Digital Amplitude Modulation and Digital Frequency Modulation | Soukacatv.com

Though based on the same concepts, digital modulation waveforms look quite different from their analog counterparts.

Though far from extinct, analog modulation is simply incompatible with a digital world. We no longer focus our efforts on moving analog waveforms from one place to another. Rather, we want to move data: wireless networking, digitized audio signals, sensor measurements, and so forth. To transfer digital data, we use digital modulation.

We have to be careful, though, with this terminology. “Analog” and “digital” in this context refer to the type of information being transferred, not to the basic characteristics of the actual transmitted waveforms. Both analog and digital modulation use smoothly varying signals; the difference is that an analog-modulated signal is demodulated into an analog baseband waveform, whereas a digitally modulated signal consists of discrete modulation units, called symbols, that are interpreted as digital data.

There are analog and digital versions of the three modulation types. Let’s start with amplitude and frequency.
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Digital Amplitude Modulation

This type of modulation is referred to as amplitude shift keying (ASK). The most basic case is “on-off keying” (OOK), and it corresponds almost directly to the mathematical relationship discussed in the page dedicated to analog amplitude modulation: If we use a digital signal as the baseband waveform, multiplying the baseband and the carrier results in a modulated waveform that is normal for logic high and “off” for logic low. The logic-high amplitude corresponds to the modulation index.

Time Domain

The following plot shows OOK generated using a 10 MHz carrier and a 1 MHz digital clock signal. We’re operating in the mathematical realm here, so the logic-high amplitude (and the carrier amplitude) is simply dimensionless “1”; in a real circuit you might have a 1 V carrier waveform and a 3.3 V logic signal.

You may have noticed one inconsistency between this example and the mathematical relationship discussed in the [[Amplitude Modulation]] page: we didn’t shift the baseband signal. If you’re dealing with a typical DC-coupled digital waveform, no upward shifting is necessary because the signal remains in the positive portion of the y-axis.

 Frequency Domain

Here is the corresponding spectrum:

Compare this to the spectrum for amplitude modulation with a 1 MHz sine wave:

Most of the spectrum is the same—a spike at the carrier frequency (f_C) and a spike at f_C plus the baseband frequency and f_C minus the baseband frequency. However, the ASK spectrum also has smaller spikes that correspond to the 3rd and 5th harmonics: The fundamental frequency (f_F) is 1 MHz, which means that the 3rd harmonic (f₃) is 3 MHz and the 5th harmonic (f₅) is 5 MHz. So we have spikes at f_C plus/minus f_F, f₃, and f₅. And actually, if you were to expand the plot, you would see that the spikes continue according to this pattern.

This makes perfect sense. A Fourier transform of a square wave consists of a sine wave at the fundamental frequency along with decreasing-amplitude sine waves at the odd harmonics, and this harmonic content is what we see in the spectrum shown above.

This discussion leads us to an important practical point: abrupt transitions associated with digital modulation schemes produce (undesirable) higher-frequency content. We have to keep this in mind when we consider the actual bandwidth of the modulated signal and the presence of frequencies that could interfere with other devices.

 Digital Frequency Modulation

This type of modulation is called frequency shift keying (FSK). For our purposes it is not necessary to consider a mathematical expression of FSK; rather, we can simply specify that we will have frequency f₁ when the baseband data is logic 0 and frequency f₂ when the baseband data is logic 1.

 Time Domain

One method of generating the ready-for-transmission FSK waveform is to first create an analog baseband signal that switches between f₁ and f₂ according to the digital data. Here is an example of an FSK baseband waveform with f₁ = 1 kHz and f₂ = 3 kHz. To ensure that a symbol is the same duration for logic 0 and logic 1, we use one 1 kHz cycle and three 3 kHz cycles.

 The baseband waveform is then shifted (using a mixer) up to the carrier frequency and transmitted. This approach is particularly handy in software-defined-radio systems: the analog baseband waveform is a low-frequency signal, and thus it can be generated mathematically then introduced into the analog realm by a DAC. Using a DAC to create the high-frequency transmitted signal would be much more difficult.

