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HEWLETT' PACKARD
JOURNAL
T E C H N I C A L I N F O R M A T I O N F R O M T H E - h p - L A B O R A T O R I E S
VOL. 3 No. 11
PUBLISHED BY THE HEWLETT-PACKARD COMPANY, 395 PAGE MILL ROAD, PALO ALTO, CALIFORNIA JULY, 1952
A Novel Impedance -Measuring System Using
Standard -hp- Instruments
WHEN an impedance is fed from a con will give even wider ranges of measurements
stant-current source, the voltage than those listed above.
across the impedance is a function of the The basic set-up for the measurements is
magnitude of the impedance. indicated in Fig. 1. The -hp- Model 200
This basic relation will permit a combina audio oscillator is converted to a constant-
tion of two standard -hp- instruments to be current source by insertion of a 1 -megohm
used to measure extremely wide ranges of resistance in series with the generator output
L and C as well as moderate ranges of R and terminals. The voltage across an unknown
Z. C's from 800 microfarads to a few micro- impedance connected into the circuit will
microfarads, L's from 200 henrys to 100 mi- then be proportional to the magnitude of
crohenrys, and R'sandZ's from 30,000 ohms the unknown impedance.
to a few ohms can be measured. A feature As part of the set-up procedure, the oscil
of the system is that, in general, the meas lator output voltage is adjusted by the panel
ured values can be read directly. controls to be 10 volts. Since the circuit im
The two instruments required for the pedance is 1 megohm, a generator voltage
measurements are an oscillator and a sensi of 10 volts will cause a constant current of
tive voltmeter. Several different combina 10.0 microamperes to flow through the un
tions of -hp- oscillators and voltmeters can known as long as it is small compared to 1
be used. Probably the combination most megohm. The significance of achieving a
commonly available is one of the -hp- 200 constant current of 10.0 microamperes is that
series audio oscillators used with the -hp- this value of current will permit R, Z and L
Model 400C VTVM or Model 330 Distor to be read directly on the -hp- voltmeter.
tion Analyzer. For these measurements the How this direct-reading feature occurs
Model 330 Distortion Analyzer is used as a can be described by considering the expres
sensitive voltmeter having a maximum sensi sion for the voltage across the unknown. If
tivity of 300 microvolts full scale. Other pos the constant current is 10 microamperes, the
sible combinations of instruments are dis voltage across the unknown will be
cussed later. Some of these combinations E| = = IZ=(10x 10'6) (Z) ==Zx 10'5
If the unknown is, say, 3,000 ohms, the
voltage across the unknown will be 0.03
volt. When the voltmeter range switch is
positioned to the 0.03 volt range, the volt
meter pointer will be deflected to read "3."
If the unknown is changed to, say, 1,500
ohms, the voltage across the unknown will
Fig. 1. Basic set-up for measuring impedances with
constant-current method. be 0.015 volt. The voltmeter pointer will
P R I N T E D I N U . S . A . C O P Y R I G H T 1 9 5 2 H E W L E T T - P A C K A R D C O .