PLL mathematical analysis to achieve capture state. Comparison between frequency modulation and phase modulation. Overview: Analog Communication System mins. Basic Communication System mins. Need of Modulation mins.
Multiplexing mins. Fourier transform mins. Band limiting process mins. Mathematical analysis of Modulation mins. Channel mins. Distortionless transmission mins. Equalisation mins. Non-linear distortion mins. Reasons for interference mins. Types of modulation mins. Amplitude modulation mins. Singletone and multitone modulation mins.
Power distribution in AM signal mins. Modulation efficiency mins. Questions on power of AM signal mins.The demodulation of single-sideband SSB signals requires special attention, because simple mixing leads to superposition of the upper and lower sidebands at audio frequencies.
The following article gives an overview of the different methods for SSB demodulation and their use in software defined radios. SSB modulators and demodulators are sometime also referred to as image reject mixers. The filter method is the traditional SSB reception method in analog superhet receivers.
Typically a first mixer translates the signal to an intermediate frequency IF first.
Single Sideband Modulation, SSB Demodulation
At the IF a sharp band-pass filter e. Then the second mixer converts the remaining sideband to audio frequencies. If required, the audio spectrum can be inverted by using high-side LO injection in one of the mixer stages. The filter method requires the availability of very sharp filters at the IF and is widely spread in the analog domain, where quartz filters are used. Sideband suppression is determined by the sharpness and attenuation of the filter at the unwanted sideband.
The phasing method uses complex IQ processing to resolve the superposition of lower and upper sideband at audio frequencies. The incoming signal is directly converted to audio frequencies by a complex mixer creating I and Q components.
The complex mixer requires a sine and cosine LO e. The following low-pass filters with a width of one sideband determines the final bandwidth. This is hardly practical with analog components.
However in the digital domain, the LO phase shift can easily be realized by a sine and cosine waveform. This makes the phasing method interesting for implementation in a software defined radio.
In practice already small phase deviations lead to prohibit very good sideband suppression, especially for frequencies at the frequency borders of the sidebands. Sideband suppression for the phasing method with different phase and amplitude errors shows the vulnerability to imbalances.
Even a tiny amplitude error of only 0. It differs a little bit from the first two methods since it does not resolve any superposition of lower and upper sideband. Instead the Weaver simply converts a portion of the spectrum to audio frequencies without any ambiguities. The Weaver demodulator uses two complex mixing stages. The first mixer stages translates the signal in order to center it at zero frequency. A second mixer stage translates the signal in order to align its frequencies with the audio frequencies.
Summing or subtracting I and Q selects the orientation of the output spectrum normal or inverted. The Weaver method is very well suited for software defined radios, because digital mixers and their sine and cosine LO signals can be generated with extremely high phase and amplitude accuracy.
The same holds for the two identical low-pass filters. Thus no gain and phase variations occur and the circuit is perfectly balanced for very high performance.The carrier does not contain any information and its transmission results in loss of power.
Thus only sidebands are transmitted that contains information. This results in saving of power used in transmission. This saved power can be inserted into the 2 sidebands. Hence, ensuring a stronger signal that transmits over long distances.
As during suppression, the baseband signal does not get affected in any way. As we know that transmission power and bandwidth are the two important parameters in a communication system. Hence, resulting in a considerable power loss. By frequency shifting property of Fourier transform.
From the above equation, it is clear that only 2 components are present in the spectrum. The mathematical representation of the signal at the output of the product modulator is given as. Thus, at the output, the DSB-SC wave contains a signal whose frequency is twice the frequency of the baseband signal. The carrier without any information content is suppressed by a balanced modulator.
Its principle of operation is such that, when two signals of the different frequency are passed through a non-linear resistance then an amplitude modulated signal with the suppressed carrier is achieved at the output. A non-linear device has the capability to produce 2 sidebands with a carrier.
On substituting the above-given value of i 1 and i 2 in the output equation, we will have. Thus, from the above expression, it is clear that output voltage is a combination of modulating signal along with the DSB-SC signal.
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Leave a Reply Cancel reply Your email address will not be published.We assume that this locally generated carrier is exactly coherent or synchronized in both frequency and phase with the original carrier wave c t used to generate the DSB-SC wave. This method of detection is therefore called as coherent detection or synchronous detection.
The output of the product modulator is applied to the low pass filter LPF which eliminates all the unwanted frequency components and produces the message signal. This phase difference has been measured with respect to the original carrier c t at the DSB-SC generator.
The above equation shows that the output of product modulator i. Hence, this is the wanted term. The second term is an unwanted one. Signal m t is then passed through a low pass filter, which allows only the first term to pass through and will reject the second term.