A more conceptually straightforward way to implement FSK is to simply have two carrier signals with different frequencies (f₁ and f₂); one or the other is routed to the output depending on the logic level of the binary data. This results in a final transmitted waveform that switches abruptly between two frequencies, much like the baseband FSK waveform above except that the difference between the two frequencies is much smaller in relation to the average frequency. In other words, if you were looking at a time-domain plot, it would be difficult to visually differentiate the f₁ sections from the f₂ sections because the difference between f₁ and f₂ is only a tiny fraction of f₁ (or f₂).

 Frequency Domain

Let’s look at the effects of FSK in the frequency domain. We’ll use our same 10 MHz carrier frequency (or average carrier frequency in this case), and we’ll use ±1 MHz as the deviation. (This is unrealistic, but convenient for our current purposes.) So the transmitted signal will be 9 MHz for logic 0 and 11 MHz for logic 1. Here is the spectrum:

 Note that there is no energy at the “carrier frequency.” This is not surprising, considering that the modulated signal is never at 10 MHz. It is always at either 10 MHz minus 1 MHz or 10 MHz plus 1 MHz, and this is precisely where we see the two dominant spikes: 9 MHz and 11 MHz.

But what about the other frequencies present in this spectrum? Well, FSK spectral analysis is not particularly straightforward. We know that there will be additional Fourier energy associated with the abrupt transitions between frequencies. It turns out that FSK results in a sinc-function type of spectrum for each frequency, i.e., one is centered on f₁ and the other is centered on f₂. These account for the additional frequency spikes seen on either side of the two dominant spikes.

 Summary

• Digital amplitude modulation involves varying the amplitude of a carrier wave in discrete sections according to binary data.
• The most straightforward approach to digital amplitude modulation is on-off keying.
• With digital frequency modulation, the frequency of a carrier or a baseband signal is varied in discrete sections according to binary data.
• If we compare digital modulation to analog modulation, we see that the abrupt transitions created by digital modulation result in additional energy at frequencies farther from the carrier.

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Source: allaboutcircuits

The difference between Digital and Analog Modulation, and Modulation and Demodulation Definition

Electronic devices produce messages like analog baseband signals in the form of audio, video or even messages can be in the form of digital bits from computer. To send these messages we must have some communication channel like wires, co-axial cable, even wireless radio waves, microwaves or infrared. We can easily transmit messages through wires or cables. Voice, Video, bit streams from computer are having lower frequency band and can travel few distance with wires but cannot be sent through wireless media. Voice signal has lower Bandwidth therefore it will not propagate through space and will be attenuated. To transmit voice signal a large size antenna is required as antenna length is proportional to half of wavelength. The size of the antenna will be more than the distance between transmitter and receiver. Again when more than one transmitter is involved all station will overlap in one frequency band. For those above reasons we choose a carrier, which is a high frequency radio wave, can travel long distance without attenuation and as the frequency is high smaller antenna is required. Selecting different carrier frequency for different transmitting stations can eliminate overlapping of frequency band.

Problem:

1. Voice, Video, bit streams from computer are having lower frequency band
2. They canntravel few distance with wires but not cannot propagate through space
3. Antenna size is half of wavelength thus antenna length for Voice, Video, bit streams would be impractical
4. Assume we transmit Voice, Video, bit streams over an imaginary antenna but being in the same frequency range all channels will overlap

Solution:

1. carrier signal is used to carry signal to long distance
2. Modulation is used with a selected carrier frequency signal to mix baseband with carrier
3. carrier frequency is in higher frequency radio wave length and thus antenna size would be smaller
4. Radio waves can travel longer distance with very less att
5. Radio wave has a wide range of frequencies to select individual non-overlapping channels

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Modulation:

Now we have to develop some way to send the information of message signal via this carrier signal. The carrier signal is a high frequency sinusoidal signal represented by amplitude, frequency and phase. We can vary one of this parameter accordingly with the message information.

What is Modulation?

Modulation is an operation of varying amplitude or frequency or phase of carrier signal according to the instantaneous amplitude of the baseband signal/modulating signal.

