Inductance meter with 1nH resolution
The principle of this
inductance meter is simple. The meter operates by forcing a constant
100kHz sinusoidal current through the inductor (Lx) and measuring the
magnitude of the quadrature voltage generated across it. This voltage
leads the current by 90º. The amplitude of the quadrature component of
the voltage across Lx may be recovered by synchronous demodulation, with
the demodulator's reference signal provided by a +90º phase-shifted
version of the current source signal. When the demodulator output is
integrated, the out-of-phase components cancel and the in-phase
(quadrature)components add to create a DC signal which is proportional
to the peack amplitude of the in-phase component.
The reference sine wave
is generated by a 100kHz crystal oscillator using a gate of an
CD4011.This reference is used by the demodulator and after amplitude
scaling by the range switch, to the voltage-controlled current source,
which drives the inductor.
The currents through
inductor is low enough to not cause saturation of sensitive ferrite
core. 20mA pk/pk on 200nH and 2uH ranges, and 200uA on the 2mH range.
The voltage generated across inductor is amplified by two amplifiers,
separated by a range switch attenuator. This amplifiers are part of a
TL084.
The synchronous demodulation is performed by
MC1496,
an RF double balanced modulator/demodulator. The phase-shifted
reference signal through a passive RLC network, drives the top
differential pairs into saturation, while the bottom differential pair
is driven linearly by the amplified signal Vx, across Lx. When the
reference signal is the same frequency as Vx (across Lx) and is
saturating the top pairs, the output of the demodulator is a function of
the amplitude and phase of Vx. The output of MC1496 is used
differentially to avoid DC drift problems, is filtered and drives the
digital voltmeter. The range of the inductance meter is 1nH to 2mH.
- References:
1. RF Design Magazine, 1987-2000 - 2. ARRL Handbook, 1980-2000
Tidak ada komentar:
Posting Komentar