EXPLANE AM/FM RADIO RECEIVER?

What is AM and FM RADIO RECEIVER- difference between AM Fm receiver ?

Radio receiver:

Radio receiver is an electronic equipment which pick ups the desired signal, reject the unwanted signal and demodulate the carrier signal to get back the original modulating signal.

Functions:

Intercept the incoming modulated signal
Select desired signal and reject unwanted signals
Amplify selected R.F signal
Detect modulated signal to get back original modulating signal
Amplify modulating frequency signal

Classification of Radio Receivers

AM Receiver
Tuned Radio Frequency Receiver (TRF)
Superheterodyne AM receiver
FM Receiver
Communication Receiver

Tuned Radio Frequency Receiver (TRF):

A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an audio frequency amplifier. This type of receiver was popular in the 1920s. These can be aligned at broadcast frequencies 535-1640 KHz.

The classic TRF receivers consisted of three sections:

One or more tuned RF amplifier stages:

These selects and amplify the desired weak signal to a level sufficient to drive the detector, while rejecting all other signals picked up by the antenna.

Detector or Demodulator:

Actually demodulation is done here in which original message signal is separated from the carrier signal.

Audio amplifier stages

which increase the power of the audio signal.

Advantages of TRF receivers:

High Sensitivity

Low cost

Disadvantages of TRF receivers:

Instable

Poor Selectivity

BW variations for different frequencies

Due to these drawbacks TRF are rarely used.

Superheterodyne AM receiver:

The AM super heterodyne receiver takes the amplitude modulated wave as an input and produces the original audio signal as an output. To heterodyne means to mix two frequencies together so as to produce a beat frequency, namely the difference between the two.

RF Tuner Section

The amplitude modulated wave received by the antenna is first passed to the tuner circuit. The tuner circuit is nothing but a LC circuit. It selects the desired frequency. It also tunes the local oscillator and the RF filter at the same time

The resultant output is a mixture of two frequencies (f1+f2), (f1−f2) produced by the mixer, which is called as the Intermediate Frequency (IF).

IF Filter

Intermediate frequency filter is a band pass filter, which passes the desired frequency. It eliminates all other unwanted frequency components present in it. This is the advantage of IF filter, which allows only IF frequency

AM Demodulator

The received AM wave is now demodulated using AM demodulator. This demodulator uses the envelope detection process to receive the modulating signal.

Audio Amplifier

This is the power amplifier stage, which is used to amplify the detected audio signal. The processed signal is strengthened to be effective. This signal is passed on to the loudspeaker to get the original sound signal

Advantages of Superheterodyne Radio Receiver:

• High Sensitivity

• High Selectivity

• High adjacent channel rejection ratio

• BW remains constant over the entire range

Disadvantages of Superheterodyne Radio Receiver:

• High cost due to the use mixer & local oscillator

Performance Characteristics of a Radio Receiver :

i. Selectivity

ii. Sensitivity

iii. Fidelity

iv. Image frequency and rejection ratio

v. S/N Ratio

vi. Double Spotting

Sensitivity:

• This is the ability of a radio receiver to amplify weak signals.

• Broadcast receivers/ radio receivers should have reasonably high sensitivity so that it may have good response to the desired signal.

• But should not have excessively high sensitivity otherwise it will pick up all undesired noise signals.

• Sensitivity of a receiver is expressed in microvolt of the received signal.

• Typical sensitivity for commercial broadcast-band AM receiver is 50 μV.

• Sensitivity of the receiver depends on :

▪ Noise power present at the input to the receiver

▪ Receiver noise figure

▪ Bandwidth improvement factor of the receiver

The best way to improve the sensitivity is to reduce the noise level.

Fidelty:

• Fidelity is defined as – a measure of the ability of a communication system to produce an exact replica of the original source information at the output of the receiver.

• Any variations in the demodulated signal that are not in the original information signal are considered as distortion.

• Radio receiver should have high fidelity or accuracy.

