Types of Modulation: Amplitude modulation: Frequency ... · PDF filevs VSB modulation Offset...
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Modulation: Modulation is a process of mixing a signal with a sinusoid to produce a new signal. This new signal, conceivably, will have certain benefits over an un-modulated signal. The modulation equation is, E=Asin(ωt+φ) We can see that this sinusoid has 3 parameters that can be altered, to affect the shape of the graph. The first term, A, is called the magnitude, or amplitude of the sinusoid. The next term, ω is known as the frequency, and the last term, φ is known as the phase angle. All 3 parameters can be altered to transmit data. Types of Modulation: There are 3 basic types of modulation: Amplitude modulation, Frequency modulation, and Phase modulation. Amplitude modulation:
A type of modulation where the amplitude of the carrier signal is modulated (changed) in proportion to the message signal while the frequency and phase are kept constant.
Frequency modulation: A type of modulation where the frequency of the carrier signal is modulated (changed) in proportion to the message signal while the amplitude and phase are kept constant.
Phase modulation: A type of modulation where the phase of the carrier signal is varied accordance to the low frequency of the message signal is known as phase modulation.
Feature AM FM PM Function Amplitude of carrier wave
varies as per amplitude or voltage of modulating signal input.
Frequency of carrier wave varies as per voltage of modulating signal input.
Phase of carrier wave varies as per voltage of modulating signal input.
frequency of carrier wave is kept constant
amplitude of carrier wave is kept constant
amplitude of carrier wave is kept constant
Types AM types include DSB-SC, SSB, and VSB etc. Refer DSB-SC vs SSB-SC and SSB
Digital FM types include FSK, GFSK,
Digital PM types include BPSK, QPSK, QAM(combination of
vs VSB modulation Offset FSK etc. Refer MSK and GMSK modulation
amplitude and phase modulation types) Refer BPSK and QPSK, QAM modulation types.
Equation =(1+msin t)sin t Δf=kf
Super heterodyne receiver: A super heterodyne receiver (often shortened to superhet) is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. Design and principle of operation: Super heterodyne receiver the diagram has blocks that are common to super heterodyne receivers. The antenna collects the radio signal. The tuned RF stage with optional RF amplifier provides some initial selectivity; it is necessary to suppress the image frequency and may also serve to prevent strong out-of-pass band signals from saturating the initial amplifier. A local oscillator provides the mixing frequency; it is usually a variable frequency oscillator which is used to tune the receiver to different stations. The frequency mixer does the actual heterodyning that gives the super heterodyne its name; it changes the incoming radio frequency signal to a higher or lower, fixed, intermediate frequency (IF). The IF band-pass filter and amplifier supply most of the gain and the narrowband filtering for the radio. The demodulator extracts the audio or other modulation from the IF radio frequency; the extracted signal is then amplified by the audio amplifier. Circuit description: To receive a radio signal, a suitable antenna is required. The output of the antenna may be very small, often only a few microvolts. The signal from the antenna is tuned and may be amplified in a so-called radio frequency (RF) amplifier, although this stage is often omitted. One or more tuned circuits at this stage block frequencies that are far removed from the intended reception frequency. In order to tune the receiver to a particular station, the frequency of the local oscillator is controlled by the tuning knob (for instance). Tuning of the local oscillator and the RF stage may use a variable capacitor, or varicap
diode. The tuning of one (or more) tuned circuits in the RF stage must track the tuning of the local oscillator. Optical isolator: An optical isolator, or optical diode, is an optical component which allows the transmission of light in only one direction. It is typically used to prevent unwanted feedback into an optical oscillator, such as a laser cavity. The operation of [some of] the devices depends on the Faraday Effect (which in turn is produced by magneto-optic effect), which is used in the main component, the Faraday rotator. Theory: The main component of the optical isolator is the Faraday rotator. The magnetic field, B, applied to the Faraday rotator causes a rotation in the polarization of the light due to the Faraday effect. The angle of rotation, β, is given by, β =vBd
Figure 2: Faraday rotator, with a polarizer and an analyzer. Where, v is the Verdet constant of the material (amorphous or crystalline; solid, liquid, or gaseous) of which the rotator is made, and d is the length of the rotator. This is shown in Figure 2. Specifically for an optical isolator, the values are chosen to give a rotation of 45°. It has been shown that a crucial requirement for any kind of optical isolator (not only the Faraday isolator) is some kind of non-reciprocal optics. Polarization dependent isolator: The polarization dependent isolator, or Faraday isolator, is made of three parts, an input polarizer (polarized vertically), a Faraday rotator, and an output polarizer, called an analyzer (polarized at 45°).
