Simplified Metering Circuit - HP 333A Operating And Service Manual

Model 333A/334A
Section IV
becomes abnormally large during anoverload, A2CR8
breaks down and provides a lower resistance charg-
ing path for A2C15, which reduces the transient re-
covery time o f the meter amplifier. Negative ac feed-
back is applied from the collector circuit of A2Q9 to
the emitter circuit of A2Q5. This feedback i s used
to
ensure f l a t frequency response, to improve linear-
ity,
and
to reduce the effect of variationof transistor
parameters with environmental changes.
In this
manner, the calibration of the instrument is made
dependent on high quality passive components.
4-47. METER RECTIFIER CIRCUIT.
4-48. The meter rectifier is connected in a
bridge
type configuration with
a
diode in each upper branch
and
a dc
milliammeter connectedacross the midpoints
of the bridge. The simplified meter rectifier i s illus-
trated in Figure 4-8.
The generator represented by
A2Q5 through A2Q9 with the internal impedance
%
provides the meter, M l , with current for full scale
deflection and develops
a
voltage across the calibra-
tion network, which closes the ac feedback loop.
Capacitors A2C27 and A2C28 are used
as
coupling
capacitors for the ac feedback loop, o u t w signal to
the OUTPUT connector, and the bridge error
signal
to the input of the automatic fine tuning loops. The
mechanical inertia of the meter and A2C26 prevents
the meter from responding to individual current
pulses, Therefore, the meter indication corresponds
to the average value of the current pulses rather than
the peak value. The meter is calibrated to indicate
the r m s value uf a sine wave. Resistor A2R45 im-
presses
a
fixed bias across diodes A2CR6 and A2CR7
(biasingthem close to the barrier voltage) to makethe
meter amplifier response linear to large variations
in signal amplitude. The linearity of this type uf cir-
cuit i s also increased by including the meter circuit
in the overall feedback loop.
4-49. POWER SUPPLY CIRCUIT.
(Refer to Figure 7-6)
4-50. The power supply circuit consists of a +25 volt
series regulated supply anda -25 volt series regulated
supply which i s the reference supply for the +25 volt
SUPPlY
4-51. The -25 volt regulated supply is oftheconven-
tional aeriesregulator type. The amplifier A1Q5
is
used to increase the loop gain o f the circuit, thus im-
proving voltage regulation. The positive feedback
applied to the junction of AlR11 and AlR12 i s used to
further improve the line frequency suppression of the
circuit.
4-52. The +25 volt regulated supply i s of the conven-
tional series regulator type and operates the same as
the -25 volt regulated supply.
4-53. RF DETECTOR
CIRCW".
(334A
only)
(Refer t o Figure 7-2)
4-54. The RF detector circuit consists of
a
rectifier,
A4CR1, and filter circuit. The
RF
signal is
applied
to the circuit through the RF INPUT connector onthe
rear panel, The rectifier diode A4CR1 recovers the
modulating signal from the RF carrier and the filter
circuit removes
any
RF components before the signal
ia applied to the impedance converter circuit through
the NORM-RF DET switch. SI.
Figure 4-8. Simplified Metering Circuit
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