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10.2 Synchronous Detection System
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0.2 Synchronous Detection System
The figure below shows an equivalent circuit for a battery. If the
measured object exhibits other electrical characteristics in addition to
resistance, as shown in this figure, we can use the synchronous detection
system to obtain the effective resistance of the object. This synchronous
detection system is also used to separate faint signals from noise.
The synchronous detection system picks up the reference signal and those
signals having the same phase components. The figure below gives a
simplified schematic diagram of the synchronous detection system. The
system consists of a multiplying circuit that multiplies two signals and a
low-pass filter (LPF) that picks up only DC components from the output.
Given "v1," a reference signal voltage for the AC current generated in the
3560, and "v2," the signal voltage for use in synchronous detection, these
parameters may be expressed by the equation given below. θof v2 shows
the phase difference against v1 and is generated by the reactance.
v1 = Asinωt
v2 = Bsin(ωt+θ)
When synchronous detection is applied to both v1 and v2, they are
expressed as follows:
v1 × v2 = 1/2ABcosθ-1/2ABcos(2ωt+θ)
The first term indicates effective resistance. The second term is
attenuated by the LPF. The 3560 displays the first term.
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