Design And Construction Of Amplitude Modulation Receiver
A receiver has the functions of selecting the desired signal from all the other available signals, amplifying and demodulating it, and displaying it in the desired manner. In the Amplitude Modulation (AM) receiver, the incoming signal voltage is combined with a signal generated in the receiver to produce a lower fixed frequency. The signal at this intermediate frequency contains the same modulation as the carrier, and it is now amplified and detected to reproduce the original information.
A constant frequency difference is maintained between the local oscillator and the Radio Frequency (RF) circuits, normally through capacitance tuning, in which all the capacitors are ganged together and operated in unison by one control knob. The IF (Intermediate Frequency) amplifier generally uses two or more transformers, each consisiting of a pair of mutually coupled tuned circuits.
It provides most of the gain and therefore sensitivity and bandwidth requirements of the receiver. The selectivity and sensitivity of the superhet are usually fairly uniform throughout its tuning range and not subject to the variations that affect the TRF receiver.
The IF signal output is an amplified composite of the modulated RF from the transmitter in combination with RF from the local oscillator. Neither of these signals is useable without further processing. The next process is in the detector stage, which eliminates one of the sidebands still presents and separates the RF from the audio components of other sideband. The RF is filtered to ground, and audio is supplied or fed to the audio stages for amplification and then to the speakers, etc.
Because of its narrow bandwidth, the IF amplifier rejects all other frequencies but 455 KHz. This rejection process is the key to the superheterodyne’s exceptional performance, which is why it is widely accepted. The process of tuning the local oscillator to a predetermined frequency for each station throughout the AM band is known as tracking and will be discussed later.
This project will cover AM receivers in general, showing why their format has been to a certain extent standardized. Each block of the receiver will be discussed in detail, as will its functions and design limitations.
Table Of Contents
- Title Page
- Approval page
- List of abbreviations
- Objective of project
- Literature Review
- Super heterodyne AM receiver
- Amplitude Modulation Receiver
- Antenna and Tuning Circuit
- Radio Frequency Amplifier
- Local Oscillator
- Mixer Network
- IF Amplifier
- Detector Circuit
- Audio Frequency Amplifier
- Design and Construction of AM Receiver
- AM receiver with additional IF amplifier
- AM receiver components
List Of Abbreviations
- AF Audio Frequency
- AFC Automatic Frequency Control
- AGC Automatic Gain Control
- AM Amplitude Modulation
- BJT Bipolar Junction Transistor
- DB Decibel
- DC Direct Current
- DMM Digital Multimeter
- IF Intermediate Frequency
- IFT Intermediate Frequency Transformer
- LO Local Oscillator
- RDF Radio Direction Finding
- RF Radio Frequency
- RFC Radio Frequency Choke
- SPK Speaker
- TRF Tuned Radio Frequency
An AM receiver must provide most of the AM transmitter functions, but in the reverse order. It must be capable of selecting one from among many rf signals, amplify it, demodulate it, and process it to recover the original intelligence in a proper form. The receiver must also cope with noise, interference sources, and a variety of other problems arising from the environment, the mode of operation, and so forth.
Superheterodyne AM Receiver
All of the basic requirements of an AM radio receiver for voice communications are met by the equipment whose block diagram appears above. A weak rf signal from the antenna is amplified and demodulated. The recovered modulation waveform is further amplified and the applied to the speaker. Tuning to a different radio frequency requires only that the rf amplifier be tuned. However, since the rf amplifier would necessarily require two or more stages of amplification, each must be individually tuned.
In the diagram above, it has only one rf amplifier stage and an intermediate frequency (IF) amplifier which normally consists of at least two stages. Although the rf amplifier stage is tuned to whichever rf carrier frequency is desired, the IF amplifier operates at the fixed frequency. The carrier frequency signal from the rf amplifier is converted to the IF in the mixer. The mixer is a nonlinear device which produces the IF by combining the carrier frequency, FC with the local oscillator frequency, FLO. The difference frequency is the IF.
Thus FIF=FLO-FC which assumes that FLO is greater than FC. However, additional frequency products, stemming from FC and FLO originates in the mixer. These include:
- The original frequencies FC and FLO.
- The sum frequency FC + FLO in addition to FIF.
- Harmonics of the frequencies in 1. and 2. above.
Only the difference frequency is accepted by the IF amplifier and other mixer products are rejected. This feature of the IF amplifier makes the superheterodyne receiver practical. It also creates new difficulties in that additional, undesirable components at, or near, the IF are sometimes produced in the mixer as the result of particular interfering frequencies that enter the receiver through the front end, i.e., from the antenna.
It will be recalled that sidebands are generated by the modulation process which are essential in the convergence of information. These side bands occupy the spectrum on one side of the carrier for SSB modulation, and on both sides for AM. During frequency translation, accomplished by the mixer, each side band frequency component, together with the carrier, is moved by exactly the same amount. Therefore, the spectra associated with the I.F signal have exactly the same shape as that transmitted with the rf carrier.
