Unit 3 – AM Transmitter & Reciever

AM
  • AM transmitter takes the audio signal as an input and delivers amplitude modulated wave to the antenna as an output to be transmitted.
AM Transmitter
Block Diagram of AM Transmitter

Working of AM Transmitter

  • The audio signal from the output is sent to the pre-amplifier, which boosts the level of the modulating signal.
  • The RF oscillator generates the carrier signal.
  • Both the modulating and the carrier signal is sent to AM modulator.
  • Power amplifier is used to increase the power levels of AM wave. This wave is finally passed to the antenna to be transmitted.

Tuned Radio Receiver (TRF)

  • TRF receiver is a receiver where the tuning, i.e. selectivity is provided by the radio frequency stages.
  • In essence the simplest tuned radio frequency receiver is a simple crystal set. Tuning is provided by a tuned coil / capacitor combination, and then the signal is presented to a simple crystal or diode detector where the amplitude modulated signal, in this case, is recovered. This is then passed straight to the headphones.
  • As vacuum tube / thermionic vale technology developed, these devices were added to provide more gain.
TRF Reciever

A TRF receiver consist of three main sections:

  • Tuned radio frequency stages: This consisted of one of more amplifying and tuning stages. Early sets often had several stages, each proving some gain and selectivity.
  • Signal detector: The detector enabled the audio from the amplitude modulation signal to be extracted. It used a form of detection called envelope detection and used a diode to rectify the signal.
  • Audio amplifier: Audio stages to provide audio amplification were normally, but not always included.

Functions of TRF receiver :

Advertisement
  • Interception: This function is performed by a receiving antenna. The radio waves coming from various transmitting stations arrive at this antenna. These radio waves contain electrical energy in the form of an electromagnetic wave.
  • When this electromagnetic wave is intercepted by the receiving antenna, a voltage is induced in it. A single antenna intercepts radio waves of all carrier frequencies. The desired frequency is selected by the selection process.
  • Selection: This is the process by which  the receiver selects a particular desired carrier frequency and rejects the others so that at any time a signal from only one transmitter is received. This is done by a tank circuit.
  • The resonant frequency of this circuit can be changed with the help of a variable capacitor or inductor. Thus a large number of modulated carriers coming from various transmitters is intercepted by  the antenna ,only one gets selected ,which is the one for which the receiver is tuned.
  • R.F. Amplification: The selected carrier is amplified by a class c tuned amplifier. This is necessary to raise the carrier voltage level so that the linear diode detector following this stage may operate in linear region.
  • Detection: Detection is the process of recovering a baseband signal from a modulated carrier.
  • Audio Amplification: The detected audio signal is further amplified so that it can derive the speaker. This stage consists of an R-C coupled amplifier followed by a class B push- pull power amplifier.
  • Reproduction: The process by which an electrical signal is converted into a desired Physical message.
  • In a commercial broadcast receiver, the output of the audio amplifier is fed into a speaker that produces sound according to the input audio signal.

Advantages of TRF Receiver:

  • Simplest type of receiver since it does not involve mixing and IF operation.
  • Very much suitable to receive single frequency.
  • TRF receives have good sensitivity.

Limitations of TRF Receiver:

  • Poor selectivity and low sensitivity.
  • Selectivity requires narrow bandwidth at a high radio frequency implies high Q or many filter sections. 
  • The bandwidth of a tuned circuit doesn’t remain constant and increases with the frequency increase.
  •  Instability due to large number of RF stages.
  • Gain is non-uniform over a wide range of frequencies.

Superhetrodyne Receiver

  • The super heterodyne receiver takes the amplitude modulated wave as an input and produces the original audio signal as an output. 
  • And have the ability of selecting a particular signal, while rejecting the others. And the capacity of detecting RF signal and demodulating it, while at the lowest power level.
Super heterodyne Reciever

Concept of RF Mixing

  • The RF mixing unit develops an Intermediate Frequency (IF) to which any received signal is converted, so as to process the signal effectively.
  • RF Mixer is an important stage in the receiver. Two signals of different frequencies are taken where one signal level affects the level of the other signal, to produce the resultant mixed output.
  • Due to this concept of mixing the name of this receiver is super heterodyne receiver. The input signals and the resultant mixer output is illustrated in the below figures.

Let the first and second signal frequencies be f1 and f2. If these two signals are applied as inputs of RF mixer, then it produces an output signal, having frequencies of f1+f2 and f1−f2.

