Specifications

CONSTRUCTION
RUPANJANA
RESONANCE TYPE L-C METER
T
he voltage developed across a ca-
pacitor or an inductor in a series-
resonant LCR circuit reaches its
maximum value at resonance. This fact
can be used to find the value of an un-
known inductance or a capacitance. The
present circuit is based on this very prin-
ciple and it may be used to measure an
inductance of even less than 1 µH, or a
capacitance of the order of a few pico Far-
ads. The quality factor ‘Q’ of the circuit
can be measured if the applied RF volt-
age and the resonant voltage, developed
across an inductor or a capacitor, are mea-
sured with the help of a sensitive RF volt-
meter, since Q is simply the ratio of these
two voltages.
Methodology
Signal from a variable-frequency RF source
is applied to the coil under test, which is
in series with a resonating network com-
ARUP KUMAR SEN
prising a known value of inductance (called
work-coil) and a standard variable cali-
brated capacitor. The resonance condition
is detected by a peak detector which de-
tects the peak voltage developed across
the capacitor at reso-
nance and gives a visual
indication of the same.
An unknown inductance
is measured in two steps
as follows:
1. The circuit is tuned
to resonance, using cali-
brated tuning capacitor,
with the work-coil in the
circuit. The in-circuit
value of calibrated capaci-
tor is noted. Let this value
be C
A
in pico Farads.
2. The unknown inductance is brought
in series with the work-coil, and the cir-
cuit is retuned to resonance using the cali-
brated capacitor. Let the new in-circuit
value of the calibrated capacitor be C
B
pico Farads.
In both the cases tuning is done by
Fig. 1: Schematic diagram of the resonance L-C meter
Fig. 2: 9V power supply
Author’s prototype
varying the value of the standard cali-
brated capacitor. Since frequency of the
RF source is the same in both the cases
(say f MHz), the unknown value of induc-
tance of the coil under test, L
X
(in micro
henries), can be calculated using the fol-
lowing relation:
When inductance L
w
(in micro Hen-
ries) of the work-coil is known, the value
of unknown inductor, L
X
, can also be cal-
culated using the relation:
Alternatively, the coil under test can
be connected directly to the circuit, with-
out any work-coil in series. In this case
the inductance is given by the relation:
where f is in MHz and C is in pF.
The value of a small unknown capaci-
tor, C
X
, can also be measured similarly in
the following two steps:
1. A resonant condition is achieved
by varying the standard capacitor against
a particular combination of a work-coil
and a crystal (forming part of crystal os-
cillator). Let the in-circuit value of tun-
ing capacitor be C
A
pF.
2. The unknown capacitor is connected
in parallel to the standard capacitor, and
decreasing the value of the standard tun-
ing capacitor brings the resonant condi-
tion back. Let this new in-circuit value of
the tuning capacitor be C
B
pF.
The value of the unknown capacitor C
X
would be the difference of these two values
of the standard variable capacitor, i.e.
41