HP 4328A Service Manual

--
Model 4328A
Section IV
Paragraphs 4-1 to 4-6
SECTION IV
THEORY OF OPERATION
4-1. INTRODUCTION
4-2. The measuring princtple of Model 4328A is, as
shown in Figure 4-1, the so -called 4-terminal method,
where constant current i Is applied to a sample Rx,
and the voltage drop VR is measured. If 1 IS known,
andresistance valueRx maybe calibrated linearly on
a
voltmeter scale. By changing i, range changing is
possible
r.c. r,.
r;,
R)) Rx
Flgure 4-1. Basic Low Resistance Measurement
4-3.. This method requires a total of 4 terminals to
be
connected to the ends of sample Rx, L e , one volt-
age and one current terminal on each end. The set of
HIGH INPliT
r-------1 IMPEDANCE
AMP
20mV
PEA~
LIMirTER
POWER
AMP
·f111
current terminals applies constant current 1 to sample
Rx, the voltage terminals detect the voltage drop
across Rx. Using this method, the resistance of the
lead wires, and the contact resistance between ter-
minals and sample will not constitute any error. The
effect of the resistance of voltage terminals rv is re-
moved by making the internal resistance of the volt-
meter (MV) sufficiently greater than rv. Resistance
rt of the current terminals can also be ignored, as it
is normally sufficiently less than the internal resist-
ance R of constant current source. In Model 4328A,
the internal impedance as seen from voltage terminals
is set at 5k0 or higher, and the internal resistance
of constant current source is set at RANGE x500.
4-4. SIMPLIFIED FUNCTIONAL DESCRIPTION
4-5. Functionally vtewed, Model 4328A may be di-
vided into two major sections. One is the 1kHz oscil-
lator sectlon serving as constant current source; and
the other is a voltmeter section of the phase-dis-
criminator type amplifying the voltage drop across
the sample, synchronously rectifying tt and deflecting
the ohm-calibrated meter.
4-6 The following is a detailed description of opera-
tionof the 4328A refering to Figure 4-2, Model 4328A
Block Diagram.
lli•Rx+JXr
V •V"R+V'x.
----Curr
Tip
v.
MODULATOR
DC
AMP
PHASE
DETECTOR
FLIP-FLOP
SCHMITT
--,
I
I
L---------------------------------------------------~~DETECTOR
__ _J
TRIGGER
Figure 4-2 .. Simplified Block Diagram
4-1

HP 4328A Specifications

General

Brand	HP
Model	4328A
Category	Measuring Instruments
Language	English