What is a Varactor Diode | How to Construct a Voltage Controlled Oscillator

A varactor is a type of diode that presents with a junction capacitance when a reverse biased voltage is applied across its terminals. This diode is also known as the varicap diode, variable capacitor diode or a tuning diode. The varactor diode is a special type of PN junction diode whose internal capacitance can be varied by the application of a reverse biased voltage.

Properties of a Varactor Diode
It is operated in the reverse biased condition. It is a voltage dependent variable capacitor.

Working Principle of a Varactor Diode
The varactor diode consists of two terminals, the anode which acts as the positive terminal and the cathode, which acts as the negative terminal. The P and the N regions act as the parallel plates of the capacitor and the region between the P and the N regions act as the dielectric.

The formula for finding out the capacitance in the varactor diode is

C = eA/d

Where epsilon (e) is the permittivity of the dielectric, A is the area of the parallel plates and d is the distance between the parallel plates.

When a reverse biased voltage is applied to a varactor diode, there will be an electric field created at the N and the P regions. The electric field direction is from the positive to the negative. In the N region, the electrons that are negatively charged get attracted to the positive terminal of the battery and in the P region, the holes which are positively charged get attracted to the negative terminal of the battery.

As the applied voltage is increased, the depletion region thickness becomes increased and the junction capacitance is decreased. When the applied voltage is decreased, the depletion region thickness becomes decreased and the junction capacitance is increased.

This change in junction capacitance of the varactor will thereby cause a variation of capacitance to the circuit that is connected to it. The capacitance value of the varactor diode is measured in picofarads or PF.

Applications of Varactor Diode

1. The varactor diode is used as a voltage controlled oscillator in receivers and transmitters.

2. They are used in tuning and signal resolution circuits.

3. They are used in variable bandpass filters.

4. They are used in frequency modulation circuit in FM transmitters.

5. They are used in frequency multipliers.

6. They are used in RF communication and microwave devices.

Watch the Video: https://youtu.be/UOt-LnEseRg

What is a Crystal Oscillator || How to Construct a Crystal Oscillator

A crystal oscillator is an electronic oscillator which uses the mechanical resonance of a vibrating crystal composed of a piezoelectric material to create an electrical signal of a constant frequency. When a small voltage is applied to a thin piece of quartz crystal, it changes its shape. This phenomenon is known as the piezoelectric effect. The most common type of piezoelectric resonator in use is the quartz crystal. The oscillators that implement these crystals for oscillation are called the crystal oscillators.

Properties of a Quartz Crystal
A quartz crystal produces a slight change in shape by application of voltage on the surface or through an electrode, called electrostriction. These piezoelectric materials that transform its shape are called as transducers. When the voltage is removed, the crystal returns back to its original shape generating a small voltage. The quartz crystal produces mechanical vibrations or oscillations at a specific resonant frequency to which it is designed. The behavior of a crystal is similar to an RLC circuit with higher quality or Q factor. Thus a quartz crystal can be used as a tuned circuit in an LC oscillator.

The quartz used in crystal oscillators is very thin and the electrical connections are taken from its two surfaces. The resonant frequency of the quartz crystal depends on the physical shape, size, thickness and elasticity of the material. The typical quartz crystal is cut and shaped to attain it's specific frequency.

The elastic vibrational nature of the quartz crystal has less dependence on the temperature, and thus has more stability. The stability of the quartz crystal makes them more acceptable as oscillators in receivers, transmitters, clocks, microcontrollers etc.

Equivalent Circuit of a Quartz Crystal
The equivalent circuit of a quartz crystal consists of a series resistance, inductance and capacitance (RLC) circuit with a low resistance Rs, large inductance Ls, and a small capacitance Cs. It also has a parallel capacitance Cp. Thus the resonant frequency of the crystal is determined by
A series resonant circuit, which is composed of Rs, Ls and the Cs.
A parallel resonant circuit, which is composed of Ls and Cs resonating parallel with the capacitor Cp. 

Calculation of Series and Parallel Resonant Frequency of a Quartz Crystal 

Calculation to find out the series resonance in a quartz crystal is given by the formula

Fs = 1/2π√LsCs

Calculation to find out the parallel resonance in a quartz crystal is given by the formula

Fp = 1/2π√Ls(CpCs/Cp+Cs)

 Applications of Crystal Oscillators

1. A crystal oscillator is used in generating an oscillator signal in radio and television.
2. It is used to generate a stable oscillator signal in transmitters.
3. It is used in communication systems, tracking and guidance systems.
4. It is used as a stable clock signal in digital frequency synthesizers, microcontrollers and other integrated circuits.
5. It is used as a local oscillator in frequency converters and doublers.
6. It is used in quartz watches and clocks for precise time measurements.

