Specifications

CONSTRUCTION
The formulae (1) to (3) are based on
the assumption that there would be no
mutual inductive coupling between the
work-coil and the coil under test, and no
stray capacitance exists to affect the reso-
nating network. The stray capacitance, if
any, is to be determined and added to both
C
A
and C
B
to get better accuracy. The stray
capacitance would have no effect upon the
results obtained using the method pertain-
ing to formula (4) above, as it would be
cancelled out by the subtraction procedure.
Description
In the present circuit shown in Fig. 1,
the RF signal source is formed using a
crystal-controlled Colpitt’s oscillator, a
buffer amplifier, and a power amplifier.
The frequency of the RF source may be
varied from 1.8 MHz to 14.3 MHz by con-
necting different crystals of known reso-
nant frequencies. (Refer parts list.)
Transistor T1, along with capacitors
C3 and C4, and crystal Xtal, forms the
Colpitts oscillator. The crystal operates
near its parallel resonant frequency. Nec-
essary positive feedback is obtained via
capacitors C3 and C4 with a feedback fac-
tor β = (C3+C4)/C3. Since the voltage gain
A of a common-collector stage is less than
unity, β is made slightly greater than unity
to sustain oscillation. (Loop gain Aβ be-
comes 1 in steady-state condition.) The
output of the oscillator is taken from emit-
ter of transistor T1 and is fed to the power
amplifier transistor T3, through a buffer
stage. Field-effect transistor T2, config-
ured as a common drain amplifier, serves
as a buffer amplifier. Due to the high in-
put impedance of the source follower, load-
ing on the oscillator is very low. Moreover,
any variation in the output load imped-
ance has no pulling effect on the oscillator
frequency. The use of common collector
configuration for the oscillator and the
power amplifier stages pro-
vides a better high-fre-
quency response.
The peak voltage at
resonance is indicated by
the intensity of LED D5
connected to the collector
terminal of transistor T4.
A milliammeter (M1) may
also be used in series with
the LED to get sharper
tuning. The base of tran-
sistor T4 is driven by the
average DC voltage devel-
oped over a complete cycle,
at the output of a half-
bridge rectifier. The recti-
fier is formed with two
point-contact RF signal diodes (1N34), D1
and D2. The input signal to the bridge is
the voltage developed across the parallel
combination of variable capacitors C
T
(cali-
brated tuning capacitor, such as the one
used in radio work with C
max
≅ 280 pF for
2J) and C
F
(calibrated fine tuning capaci-
tor 0-10 pF). Diode D1 conducts only dur-
ing positive half cycle of the input signal
and causes a base current proportional to
the average value of the signal voltage to
flow through the B-E (base-emitter) junc-
tion of transistor T4. On the other hand,
diode D2 conducts heavily during nega-
tive half cycle and ensures that no re-
verse-bias leakage current flows through
diode D1 via B-E junction. (The leakage
current reduces the average voltage de-
veloped over a complete cycle, and hence,
reduces the intensity of the LED.) If a
second identical detector stage, built
around transistor T5, is connected to the
power amplifier output (emitter of tran-
PARTS LIST
Semiconductors:
IC1 - 7809, 3-terminal +9V
regulator
T1, T3 - BF194B npn transistor
T2 - BFW10 field-effect transistor
T4, T5 - BC147 npn transistor
D1-D4 - 1N34/OA79 point-contact
signal diode
D5, D6 - LED, 5mm
D7, D8 - 1N4002 rectifier diode
Resistors (all ¼W, ±5% metal carbon film,
unless stated otherwise)
R1, R7 - 33-kilo-ohm
R2 - 22-kilo-ohm
R3 - 2.7-kilo-ohm
R4 - 330-ohm
R5 - 1-meg-ohm
R6 - 4.7-kilo-ohm
R8 - 620-ohm
R9, R10 - 1-kilo-ohm
VR1, VR2 - 470-kilo-ohm, variable (linear)
Capacitors:
C1, C9 - 220µF, 25V electrolytic
C2, C11 - 0.2µF ceramic disc
C3 - 100pF polyester
C4 - C8, C10 - 820pF polyester
C12 - 1000µF, 25V electrolytic
C
T
- 10pF - 280pF 2J type
C
F
- 0pF - 10pF trimmer
Miscellaneous:
X1 - 230V AC primary to 12V-0-
12V, 250mA secondary
transformer
LW - Work-coil (refer Table I)
M1, M2 - DC mA meter, 1 mA F.S.D.
Xtal - 1.8, 3.5, 6, 10, 12, 14.3 MHz
quartz crystal
SW1, SW2,
SW4 - ON/OFF switch
SW3 - SPDT switch
- BNC and SIP connectors
- Snap connectors for coils
(male/female)
Fig. 3: Single-layer coil
Fig. 4: Actual-size, single-sided PCB layout for the circuit
TABLE I
Work-coil Specifications for Different Crystal Frequencies
Crystal SWG Turns Coil No. of Inductance
frequency enameled /inch diameter turns closely (µH)
(MHz) copper wire (in inch) wound
1.8 36 120 0.4 88 45
3.5 36 120 0.3* 42 15
6.0 36 120 0.3* 30 10
10.0 28 63 10/16 10 2.3
12.0 28 63 1016 10 2.3
14.3 28 63 10/16 10 2.3
*Note. The coils are to be wound on standard ebonite coil formers (ferrite cored) that
are used in radio work. The listed inductance values are with the core almost
completely dipped in the hole of the former. The values, without a core, would be
approximately half of the listed values.
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