteresis to the Comparator to assure a complete and rapid
transition
of
the output once it begins to change.
8B-49
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