Watch the Video: https://youtu.be/W2Gg8Ot6CsA

 

 

 

What is a Colpitts Oscillator | How to Construct a Colpitts Oscillator

Colpitts oscillator is a linear oscillator and it generates radio frequency signals in the range of 20 kHz to 300 MHz. The Colpitts oscillator was invented in 1918 by an American engineer called Edwin H Colpitts. Colpitts oscillator consists of an amplifier and a positive feedback circuit for its functioning. The amplifier uses an active device such as a bipolar transistor, field effect transistor or vacuum tube as an amplifier and uses a positive feedback from its output connected to the input by means of a parallel LC tuned circuit or tank circuit, which determines the frequency of oscillation.

Operation of a Colpitts Oscillator
In an oscillator, the amplification of the active device should be larger than the losses due to the attenuation at the capacitive voltage divider network for the oscillator to work constantly.

 

In an oscillator, the thermal noise in the amplifier gets amplified, which contains a number of different frequency components. The positive feedback circuit or the LC tank circuit selects a particular frequency from this range of frequencies and feeds it back to the input of the amplifier. In a Colpitts oscillator, the LC circuit consists of a series combination of two capacitors in parallel with the inductor. The frequency of the oscillator is approximately the resonant frequency of this LC tank circuit.

 

Working of Colpitts Oscillator
The voltage gain of the amplifier is denoted as A. The portion of the output signal of the amplifier that is fed back to the input of the amplifier is called the feedback factor which is denoted as beta. The total loop gain calculated as the product of A and beta is called as the A beta. The feedback circuit provides 180 degree phase shift and the amplifier provides another 180 degree of phase shift. So, there will be either a 0 degree or 360 degree phase shift happening in the signal during this process. The oscillator works only when the A beta becomes equal to 1 and there is 0 degree or 360 degrees phase shift in the signal. This is called the the Barkhausen criteria.

Barkhausen criteria is satisfied by the formula

A beta = 1
Beta = C1/C2
A > C2/C1

Frequency of the Colpitts Oscillator
The resonant frequency of the Colpitts oscillator is determined by the formula:

Fr = 1/2π√LC

where C= C1C2/C1+C2

In this oscillator, the frequency can be changed by varying the inductance using a variable inductor or by means of adding a variable capacitor connected parallel to the inductor. There are two types of Colpitts oscillator, the common base Colpitts oscillator and the common collector Colpitts oscillator.

Applications of Colpitts Oscillator

1. It is used as an oscillator in radio receivers.
2. It is used as a beat frequency oscillator in single side-band receivers.
3. It is used as a local oscillator in transceivers.
4. It is used as a carrier oscillator in transmitters.
5. It is used in signal generators.
6. It is used in radio direction finders and sensors.

Watch the Video: https://youtu.be/8xA2s3ibPfM

What is an LC Tank circuit | How to Construct an LC Tank Circuit

An LC tank circuit is an electrical circuit which consists of an inductor and a capacitor connected together. The letter L represents an inductor or a coil of wire and the letter C represents a capacitor. This circuit forms a resonant circuit that stores energy and resonates at its resonant frequency, which is determined by the value of the inductor and the capacitor. Its function is similar to a tuning fork.

Working of an LC Tank Circuit
An LC tank circuit works by storing electrical energy alternately in a capacitor and an inductor. The capacitor stores energy in the form of an electric field or charge between its plates depending on the voltage across it, whereas an inductor stores energy in the form of a magnetic field around it depending on the current through the inductor.

When an inductor is connected in parallel to a charged capacitor, the voltage across the capacitor will produce a current through the inductor that builds up a magnetic field around it. The voltage in the capacitor drops to zero as the charge is used up by the current flow through the inductor. At this moment, the energy is stored in the inductor as a magnetic field and induces a voltage across the coil, as it opposes the change in current through it. The induced voltage across the inductor creates a current to flow across the discharged capacitor with a polarity opposite to that of its previous charge. The energy required by the capacitor to charge is drawn by the magnetic field across the inductor. The current flow stops as the magnetic field collapses and completely dissipates. The capacitor is now charged with a polarity that is opposite and at a strength lesser than its previous charge.

In the next cycle, discharging of the capacitor occurs into the inductor inducing a magnetic field across the coil with a current flow opposite in direction to the previous one. The charge flows, oscillating between the capacitor plates and the inductor coil until it finally dies out completely. There is energy flow back and forth until the internal resistance of the circuit such as the resistive losses of the inductor and the dielectric losses of the capacitor dampens the oscillation.

Frequency of the LC tank circuit
When an LC circuit is driven from an external source at an angular frequency, resonance happens when the inductive and capacitive reactance become equal in magnitude. The frequency at which this equality of inductive and capacitive reactance in that particular circuit is called the resonant frequency of the circuit.

