Since when judging battery wear it is important to ascertain changes arising from the passage of
time, use the same measurement lead for each measurement.
10.5 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 the below image, 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 instrument, and ‘V2’, the
signal voltage for use in synchronous detection, these parameters may be expressed by the equation
given below. Θ of 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 x v2 =
1
2
ABcosθ -
1
2
ABcos (2ωt + θ)
the first term indicates effective resistance. The second term is attenuated by the LPF. The
instrument displays the first term.
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