Specifications

CONSTRUCTION
of LED D5, or peak on the meter. The
intensity of LED D5, or the deflection of
the meter pointer, would be the maxi-
mum at resonance. If no peak is found, it
might be due to low signal input to the
peak detector. Gradually decrease the in-
circuit resistance value of potmeter VR1.
If still no peak is found, it would mean
that crystal frequency is not appropriate.
Try with another crystal.
5. Note down the capacitance values
from the dials of C
T
and C
F
. The total
value of capacitance C is given by C =
C
T
+C
F
+C
Stray
.
The value of C
Stray
may be in the range
of 10-30 pF. If C
T
is sufficiently high, C
Stray
may be dropped from the calculation.
6. Calculate the value of L
X
using
equation (7).
2. Series connection. This includes
following steps:
1. Insert a crystal in the crystal socket.
Connect the coil under test across termi-
nals X1-Y and a suitable work-coil across
X2-Y (Table I). Initially short X1-Y termi-
nals using a small wire. Tune C
T
and C
F
to get a peak on LED D5 (or meter). Note
the capacitance values from the dials of
tuning capacitors C
T
and C
F
. Let this value
be C
A
. If no peak is obtained, try with
another crystal and work-coil combination.
2. Remove the short across X1-Y and
retune the circuit by rotating C
T
(and/or
C
F
) towards lower capacitance value to
establish the peak once again. Make a
fine adjustment using C
F
. Note the new
positions on the dials. Let the new sum
of C
T
and C
F
be C
B
.
3. Calculate the value of the unknown
inductance from Eqs (1) or (2) given above.
3. Two-frequency method. If a coil
of inductance L
X
gives peak responses on
two different frequencies—f
A
at C
A
and f
B
at C
B
on tuning the dial—then L
X
can be
calculated from the formula given below:
Method to find the value of capaci-
tance. Follow the steps given below:
1. Tune the circuit to resonance
against a particular crystal and work-coil
combination.
2. Read the capacitance values from
the dials. Let the sum be C
A
pF.
3. Connect the capacitor under test in
parallel with C
T
.
4. Retune the circuit to get the peak
back.
5. Note the new values of capacitance
from the dials. Let the sum be C
B
pF.
6. Calculate the unknown capacitance
from the relation:
C
x
= (C
A
– C
B
) pF
Method to determine an unknown
frequency. Frequency of an unknown RF
sinewave signal may be measured by fol-
lowing the steps given below:
1. Connect a suitable work-coil in the
circuit.
2. Connect the RF signal source (with
unknown frequency F
X
) to the gate of
buffer transistor T2, after disconnecting
the crystal oscillator from it, with the help
of switch S3.
3. Apply power to the meter by turn-
ing switch S1 ‘on’.
4. Apply RF power to the resonant
circuit after turning switch S2 ‘on’.
5. Tune the resonant circuit to get a
peak. If no peak is obtained, try with an-
other work-coil.
6. Note the capacitance value from
the dials of the tuning capacitors. Let the
sum be C.
7. Calculate the frequency of the in-
coming RF signal using the following re-
lation:
MHz
Here, L
w
is in µH and C in pF.
Calibration
If a standard variable capacitor is not
available, we may use, after proper cali-
bration, a 2J type variable capacitor which
is generally used for radio work. To cali-
brate the same, follow the steps given be-
low:
1. Switch on the RF source with a 3.5
MHz crystal, 17µH work-coil, and a 2J
capacitor for C
T
.
2. Rotate the tuning capacitor towards
its maximum capacity (approx. 280 pF).
3. Tune by varying the slug of the coil
to get a peak on the meter or LED.
4. If you require a resolution of 10
pF, connect a standard low-tolerance 10
pF capacitor in parallel with C
T
. Instantly
the circuit would be out of tune.
5. Rotate capacitor C
T
to get back the
peak again. Mark the new position of C
T
.
It would be C
max
=10 pF.
6. Redo steps 4 and 5 to cover the
entire angular span (=180
o
). Each time
the new position of C
T
would be 10pF less
than its previous value.
Limitations
While tuning with C
T
to get a resonance,
the LCR circuit may produce a peak volt-
age at the harmonic frequency of the crys-
tal used, which would give misleading re-
sults. To avoid this situation, the posi-
tions of those harmonic frequencies on the
dial of C
T
, for a particular crystal and
work-coil combination, should be spotted
first, by rotating the capacitor over its
full swing.
During tuning, if a conductive body is
brought near the resonant network of the
variable capacitor, interference would be
produced.
Results
Coils practically wound, using the formu-
lae given in the article, and inductance
practically determined, using two-fre-
quency method and series connection
method, are tabulated in Tables II, III,
and IV respectively.
Power supply may be assembled sepa-
rately on the PCB, which may be cut out
from the main PCB. Variable tuning ca-
pacitors (C
T
and C
F
), snap connectors for
coils (L
W
and L
X
), switches, LEDs,
potmeters, etc may be mounted on front
panel.
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