RCL meter 1_10RCL-meter is a low cost and minimal hardware solution for measuring:
– capacitances (5pF to 5uF)
– inductances (5uH to 50mH)
– resistances (5 Ohm to 50 MOhm)
using the PC soundcard.
The required external hardware is just 2 resistors and 2 capacitors.
Version 1.10 includes an improved phase shift measurement, making the software less critical toward the quality of the soundcard.
How it works
The working principle is simple: an AC voltage, with a known frequency, is applied over a known resistor (R) in series with an unknown impedance (X). Based on the voltage ratio (Ux/Ur), the phase shift between Ux and Ur the unknown impedance (X) can be determined.
However, implementing this principle using a soundcard is not so easy. At high impedances (X) Ur is close to 0 while at low impedances Ux is close to 0. In both case it is difficult to get a sufficient accuracy. Mainly because of the inaccurate phase measurement.
In addition the soundcard inputs have a rather low input resistance and a significant input capacitance, appearing in parallel to X.
In fact a soundcard is not really suited to build a RCL-meter, in particular due to the low input impedance and relatively large input capacitance. In addition there is a lot of variation between soundcards in regard with the input impedance, input capacitance, line-in sensitivity and speaker out level.
But it is probably the nature of a radio amateur to try to use things for purposes they are not designed for …
These problems were solved by taking the Fourier Transforms of Ur and Ux and use these to calculate the voltage ratio and phase shift. After using some tricks and a lot of calculations it seems possible to build a relative accurate (and very cheap) RCL-meter.
– As a lot of math is involved (3 FFT’s per measurement) at least a Pentium 200MHz with 8MB RAM (16MB or more preferred) is needed.
– Of course a soundcard that can handle 16 bit ADC / 44kHz sample rate. With older / cheaper soundcards the measurement accuracy can be limited, due to a strong internal coupling between speaker out and line in (see help file for details). In addition the soundcard MUST have a line-in input, as 2 signals must be measured at the same time the microphone input (as it is mono) cannot be used.
– At least Win98.