OSCILLATOR

 Oscillator is a non-rotating electronic device. Oscillators can produce oscillations ranging from a few Hz to several MHz.

A transistor amplifier with proper positive feedback can act as an oscillator.

An electronic device that generates sinusoidal oscillations of desired frequency is known as a sinusoidal oscillator. 

It does not create energy, but merely acts as an energy converter. 

It receives d.c. energy and changes it into ac energy of desired frequency. The frequency of oscillations depends upon the constants of the device. 

BAND-STOP FILTERS

Band-stop filters are also called band-elimination, band-reject, or notch filters; this kind of filter passes all frequencies above and below a particular range set by the component values. This type of filters can be made out of a low-pass and a high-pass filter, just like the band-pass design, except that the two filter sections are connected in parallel with each other instead of in series.

The low-pass filter section is comprised of R1, R2, and C1 in a “T” configuration. The high pass filter section is comprised of C2, C3, and R3 in a “T” configuration as well. Together, this arrangement is commonly known as a “Twin-T” filter, giving sharp response when the component values are chosen in the following ratios:

Component value ratios for the "Twin-T" band-stop filter

R1 = R2 = 2(R3)

C2 = C3 = (0.5) C1

The frequency of maximum rejection (the “notch frequency”) can be calculated as follows:

fnotch =1/4*3.14*R3C3

BAND-PASS FILTERS

In applications such as a particular band, or spread, or frequencies need to be filtered from a wider range of mixed signals. This can be achieved by combining the properties of low-pass and high-pass into a single filter. The result is called a band-pass filter.

The series combination of these two filter circuits is a circuit that will only allow passage of those frequencies that are neither too high nor too low. This type of band-pass filter works by relying on either section to block unwanted frequencies, it can be difficult to design such a filter to allow unhindered passage within the desired frequency range. Both the low pass and high-pass will always block signals to some extent, and their combined effort makes for an attenuated signal at best, even at the peak of the “pass-band” frequency range.

HIGH-PASS FILTERS

A high-pass filter’s task is just the opposite of a low-pass filter it offers easy passage of a high-frequency signal and difficult passage to a low-frequency signal. The capacitor’s impedance increases with decreasing frequency. This high impedance in series tends to block low-frequency signals from getting to load.

The inductance impedance decreases with decreasing frequency. This low impedance in parallel tends to short out low-frequency signals from getting to the load resistor. As a consequence, most of the voltage gets dropped across series resistor R.

LOW-PASS FILTERS:

A low-pass filter is a circuit that offers easy passage to low-frequency signals and difficult passage to high frequency signals. There are two basic kinds of circuits in low pass filters,

The inductive low-pass filter and the capacitive low-pass filter,

The inductance impedance increases with increasing frequency. This high impedance in series tends to block high-frequency signals from getting to the load.

The capacitor’s impedance decreases with increasing frequency. This low impedance in parallel with the load resistance tends to short out high-frequency signals, dropping most of the voltage across series resistor R.

All low-pass filters are rated at a certain cutoff frequency. That is, the frequency above which the output voltage falls below 70.7% of the input voltage. In a simple capacitive/resistive low-pass filter, it is the frequency at which capacitive reactance in ohms equals resistance in ohms. In a simple capacitive low-pass filter (one resistor, one capacitor), the cutoff frequency is given as:

fcutoff =1/2*3.14*RC

FILTER:

There are circuits that are capable of selectively filtering one frequency or range of frequencies out of a mix of different frequencies in a circuit. A circuit designed to perform this frequency selection is called a filter circuit, or simply a filter. A common need for filter circuits is in high-performance stereo systems, where certain ranges of audio frequencies need to be amplified or suppressed for best sound quality and power efficiency. Equalizers and crossover networks are examples of filters, designed to accomplish filtering of certain frequencies.

Filters are of different types;

• Low-pass filters
• High-pass filters
• Band-pass filters
• Band-stop filters
• Resonant filters

REACTIVE POWER , TRUE POWER, APPARENT POWER

The reactive loads such as inductors and capacitors dissipate zero power, the voltage drop and draw current gives the deceptive impression that actually do dissipate power. This is called reactive power, and it is measured in a unit called Volt-Amps-Reactive (VAR), rather than watts. The mathematical symbol for reactive power is (unfortunately) the capital letter Q.

The actual amount of power being used, or dissipated, in a circuit is called true power, and it is measured in watts (symbolized by the capital letter P, as always).

The combination of reactive power and true power is called apparent power, and it is the product of a circuit’s voltage and current, without reference to phase angle. Apparent power is measured in the unit of Volt-Amps(VA) and is symbolized by the capital letter S.

The true power is a function of a circuit’s dissipative elements, usually resistances (R). Reactive power is a function of a circuit’s reactance (X). Apparent power is a function of a circuit’s total impedance (Z).