A better series-inductance measurement technique uses a pulse generator. In the time domain, R_s can be eliminated from its effect on the measurement, a big advantage over the previous approach. Using the same setup as before, except with a pulse generator as an open source, the generator voltage pulse rising transition is adjusted to be, in this case, 10 V in 50 ns, or 200 V/μs. Then the source - still considered to be a current source (because R_g >> R_s) - drives the sense resistor with a current ramp of

(The ¸ 2 is due to the 50 W termination divider.) The inductance follows from the v-i relationship for inductance:

where v_L is the constant inductor voltage due to the current ramp.

When driven by a Tektronix PG508 pulse generator, the following waveforms were observed.

The top waveform (A) is the open-source voltage without sense resistor; the ch 2 waveform is the response with resistor, though only approximately time-aligned with the (stored) A waveform. (Note the ´1 setting for ch 2, which is 50 W terminated.)

The voltage, v_o, steps up to v_L, a value of about 100 mV (on ch 2, at 20 mV/div). The current-ramp voltage drop across R_s causes a ramp-up superimposed on v_L, which is negligible and undiscernible amidst waveform ringing. The pulse flattens on top to a constant voltage, leaving (on ch 2) a constant-current drive of R_s, producing about 6 mV (a fourth div). The value of R_s can be calculated from the voltage-divider formula, where V_g (= 12.5 V) drives R_g = 25 W in series with R_s to produce about 6 mV across it:

For this measurement, the resulting value of R_s is about 24 mW , 4 % low from the approximately 25 mW measured with an RLC meter.

The inductance is calculated from the measured voltage to be

a reasonable value based on geometry.

Measurement of the series inductance of small-value resistors is difficult. An approximate value can be measured, however, using a very simple setup, with a pulse generator and an oscilloscope. This approach will result in the small values not measurable with most RLC meters. A network analyzer can provide more accurate measurements at a much higher price.

Ó Dennis L. Feucht, 2000