Here baseband signals comes from a audio/video or computer. Baseband signals are also called modulating signal as it modulates carrir signal. career signals are high frequecy radio waves it generally comes from a radio frequency oscillators. These two signls are combined in modulator. Modulator takes the instantenious amplitute of baseband signal and varies amplitute/frequency/phase of career signal. Resultant signal is a modulated signal. It goes to an RF-amplifier for signal power boosting and then feed to antenna or a co-axial cable.

There are two types of modulation analog and digital. Analog modulation delas with the voice, video and regular waves of base band signals. Where as digital modulations are with bit streams or symbols from computing vevices as base band signals.

DeModulation:

Demodulation is the opposite process of modulation. Modulator is a part of signal transmitter where as demodulator is the receiving side. In broadcast system radio tranmitting station does to modulation part. A radio receiver acts as a demodulator. A modem receives signals and also transmits signals thus it does modulation and demodulation at the same time. Thus the name modem has been given. A radio antenna receives low power signal. A co-axial cable end point can also taken as an signal input. An RF amplifer boosts the signal amplitude. Then the signal goes to a demodulator. demodulator does the reverse of modulation and extracts the backband signal from career. Then the base band signal is amplified to feed a audio speaker or video moitor or TTL/CMOS signal levels to match computer inpts.

What is De-modulation?

Demodulation is the opposite process of modulation where the varying amplitude, frequency or phase of carrier signal is extracted to construct the original the message signal.

Analog Modulation:

Analog modulation refers to the process of transferring analog low frequency baseband signal, like an audio or TV signal over a higher frequency carrier signal such as a radio frequency band. Baseband signal is always analog for this modulation.

There are three properties of a carrier signal amplitute, frequency and phase thus there are three basic types of analog modulations.

1. Amplitude Modulation (AM)
2. Frequency Modulation (FM)
3. Phase modulation (PM)
   

Digital modulation:

Analog modulation refers to the process of transferring digital low frequency baseband signal, like digital bitstream from computers over a higher frequency carrier signal such as a radio frequency band. Digital modulation in somewhat similar to the analog modulation except base band signal is of discrete amplitude level. For binary signal it has only two level, either high or logic 1 or low or logic 0. The modulation scheme is mainly three types.

1. ASK or Amplitude shift Key
2. FSK or Frequency shift key
3. PSK or Phase shift key

ASK or Amplitude shift Key:

When the carrier signal's instantaneous amplitude is varied in proportion to message signal m(t). We have the modulated carrier m(t)cosw_ct where cosw_ct is the carrier signal. As the information is an on-off signal the output is also an on-off signal where the carrier is present when information is 1 and carrier is absent when information is 0. Thus this modulation scheme is known as on-off keying (OOK) or amplitude shift key.

Application:

1. Used in our infrared remote controls
2. Used in fibre optical tranmitter and receiver.

FSK or Frequency shift key:

When Data are transmitted by varying instantaneous frequency of the carrier, we have the case of frequency shift key. In this modulation carrier has two predefined frequency w_c1and w_c2. When information bit is 1 carrier with w_c1 is transmitted i.e. cosw_c1 and When information bit is 0 carrier with w_c0 is transmitted i.e. cosw_c0

Application:

1. Many modems used FSK in telemetry systems

PSK or Phase shift key:

The instantaneous phase of the carrier is shifted for this modulation. If the base band signal m(t) =1 carrier in phase is transmitted. If m(t)=0 carrier with out of phase is transmitted i.e. cos(w_ct+П). If phase shift is done in 4 different quadrants then 2bit of information can be sent at a time. This scheme is a special case of PSK modulation known as QPSK or Quadrature Phase Shift Key.

Application:

1. Used in our ADSL broadband modem
2. Used in satellite communication
3. Used in our mobile phones

Established in 2000, the Soukacatv.com (DSW) main products are modulators both in digital and analog modulators, amplifier and combiner. We are the leading communication supplier in manufacturing the headend system in China. Our 16 in 1 and 24 in 1 now are the most popular products all over the world. For more, please access to https://www.soukacatv.com/.

Source: http://www.equestionanswers.com/notes/modulation-analog-digital.php