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1693 RLC Digibridge
4.1 Introduction
4.1.1 General
This instrument uses an unusual method of measure-
ment, which is quite different from those used in most
previous impedance meters or bridges. A thorough
understanding of this method will be helpful in un-
usual applications of the instrument. The following
paragraph gives a brief overall description outlining
the measurement technique to one familiar with
impedance measurement methods. A more detailed
description of operation, specific circuitry, and control
signals is given later.
4.1.2 Brief Description of the 1693
Digibridge
The Digibridge RLC tester uses a patented measure-
ment technique, in which a microprocessor calculates
the desired impedance parameters from a series of 6
or 8 voltage measurements (6 for FAST, 6 for MED,
and 8 for SLOW measurement rates). These measure-
ments include quadrature (90 degree) and inverse (180
degree) vector components of the voltages across the
device under test Zx (the DUT), and across a standard
resistor Rs carrying the same current as the DUT. Each
of these voltage measurements is meaningless by it-
self, because the reference signals have no particular
phase relationship to the measured analog signals,
and because the current through Zx is not controlled.
Each set of voltage measurements is made in rapid
sequence with the same -phase-sensitive detector and
analog-to-digital converter. Therefore properly cho-
sen differences between these measurements subtract
104
Chapter 4
THEORY
out fixed offset errors, and ratios between them cancel
out the value of the common current, the scale factor
of the detector-converter, and the effect of the relative
reference-to-signal phase angle.
The phase-sensitive detector uses 4 reference sig-
nals, precisely 90 degrees apart, that have exactly
the same frequency as the test signal , but whose
phase relationship to any of the analog voltages or
currents (such as the current through Zx and Rs) is
incidental. Therefore, no precise analog phase shifter
or waveform squaring circuit is required. Correct
phase relationships are maintained by generating
test signal and reference signals from the same high-
frequency source.
Because of the measurement technique and circuitry,
the only calibration adjustment in the Digibridge is the
factory setting of the test-voltage-level reference. The
only precision components in this instrument are four
standard resistors and a quartz-crystal stabilized oscil-
lator. There is no reactance standard. For example, C
and D are calculated by the microprocessor from the
set of voltage measurements, the predetermined fre-
quency, and the calibrated R and Q of the applicable
standard resistor.
In these calculations, the microprocessor automati-
cally removes from the measured result the param-
eters of the test connection ("stray• capacitance and
conductance and series resistance and inductance), if
simple open circuit and short-circuit "ZERO" calibra-
tion measurements have been performed by the opera-
Theory
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