Phase sensitive voltmeter

Tlie use of a phase-sensitive voltmeter for the study of the electrical response of the interpliase was mentioned in Section 16.8 as an accurate method for the measurement of the double-layer capacitance. But this instrument has far more important uses in electrochemistry than just the measurement of capacitance, by combining a phase-sensitive voltmeter (also called a lock-in amplifier) with a variable frequency sine-wave generator, one obtains an electrochemical impedance spectrometer. Such instruments are commonly combined with a microcomputer,... 

The best method to measure the double-layer capacitance is to use a phase-sensitive voltmeter. This instrument is sometimes incorporated into a frequency response analyzer, designed to make electrochemical impedance spectroscopy measurements, but it can also be used independently. In Part Two we devote a full section to the operation of such instruments and the analysis of results obtained by them. Here we shall limit the discussion to the measurement of capacitance. 

For an ideally polarizable interface with negligible solution resistance, the phase angle is —90°. For an ideally non-polarizable interface the phase angle is zero. Real systems do not behave ideally of course. The actual phase angle will, therefore, be somewhere in between, and it will depend on frequency in most cases. The phase-sensitive voltmeter can measure the absolute value of the impedance vector Z and the phase angle simultaneously... 

It is possible to connect the microphone directly to a sensitive voltmeter and watch the variation in the intensity of the received signal as the detector is moved along the tube. The intensity will decrease to a minimum when an antinode is reached and then increase to a maximum at a node. However, it is more precise and more convenient to use an oscilloscope to indicate the phase relationship between the speaker input and the microphone... 

Fig,4 Schematic diagram of an intracavity laser Stark spectrometer. PSD stands for phase sensitive detector, DVM for digital voltmeter, HV for high voltage, and MOD for modulation source. (Reproduced by permission from Journal of Chemical Physics, 1977 66, 127)... 

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