HP 4328A Service Manual

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Sectwn IV
Paragraphs 4-7 to 4-17
4-7 .. OSCILLATOR CffiCUIT
4-8.. Assuming that a sample with a resistance value
Rx is connected between the current terminals only,
a closed current loop is formed bv Reference Resist-
ance Rs of the instrument, Rx and secondary winding
of Output Transformer Tl. A positive feedback loop
is formed as Rs is connected to the mput terminal of
High Input Impedance Amp I ifier, and through It to the
Power Amplifier of the next stage and the Output
Transformer T 1. This loop Will oscillate depending
on the range setting (Rs value). The oscillation fre-
quency, which IS determined by capacity Co and in-
ductance Lo of T1, is 1kHz ±100Hz ..
4-9. The value of Rs is selected by RANGE switch,
to be 500 times full scale RANGE value. Th.is oscil-
lator acts as a constant current source, as viewed
from sample Rx. Also, the voltage VI at both ends of
Reference Resistance Rs is always 100mV peak, thus,
when the value of Rx is equal to full-scale value, the
1kHz voltage applied to Rx wlll be 0. 2mV peak. In
other words, the voltage applied to the sample is of
a verv low level, 0. 2mV peak or lower, at normal
measurement. The input impedance of High Input
Impedance Amplifier is set sufficiently high so that
Rs is unaffected by it.
4-10. Schmitt Trigger Ctrcuit
4-11. When the value of sample Rx is greater than
set range, or specifically, Rs 2 Rx, the loop gain
of this positLve feedback loop becomes 1 or smaller,
so thatoscillation stops .. (f the voltage terminals are
connected to both ends of sample Rx at this time, the
voltage drop at both ends of Rx is zero. A trigger
circuit is added to cause the meter needle -to read
above full-scale whenever the value of Rx IS greater
than the set range. In other words, when the oscil-
lator's power decreases to below a certain level, the
Schmitt Trigger circuit provides a DC voltage to the
DC Amplifier m the input stage of the Meter, causing
it to deflect off-scale,.
4-12. 20mV Peak Limiter
4-13. The 20mV peak Limiter connected to the cur-
rent termmals limits the a. c. voltage applied to the
sample to 20mV peak.. It does this by comparing the
1kHz output level to a reference diode. When a given
level is exceeded, the 1kHz signal is amplified and
rectified and the resultant de level is used to reduce
the oscillator gain
4-14. PHASE SENSITIVE VOLTMETER
4-15. When the current terminals and the voltage
terminals are connected to the sample Rx (It is as-
sumed that the range-setting of 4328A at this time is
correct), the voltage drop at both ends of Rx is applied
to AC Amplifier via Input Transformer T2, where it
is amplified Its output IS connected to the input of
the Phase Detector. The other input of the Phase
Detector is driven by a Flip- Flop driven by 1kHz from
the oscHlator section. That is, the output signal of
the AC Amplifier IS synchronously rectified by a volt-
age which is in-phase with the current flowing in the
Reference Resistance Rs (L e .. current flowing in sam-
ple Rxl. Thus, a sample Zx = Rx + ]Xx, the voltage
4-2
Model 4328A
drop across the sample will be v = VR + jvx The In-
put of the AC Amplifier will also be the voltage drop
vx whose phase differs from vR by 90°. The Phase
Detector detects only the voltage drop vR which is in
phase with the current in Rs and provides DC voltage
output proportional to vR only.. Therefore, no error
is present i.n the meter reading due to Vx, or effec-
tively, the reactance Xx gives rise to no error.
lt
1/x,
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Zx
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Xx
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VR
(a)
(b)
Flip-Flop
(c)
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Vx=O
Figure 4-3. Operation of Phase Detector
4-16. The dynamic ranges of the AC Amplifier and
Phase Detector of the next stage are sufficiently great
that errors due to X11: will be less than 100 as long as
the value of reactance Xx at lkHz does not exceed
twice the range setting This is an important feature,
espectally with reference to AC measurement of low
resistance where1n the series inductance component
in the sample is a sigmftcant part of the total ac im-
pedance.
4-17 DC signal VR which was synchronously recti-
fied by Phase Detector is amplified and appHed to the
Modulator of the next stage This Modulator has two
inputs and one output -an AC signal vR corresponding
to the product of VR (proportional to the resistance
value Rxl and the 1kHz AC signal v3 (which is in phase
with the current ln Rs) The AC output signal vR is
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HP 4328A Specifications

General

Brand	HP
Model	4328A
Category	Measuring Instruments
Language	English