Let us consider the expression for the output of coherent detector is given by :. In practice, the phase error varies randomly with time due to the random variations taking place in the communication channel. This is undesirable.
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Double Sideband Suppressed Carrier (DSB-SC) Modulation
Sasmita Hi!You can understand the background to the characteristics of the signal. To run the applet, click the picture at the bottom of this page. When amplitude modulation is carried out, the modulating signal spectrum is shifted to the carrier frequency band.
DSB-SC is so named because the modulating signal extends symmetrically on both sides of the carrier wave. There is some confusion surrounding the various systems of amplitude modulation. The types can be summarized as follows. If the amplitude of the carrier wave is simply changed and mathematised, it is as follows.
The first equation shows a spectrum where only the modulation signal frequency Fm is separate on either side of the carrier frequency Fc. However, synchronous detection is necessary on the receiving end, which involves a system with advanced technology and the associated costs. It offers even better electrical efficiency and frequency band efficiency than DSB.
In addition, with digital modulation, modulation is performed at relatively low frequency, and this method is also used to up-convert the signal to a radio frequency. As with DSB-SC, the modulating signal spectrum is shifted directly to the carrier frequency band without loss.LECT-13 : DEMODULATION OF SSB WAVE
The symbol for amplitude modulation is a multiplier, but actual circuits use the nonlinearity of transistors or diodes, or switching operations. For example, if a modulating signal is input to a circuit using non-linear amplification, several higher harmonics are generated in the output due to the amplification characteristics of the circuit.
This higher harmonic contains a sum or difference component of the information signal frequency and carrier frequency, and if a suitable filter is used to eliminate the unwanted components, the correct amplitude modulation waveform can be obtained. Synchronous detection demodulates the received signal by multiplying it with a carrier frequency which has the same frequency and phase as the transmission carrier wave. Asynchronous detection includes envelope detection and rectification detection.
With asynchronous detection, the information signal m t must be incorporated in the envelope of the receiving waveform. However, synchronous detection requires complex, costly circuits. With DSB-SC modulation, the amplitude of the carrier wave is shifted proportionally to the amplitude of the modulating signal using a modulator. A transistor or diode can be used as a modulator.Since, the two sidebands carry the same information, there is no need to transmit both sidebands.
We can eliminate one sideband. The process of suppressing one of the sidebands along with the carrier and transmitting a single sideband is called as Single Sideband Suppressed Carrier system or simply SSBSC. It is plotted as shown in the following figure.
In the above figure, the carrier and the lower sideband are suppressed.
Hence, the upper sideband is used for transmission. Similarly, we can suppress the carrier and the upper sideband while transmitting the lower sideband. This SSBSC system, which transmits a single sideband has high power, as the power allotted for both the carrier and the other sideband is utilized in transmitting this Single Sideband.
Let us consider the same mathematical expressions for the modulating and the carrier signals as we have considered in the earlier chapters. The quality of the signal gets affected unless the SSB transmitter and receiver have an excellent frequency stability.
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Previous Page Print Page.To demodulate SSB transmissions a beat frequency oscillator or carrier insertion oscillator is used along with a mixer or product detector. As described in previous pages, single sideband is a form of amplitude modulation where the carrier and one sideband have been suppressed or reduced in level. In order to demodulate single sideband, SSB, it is necessary to reintroduce the carrier.
To achieve this two main elements are required:. When receiving SSB it is necessary to have a basic understanding of how a receiver works.
Single Sideband Modulation, SSB
Most radio receivers that will be used to receive SSB modulation will be of the superheterodyne type. Here the incoming signals are converted down to a fixed intermediate frequency. Many radio receivers will have a switch to select this, other receivers will have a BFO pitch control which effectively controls the frequency.
The BFO needs to be positioned to be in the correct position for when the signal is in the centre of the receiver passband. This typically means that it will be on the side of the passband of the receiver. Once this has been done, then the main tuning control of the receiver can be used, and once a signal is audible with the correct pitch, then it is also in the centre of the receiver passband. Adjust the main tuning control so that the pitch is correct, and the signal should be comprehensible.
SSB demodulation basics As described in previous pages, single sideband is a form of amplitude modulation where the carrier and one sideband have been suppressed or reduced in level. Single sideband modulation In order to demodulate single sideband, SSB, it is necessary to reintroduce the carrier. The same type of oscillator was used for SSB demodulation and the term persisted.
It is a more correct term than BFO, but not was widely used. The output from the mixer is the demodulated audio signal. Producing a product or multiplication of the value of the inputs is the function of a mixer, but the term product detector was used in the early days of the use of SSB, and has remained in use, although it is less widely used than it was previously.
With a little practice it should be possible to easily tune in SSB signals. Supplier Directory For everything from distribution to test equipment, components and more, our directory covers it.