Specifications

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CONSTRUCTION

sistor T3), it would show a dip in the in-

tensity of the LED, as the resonant cir-

cuit is tuned to a peak.

Positive 9V supply, for the meter cir-

cuit, is derived using a conventional three-

terminal fixed-voltage regulator IC 7809.

AC mains voltage is stepped down by

power transformer X1 from 230V AC to

12V AC, which is then rectified to deliver

the unregulated DC input to the regula-

tor IC. (Refer Fig. 2.)

Construction

During construction, special care is to be

exercised towards lead dressing. Any stray

coupling from output to input through a

stray capacitance, or an unwanted mu-

tual inductance, may produce unwanted

oscillations, which would hamper the re-

liability of the meter. Since stray capaci-

tances play a very adverting role in a mea-

surement, the same should be minimised

by keeping the length of the connecting

wires as short as possible.

The voltage developed across tuning

capacitors (C

and

) gradually increases

as the frequency of oscillation is lowered.

Hence, the input to the peak detector

must be controlled accordingly for its safe

operation. On the other hand, a reduc-

tion in the applied voltage to the peak

detector reduces the current through the

LED, which again enhances the sharp-

ness in tuning. The peak detector input

should not have any direct coupling with

the power amplifier via a stray resistance

or capacitance, as it would cause hin-

drance during the search for a peak.

To get better results, different work-

coils are to be used with different crystal

frequencies. The details of these coils are

given in Table I.

If a continuously variable RF source

is desired, one may substitute the crystal

in the oscillator circuit with a suitable

combination of an inductance and a vari-

able condenser. The approximate fre-

quency of oscillation of the circuit can be

determined with the help of a radio re-

ceiver—by bringing it close to the meter

circuit. The frequency on the dial at which

maximum reception would be heard is the

desired frequency.

The required inductance for a particu-

lar frequency range can be calculated from

the following relation:

Here, C is in µF and C

max

≅ 280 pF for

2J. λ is the wavelength in metres corre-

sponding to a particular

frequency, and is given by

the relation:

metres,

where f is the frequency in

MHz.

To wind a coil of re-

quired inductance, we may

use the following equation:

Here, r is the mean ra-

dius, l is the length of the

coil in inches, and n is the

number of turns per inch

of the selected SWG (as per

wire tables). Initially, a table of l-vs-L is

to be generated for a particular SWG, by

putting various values of winding length

l in equation (6) and finding the resultant

L. The winding length, and consequently

the number of turns required for the in-

ductance calculated above, may then be

found from this table. However, it is to

be noted that formula (6) above could only

help us to get close to the target induc-

tance. The desired value is then achieved

by adjusting its core, after connecting the

coil in the circuit.

In equation (6) we may substitute N

(total number of turns) for product l.n, if

desired.

Methods

Some methods to find the value of induc-

tance are given below:

1. Direct connection. Most of the un-

known inductances may be measured by

connecting them directly to the circuit and

using the relation:

Here, f is in MHz and C is in pF. The

steps to be followed are given below:

1. Switch on the RF generator

(switch S1) with the coil under test

connected to the circuit across points

X1-X2.

2. Switch on S2 to apply RF power

to the resonating network.

3. Rotate LED-intensity-control

potmeter VR1 to obtain the maximum

intensity of LED D5.

4. Tune calibrated tuning capaci-

tor C

(and/or fine-tuning calibrated

capacitor C

) for maximum intensity

TABLE II

Construction Details of the Coils

Coil Radius(r) Length(l) Turns(N) L*

(inch) (inch) (µH)

10.4/16 5/16 20 7

10.4/16 3/16 10 2.2

0.25 1.25 77 25

0.25 13/16 50 15

10.4/16 2/16 4 0.4

Fig. 5: Component layout for the PCB

TABLE III

Determination of the Unknown

Inductances by Two-frequency

Method

Coil Peak response C(pF) Results

obtained at L(µH)

osc

(MHz)

3.5 265.2

6 74.8 7.16

10 81.6

14.3 20.4 2.1

1.8 265.2

3.5 37.8 25.2

TABLE IV

Determination of Unknown Inductances

by Series Connection Method

Coil Peak response C(pF) Results

obtained at (µH)

osc

(MHz)

)665

)+L

) 6 40.8 L

=2.3

) 1.8 265.2

)+L

) 1.8 156.4 L

=13.6

) 14.3 20.4

)+L

) 14.3 10.3 L

=0.43