RF Tuner Section

  • The amplitude modulated wave received by the antenna is first passed to the tuner circuit through a transformer.
  • The tuner circuit is nothing but a LC circuit, which is also called as resonant or tank circuit.
  • It selects the frequency, desired by the AM receiver. It also tunes the local oscillator and the RF filter at the same time.

RF Mixer

  • The signal from the tuner output is sent to the RF-IF converter, which acts as a mixer. It has a local oscillator, which produces a constant frequency. The mixing process is done here, having the received signal as one input and the local oscillator frequency as the other input.
  • The resultant output is a mixture of two frequencies f1+f2 and f1−f2  as discussed above produced by the mixer, which is called as the Intermediate Frequency (IF).The production of IF helps in the demodulation of any station signal having any carrier frequency.
  • Hence, all signals are translated to a fixed carrier frequency for adequate selectivity.

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 Super heterodyne Reciever –

  • It converts high frequency to low frequency, all processing takes place at lower frequencies. The devices are cheaper at such lower frequencies compare to higher frequencies.
  • It is easy to filter IF signal compare to RF signal.
  • It offers better sensitivity as compare to TRF receiver. 
  • Heterodyne uses single conversion and super heterodyne uses double conversion.
  • The super heterodyne receiver prevents image noise fold over due to use of two IF frequencies before conversion to baseband. 

Drawbacks

  • It requires additional Local Oscillators and RF Mixers to convert signal from RF to IF before conversion to baseband. This increases cost of overall receiver. 
  • Filters are needed to remove LO leakage as well as undesired frequency components to prevent image frequencies. This also increases cost as well as complexity of the receiver.

IF Frequency

  • When the receiver demodulates the incoming desired signal at fRF, unfortunately it demodulates down to IF also an unwanted signal at fRF+2fIF.This frequency is called image frequency.
  • To reduce the design complexity of the receivers the IF frequency is chosen in such a way that the signal at fRF+2fIF can be rejected by a simple tunable RF band pass filter such as a tank circuit with a variable capacitor.

Image Signal Rejection

  • The basic concept of the superheterodyne receiver is that it is possible for two signals to enter the intermediate frequency.
  • It is clearly unacceptable to receive signals on two frequencies at the same time and it is possible to remove the unwanted one by the addition of a tuned circuit prior to the mixer.This tuned circuit does not need to be excessively sharp. It does not need to reject signals on adjacent channels, but instead it needs to reject signals on the image frequency.
  • These will be separated from the wanted channel by a frequency equal to twice the IF. In other words with an IF at 1 MHz, the image will be 2 MHz away from the wanted frequency.
  • The image rejection of a receiver will be specified as the ratio between the wanted and image signals expressed in decibels (dB) at a certain operating frequency. For example it may be 60 dB at 30 MHz.
  • This means that if signals of the same strength were present on the wanted frequency and the image frequency, then the image signal would be 60 dB lower than the wanted one.

When a radio receiver has a poor level image rejection signals which should not be received as they are on the image will pass through the IF stages along with the required ones.

This means that unwanted signals are received along with the wanted ones and this means that the levels of interference will be higher than those with a high level of image rejection.

Selectivity

  • It is defined as its ability to accept or select the desired band of frequency and reject all other unwanted frequencies which can be interfering signals.
  • Adjacent channel rejection of the receiver can be obtained from the selectivity parameter.
  • Response of IF section, mixer and RF section considerably contribute towards selectivity.
  • The signal bandwidth should be narrow for better selectivity.

Sensitivity

  • It is ability to identify and amplify weak signals at the receiver output.
  • It is often defined in terms of voltage that must be applied to the input terminals of the receiver to produce a standard output power which is measured at the output terminals.
  • The higher value of receiver gain ensures smaller input signal necessary to produce the desired output power.
  • Thus a receiver with good sensitivity will detect minimum RF signal at the input and still produce utilizable demodulated signal.
  • Sensitivity is also known as receiver threshold.
  • It is expressed in microvolts or decibels.
  • Sensitivity of the receiver mostly depends on the gain of IF amplifier.
  • It can be improved by reducing the noise level and bandwidth of the receiver.

Fidelity

  • Fidelity of a receiver is its ability to reproduce the exact replica of the transmitted signals at the receiver output.
  • For better fidelity, the amplifier must pass high bandwidth signals to amplify the frequencies of the outermost sidebands, while for better selectivity the signal should have narrow bandwidth. Thus a trade off is made between selectivity and fidelity.
  • Low frequency response of IF amplifier determines fidelity at the lower modulating frequencies while high frequency response of the IF amplifier determines fidelity at the higher modulating frequencies.