• Example- In an A.M. broadcast the maximum audio frequency is 5 KHz hence receiver with good fidelity must produce entire frequency up to 5 KHz.

IMAGE FREQUENCY AND REJECTION RATIO:

• In radio reception using heterodyning in the tuning process, an undesired input frequency that is capable of producing the same intermediate frequency (IF) that the desired input frequency produces.

• Image frequency – any frequency other than the selected radio frequency carrier that will produce a cross-product frequency that is equal to the intermediate frequency if allowed to enter a receiver and mix with the local oscillator.

• It is given by signal frequency plus twice the intermediate frequency

fsi = fs + 2fIF

• It is equivalent to a second radio frequency that will produce an IF that will interfere with the IF from the desired radio frequency.

◦ if the selected RF carrier and its image frequency enter a receiver at a same time, they both mix with the local oscillator frequency and produce different frequencies that are equal to the IF.

◦ The higher the IF, the farther away the image frequency is from the desired radio frequency. Therefore, for better image frequency rejection, a high IF is preferred.

S/N Ratio:

Signal Power/Noise Power = Ps/Pn

S/N ratio can also be expressed in decibels:

S/N = 10 log10 (Ps/Pn) dB

Selection criteria for IF:

Type of Receiver Band Used IF Range

AM receiver 540-1640 KHz 455 KHz

FM receiver 88-108 MHz 10.7 MHz

TV receiver VHF(54-225 MHz) 30-46 MHz

UHF (450-940 MHz)

Automatic Gain Control (AGC): AVC

An AGC circuit compensate for minor variations in the received RF signal level. It automatically increases receiver gain for weak RF input signal. And automatically decrease the receiver gain when strong RF signal is received

i. Simple AGC:

ii. Delayed AGC

Simple AGC:

The AGC circuit monitors the received signal level & sends a signal back to the RF & IF amplifiers to adjust their gain automatically. The AGC circuit produces a voltage that adjusts the receiver gain and keeps the IF carrier power at the input to the detector constant.

Delayed AGC:

With simple AGC, the AGC bias begins to increase as soon as the received signal level increases. But In delayed AGC, bias is not applied to the amplifiers until signal strength crosses a predetermined level. This type of system develops no AGC feedback until an established received signal strength is attained. For signals weaker than this value, no AGC is developed. For sufficiently strong signals, the delayed AGC circuit operates essentially the same as ordinary AGC

FM Receiver:

COMMUNICATION RECEIVER:

• Image rejection (HF-IF) & adjacent channel rejection (LF-IF) is achieved at high level through this receiver.

• Although the basic idea of superheterodyne radio receiver works very well, but an extension of the principle known as Double conversion superheterodyne radio receiver may be used.

• A Communication Receiver is one whose main function is the reception of signals used for communication rather than for entertainment.

• It improves the performance in a number of areas including stability, image rejection and adjacent channel filter performance.

Receiver converts the incoming signal down to a relatively high 1st IF. This enables the high levels of image rejection to be achieved. It is then passed through a 2nd mixer to convert it down to a lower IF where the narrow band filtering is accomplished so that the adjacent channel signals can be removed.

BFO:

Communication receivers can also receive telegraphic signals that use Morse code. These codes are transmitted as dots, dashes and spaces. To detect a Morse code BFO is used. A beat frequency oscillator or BFO is an LC oscillator used to create an audio frequency signal from Morse code radiotelegraphy (CW) transmissions to make them audible. The 2nd IF signal and the o/p of the BFO together generate whistles that indicate the presence of a dot, a dash or a space. A switch is provided in the receiver to select the option of receiving an audio signal or a telegraph signal. This switch remains off if telegraph signals are not received.

Squelch Circuit: When the communication transmitter does not transmit any signal, the receiver receives only the noise present at its input. A good quality communication receiver can amplify this noise. A squelch or a muting circuit actually eliminates the noise here.

THANKS FOR READING THIS BLOGGES