The resonant frequency is measured by the formula
Fr = 1/2π√LC

Applications of LC tank circuit

1. LC circuits are used for generating signals of a particular frequency such as oscillators, signal generators etc.
2. It is used in selecting a specific frequency from a band of frequencies, where it is commonly used as band-pass filters and tuned radio frequency amplifiers in radios, television receivers, communication equipment etc.
3. LC circuit acts as an electronic resonator in oscillators, filters, tuners, mixers, discriminators etc. in various gadgets.
4. The parallel resonant circuit is used as a load impedance in RF amplifiers. The gain of the amplifier is maximum at its resonant frequency.
5. Series and parallel resonant circuits are used in induction heating.

Watch the Video: https://youtu.be/fjq9I7TVLes

What is a Tuned Capacitance Box | How to Construct a Tuned Capacitance Box

A tuned capacitance box is an electronic tool, which helps to find out small capacitance values in a circuit. This is an important tool in radio frequency electronics where capacitance values are very small, which are in the picofarad range. This instrument is different from a capacitance decade box in that it is variable and it measures smaller capacitance values. This capacitance box helps in finding out the ideal capacitance value in place of an actual capacitor in a circuit. The physical isolation of the variable capacitor in the box helps to prevent introduction of hand capacitance to the circuit. The connecting leads to the circuit are shortened so as not to introduce additional inductance or load to the circuit under test.

Capacitance Box Tuning an LC Tank Circuit

Construction and Working of Tuned Capacitance Box
The device consists of a variable capacitor which is a 2J gang capacitor; the two sections of it are connected in parallel. The variable capacitor measures between 5 PF to 500 PF. It is housed inside a box so that the shaft can be manually rotated from outside the box. Two screw terminals are taken out from the leads of the variable capacitor through which the circuit to be tested is connected. One end of the connecting wire is attached to the connector by means of screws and the other end of the wire is connected to the circuit under test. The variable capacitor is rotated to find out the ideal value of capacitance in that circuit. This specific capacitance that was found out, is measured using an LC meter and an appropriate fixed value capacitor of high Q is replaced in the circuit.

Completed Capacitance Box

 

Capacitance Box Tuning to the Resonant Capacitance Value

 

Measuring the Capacitance Value With an LCR Meter

Uses of a Capacitance Box

1. The capacitance box helps to measure small capacitance values, particularly in an RF circuit.
2. It helps to find out the capacitance value of a tuned circuit or tank circuit in an oscillator, RF bandpass filter, IF tuned amplifier, antenna input tuned circuits etc.of a radio receiver.
3. It helps to find out the capacitance value of a tuned circuit in an oscillator, bandpass filter, tuned amplifier stages, RF output filter, antenna tuner etc. of a transmitter.

 Watch the Video: https://youtu.be/yRbFg5q31Qw

 

 

 

 

 

What is an RF Probe | How to Construct an RF Probe

RF probe is an electronic test equipment which is used to measure the strength of radio frequency signals in an electronic circuit. The RF probe converts the high frequency RF signal of a testing circuit into a DC signal, which can be read and measured using a multimeter or an oscilloscope. It gives a relative reading of the RF amplitude. RF probe is used only in low power radio frequency signals. It is used at a frequency range of 100 kHz up to 3 GHz.

 

 

 

Uses of an RF Probe
This probe is used for testing and doing adjustments in oscillators, receivers, modulators, transmitters etc. It is used in radios for tuning IF stages, adjust filter responses, aligning the radio, peaking RF stages, examining the amplitude modulation etc.

Challenges for Measuring an RF signal
There are challenges in measuring an RF signal because of the high frequency involved, low power, the nature of the circuit to be measured such as an oscillator, that can overload when attaching a testing lead etc. In circuits where the loading of a section is critical, high impedance probes are used which extracts a very small amount of RF energy, amplifies it to a suitable level and converted into DC, which can be measured using a suitable test instrument.

RF Probe Circuit
The RF probe circuit consists of two diodes, two capacitors, a resistor and a variable resistance. Use of the germanium diode gives the best response to the circuit. A multimeter or an oscilloscope helps to measure the detected DC signal.

 

 

1N64 Germanium Diode

Working of the Circuit
The input capacitor of the probe collects an RF sample of the circuit to be tested. When the voltage falls, it eventually turns on the first diode and builds up a voltage across the node. When the voltage rises up, it turns on the second diode charging the second capacitor. The capacitor charges close to the peak of the amplitude of the signal. A 10k resistor is used to release the charge from the capacitor. The rectified DC signal is allowed to pass through the circuit, which is read using a multimeter or an oscilloscope.

Measuring the signal output from a VFO

Watch the Video: https://youtu.be/DMF4Fp1xT0Q