In other that the superheterodyne AM receiver functions properly, FLO, as produced by the local oscillator, must be positioned so that it is greater than FC by the same amount for any FC to which the receiver is tuned. This means that the tuning of all circuits preceding the mixer, including those in the rf amplifier, and those circuits of the local oscillator, be coordinated to maintain this frequency difference. Following amplification by the I.F amplifier stages, demodulation occurs, of course, the method for demodulation will depend on the form of modulation employed.
However, in simple superheterodyne receivers having one mixer, one local oscillator and one I.F amplifier, there is a pronounced technical advantage in positioning the local oscillator frequency above the carrier frequency. This occurs because the ratio of the highest to the lowest local oscillator frequencies required over the receiver tuning range is less than would otherwise be the case. Similarly, the relative range of tuning capacitance for the local oscillator is less a decided advantage.
General Performance Specifications / Objective of Project
The purpose of this project is to provide a general overview of the basic receiver functions expressed in terms of performance.
Selectivity is a measure of the receiver’s ability to select the desired, modulated rf signal to the exclusion of undesired emissions in the same general frequency range. As a familiar example, a standard AM broadcast receiver will be tuned to a specific carrier frequency in the 540Hz to 1600Khz range. Assume that this frequency is 1000Khz to be specific. Elsewhere there might be one or more stations broadcasting on 990Khz and others on 1010Khz. The receiver must be capable of rejecting these neighboring transmissions without significant interference to the selected channel centered at 1000Khz.
Sensitivity is the ability of the receiver to correctly receive weak signals. This would appear to be a question of designing sufficient gain into the receiver to amplify the received signal from any transmitter, however distant it might be. The key to achieving the best sensitivity possible is the reduction of the internally generated receiver noise to the lowest amount possible. Noise performance of an AM receiver, in as much as it is influenced by design, is established by those parts of the receiver preceding the I.F amplifier.
Practical radio receivers and especially communications type receivers, embody a substantial number of control and other functions that improve their operations in some way. These additional functions, together with refinements in the consideration of the basic receiver, require the definition of more performance specifications which will be encountered later in the chapters.
Finally, an ideal amplifier will reproduce at its output an amplified, but otherwise identical, version of the waveform at the input terminals. There should be no additional noise in the output. There should be no distortion of the waveform. Additionally, there should be no interaction of different frequency signals caused by amplifiers nonlinearities so that different and unwanted products appear in the output.
Summary Conclusion And Recommendation
Amplitude Modulation (AM) receiver has the functions of selecting the desired signal from all the other available signals, amplifying and demodulating it, and displaying it in the desired manner. In the Amplitude Modulation (AM) receiver, the incoming signal voltage is combined with a signal generated in the receiver to produce a lower fixed frequency. The signal at this intermediate frequency contains the same modulation as the carrier, and it is now amplified and detected to reproduce the original information.
In the mixer stage, the input frequencies are combined with the output of the local oscillator, at a frequency Fo, to generate components at a large number of few frequencies. The frequencies generated are components at the sum and the difference of the wanted signal and the local oscillator frequencies i.e., FO+FS and FO-FS. The difference frequency is known as the IF and is selected by the IF amplifier.
The amplified output of the IF amplifier is applied to the detector circuit where the information contained in the modulation signal is recovered. The detected signal is amplified to the required power level by the audio-frequency amplifier and is then fed to the loudspeaker, earphone or other output device.
An AM receiver covers the broadcast band from 550Khz to 1650Khz.
A typical AM receiver consist of a tuning circuit, an rf amplifier, mixer, local oscillator, IF amplifier, detector incorporating AGC line, audio frequency amplifier and a loudspeaker.
The bandwidths of the rf and the IF amplifiers are considerably different.
In AM receiver, only the difference frequency is accepted by the IF amplifier and the other mixer products are rejected.
The IF signal output is an amplified composite of the modulated rf from the transmitter in combination with the local oscillator signal.
In other for AM receiver to function properly, the local oscillator’s frequency must be positioned so that it is greater than the carrier frequency by the same amount for any carrier frequency to which the receiver is tuned.
The circuit should be assembled on a solderless breadboard first, so that changes can easily be made.
Short connection must be provided for the rf section with the antenna loop a couple inches away from the board.
The output of a radio receiver must always contain some noise, therefore the AM receiver must be designed so that the output signal-to-noise ratio is always at least as good as the minimum figure required for the system.
The rf stage must couple the aerial to the receiver in an efficient manner. It must suppress signals at or near the image on the intermediate frequencies. It must provide and operate linearly to avoid the production of cross modulation.
Design And Construction Of Amplitude Modulation Receiver
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