| 摘要 |
Known measuring instruments generate a test signal for application to a target device. Linear analogue to digital converters are then used to produce data samples representing the voltage across and the current through the device and these data samples are used to calculate impedance as a function of frequency. The A/D conversion process can be expected to worsen with reducing signal levels. This problem is overcome, in the invention, by using a logarithmic level detector (9) to produce analogue signals that represent logarithms of the voltage across and current through the device (7). These signals are passed to a processor (1) that performs a digital conversion; and, because this takes place after logarithmic conversion, it becomes possible to obtain an appropriate resolution for all signal levels.
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