Light traveling in the forward direction becomes polarized vertically by the input polarizer. The Faraday rotator will rotate the polarization by 45°. The analyzer then enables the light to be transmitted through the isolator. Light traveling in the backward direction becomes polarized at 45° by the analyzer. The Faraday rotator will again rotate the polarization by 45°. This means the light is polarized horizontally (the rotation is sensitive to direction of propagation). Since the polarizer is vertically aligned, the light will be extinguished. Figure 2 shows a Faraday rotator with an input polarizer, and an output analyzer. For a polarization dependent isolator, the angle between the polarizer and the analyzer, β, is set to 45°. The Faraday rotator is chosen to give a 45° rotation. Polarization dependent isolators are typically used in free space optical systems. This is because the polarization of the source is typically maintained by the system. In optical fiber systems, the polarization direction is typically dispersed in non-polarization maintaining systems. Hence the angle of polarization will lead to a loss. Tuned radio frequency receiver:
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. Early examples could be tedious to operate because when tuning in a station each stage had to be individually adjusted to the station's frequency, but later models had ganged tuning, the tuning mechanisms of all stages being linked together, and operated by just one control knob.
How it works:
Block diagram of TRF receiver The classic TRF receivers of the 1920s and 30s usually consisted of three sections:
one or more tuned RF amplifier stages. These amplify the signal of the desired station to a level sufficient to drive the detector, while rejecting all other signals picked up by the antenna.
a detector, which extracts the audio (modulation) signal from the radio carrier signal by rectifying it.
optionally, but almost always included, one or more audio amplifier stages which increase the power of the audio signal.
Leutz 9-tube receiver from 1927 clearly shows the component parts of a TRF set. Each RF stage is in a separate compartment. Within each compartment can be seen (from top): the triode tube, the interstage coupling coil, and the capacitor attached to its front panel tuning dial. The compartments contain (from left): the 4 RF stages, the detector stage, and the 4 tube audio amplifier. The capacitors could be linked to a common shaft and tuned together, or tuned separately. Each tuned RF stage consists of an amplifying device, a triode (or in later sets a tetrode) vacuum tube, and a tuned circuit which performs the filtering function. The tuned circuit consisted of an air-core RF coupling transformer which also served to couple the signal from the plate circuit of one tube to the input grid circuit of the next tube. One of the windings of the transformer had a variable capacitor connected across it to make a tuned circuit. A variable capacitor (or sometimes a variable coupling coil called a variometer) was used, with a knob on the front panel to tune the receiver. The RF stages usually had identical circuits to simplify design.
Each RF stage had to be tuned to the same frequency, so the capacitors had to be tuned in tandem when bringing in a new station. In some later sets the capacitors were "ganged", mounted on the same shaft or otherwise linked mechanically so that the radio could be tuned with a single knob, but in most sets the resonant frequencies of the tuned circuits could not be made to "track" well enough to allow this, and each stage had its own tuning knob.
The detector was usually a grid-leak detector, consisting of a triode tube biased near cutoff so it would only conduct on the positive half of the RF cycles. Some sets used a carborundum crystal detector (semiconductor diode) instead. Occasionally, a regenerative detector was used, to increase selectivity.
Some TRF sets that were listened to with earphones didn't need an audio amplifier, but most sets had one to three transformer-coupled or RC-coupled audio amplifier stages to provide enough power to drive a loudspeaker.
The schematic diagram shows a typical TRF receiver. This particular example uses six triodes. It has two radio frequency amplifier stages, one grid-leak detector/amplifier and three class ‘A’ audio amplifier stages. There are 3 tuned circuits T1-C1, T2-C2, and T3-C3. The second and third tuning capacitors, C2 and C3, are ganged together (indicated by line linking them) and controlled by a single knob, to simplify tuning. Generally, two or three RF amplifiers were required to filter and amplify the received signal enough for good reception.