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107
1693 RLC Digibridge
Theory
4.2 Principal Functions
4.2.1 Elementary Measurement
Circuit
The measurement technique is illustrated in Figure
4-2 by the accompanying simplified diagram, which
can be correlated with the previous (block) diagram.
A sine-wave generator drives current Ix through the
DDT Zx and standard resistor Rs in series. Two dif-
ferential amplifiers with the same gain K produce
voltages e1 and e2. Simple algebra, some of which
is shown in the figure, leads to the expression for the
“unknown” impedance:
Zx = Rs [e(1)/e(2)J
Notice that this ratio is complex. Two values (such
as C and D or L and Q) are automatically calculated
by the microprocessor from Zx, frequency, and other
information.
4.2.2 Frequency and Time Source
A necessary standard for accuracy is the frequency
of the test signal; and equally important are the gen-
eration of multi-phase references for detection and
clocks for the microprocessor. Frequency and timing
requirements are implemented by derivation from a
single very accurate oscillator, operating at 38.4 MHz.
Digital dividers and logic circuitry provide the many
clocks and triggers , as well as driving the sine-wave
generator described below.
Figure 4-3 shows several clocks and synchronizing
pulses as well as the measurement signal f are derived
from the accurate time-base signal.
4.2.3 Sine-Wave Generation
Source of the Test Signal
. Starting with a digital signal
at 64 times the selected test frequency, the sine wave
generator provides the test signal that drives a small
but essential current through the DUT. The sine wave
is generated as follows.
Binary dividers count down from 64 I, providing sig-
nals at 32 f, 16 f, ... 2f, f. This set of signals is used to
address a read-only memory which contains a 64-step
approximation to a sine function. The RO:-.1 output
(as an 8-bit binary number) is converted by a D/A
converter to a stepped approximation of a sine-wave,
which is then smoothed by filtering before its use in
the measurement of a DUT. The filter is switched ap-
propriately, according to the selected test frequency.
Source of the Reference Sine Wave for the Multiplying
Detector . Another sine-function ROM is addressed by
the same digital signals (64 f through f) to produce
another stepped approximation of a sine wave at 0
degrees. Suitable inversions of signals 2f and/or f
serve to shift the phase of the output sine wave, under
microprocessor control, by 90, 180, or 270 degrees.
Figure 4-4 shows how given square waves at fre-
quencies of 64 f, 32 f, 16 t, 8 f, 4 f, 2 f, and I, a ROM
containing the mathematical sine function drives a
D/A converter to form a finely stepped approxima-
tion to a sine wave at frequency f. The filter provides
smoothing of the test signal.