Frequency Modulation: History, definition, Equation, Advantages
Table of Contents
Although it only has a few problems, FM, or frequency modulation, has been around roughly since AM (Amplitude Modulation). Apart from our inability to recognize the FM transmitter potential, FM itself had no issues. In the early days of wireless communication, it was determined that a smaller bandwidth was needed in order to reduce interference and noise. Such a measure reduced frequency modulation while increasing AM. After that, an American engineer named “Edwin Armstrong” completed a deliberate effort to determine the power of FM transmitters. Edwin came up with the idea of using FM for transmitting, which was against the current fashion at the time.
What is a Frequency Modulation?
The carrier signal’s frequency is changed proportionally to (in accordance with) the input modulating signal’s amplitude, which is the definition of frequency modulation. A single tone sine wave serves as the input. The following figure also displays the carrier and the FM waveforms.
As the amplitude of the modulating (input) signal increases, a carrier’s frequency (fc) will rise. When the input signal is at its strongest, the carrier frequency (fc max) will be at its highest level. Maximum deviation from the carrier’s normal value is observed.As the amplitude of the modulating (input) signal decreases, the carrier frequency will also decrease. When the input signal is at its weakest, the carrier frequency (fc min) will be at its lowest value. Minimum deviation from the carrier’s typical value. When the input signal value is 0V, the carrier’s frequency will be at fc, which is its normal value (free-running). The carrier doesn’t deviate in any way. Figure depicts the FM wave’s frequency at the input’s maximum, 0V, and minimum.
Below is a block diagram of frequency modulation. While the carrier signal comes after the message signal and does not contain any data, the message signal contains the specific data. These signals will be modulated, producing an FM modulated signal. This signal has a higher level of importance because its frequency fluctuates according to its amplitude. The unit of measurement for this frequency change is kHz (kilohertz). For instance, if there is a 3 kHz up/down frequency variation, it is denoted as 3 kHz.
When radio first came out, static was the main issue, and everyone tried to reduce the effects of static by narrowing the bandwidth. Therefore, less noise was transmitted to the receiver using this method. American engineer Edwin Armstrong studied this issue and whether FM rather than AM might provide a benefit. With the aid of FM, he simply began to develop the concept in 1928, and rather than reducing the BW, he increased it. However, for a variety of reasons, others do not accept his ideas. He immediately approached Radio Corporation of America (RCA), who were impressed by his concepts but didn’t want to devote any resources to a new form of broadcasting because RCA was solely focused on television.
He finally managed to launch a radio station in 1939 to demonstrate the effectiveness of FM. to maintain this station along with others using a range of frequencies between 42 MHz and 50 MHz. However, the FCC in the USA changed the designated frequency band between 88 MHz and 108 MHz after the war.
Even so, there was some minor discomfort because thousands of radios had been sold and the current VHF FM band was widely accepted. Additionally, a specific type of narrowband FM rose to fame for UHF and VHF mobile communications.
- Frequency deviation is the degree of the modulating signal’s amplitude-induced change in the carrier frequency.
- As the input’s amplitude changes, the carrier frequency fluctuates between fmax and fmin.
- The difference between fmax and fc is known as frequency deviation. fd = fmax – fc
- Similarly, the difference between fc and fmin also is known as frequency deviation. fd = fc –fmin
- It is denoted by Δf. Therefore Δf = fmax – fc = fc – fmin
- Therefore fd = fmax – fc = fc – fmin
|Modulating Signal Amplitude
|Frequency of Carrier
|Nil (Center frequency)
|+ 5 MHz
|─ 2 V
|– 5 MHz
Freq deviation = 105 -100 = 5 MHz (or) Freq deviation = 95-100 = -5 MHz
Frequency Modulation Equation
The FM equation include the following
v = A sin [ wct + (Δf / fm) sin wmt ]
= A sin [ wct + mf sin wmt ]
A = Amplitude of the FM signal. Δf = Frequency deviation
mf = Modulation Index of FM
mf = ∆f/fm
mf is called the modulation index of frequency modulation.
wm = 2π fm wc = 2π fc
What is the Modulation Index of Frequency Modulation?
The FM modulation index is calculated as the carrier’s frequency deviation divided by the modulating signal’s frequency.
mf = Modulation Index of FM = ∆f/fm
Frequency Modulation in Communication Systems
Analog frequency modulation and digital frequency modulation are the two types of frequency modulation techniques used in telecommunications. A sine carrier signal that is continuously changing can be used to modulate the data signal in analog frequency modulation. This carrier signal has various characteristics, such as frequency, amplitude, and phase, which are primarily used to produce AM and PM.
There are three types of digital frequency modulation: FSK (Frequency Shift Key), ASK (Amplitude Shift Key), and PSK (Phase Shift Key), which functions analogly. In contrast to digital modulation, which is used to transmit binary signals like 0 and 1, analog modulation is typically used for AM, FM, and short-wave broadcasting. The phase-locked loop and varactor diode oscillator are FM techniques. The diode is arranged in the circuit to change the frequency in the varactor diode oscillator technique. Therefore, this method just offers narrowband transmissions. It offers an excellent FM when using the PLL technique. Phases are therefore restricted in the loop in this method to change the frequency.
FM in Vibration Analysis
the process of gauging the strength of the entire machine as well as its individual components by measuring and analyzing the vibration signals’ intensities, patterns, and frequencies. This type of analysis is especially beneficial for rotating machinery where there may be defective devices that are the cause of amplitude and frequency modulation deviations. The demodulation technique is used to extract the data from the modulated carrier signal because it can detect these modulation frequencies directly.
The Bandwidth of Frequency Modulation Signal
One of the key components of an FM signal is bandwidth. Although the sidebands in an FM signal will extend to infinity on either side, their strength will gradually wane. Fortunately, it has the potential to limit the BW of an FM signal without significantly altering its value. Remember that the bandwidth of an FM signal, which is expressed in Hertz (Hz), is the difference between its highest and lowest frequency components. With bandwidth, only frequencies are discussed. AM has been found to have a 2 fm bandwidth and only two sidebands (USB and LSB).
It is not as easy in FM. The figure depicts the FM signal spectrum, which is quite complicated and has an infinite number of sidebands.This illustration illustrates how the spectrum widens as the modulation index rises. Sidebands are separated from the carrier by fc ± fm, fc ± 2fm, fc ± 3fm, and so on.
Only the first few sidebands are regarded as significant sidebands because they are the ones that will carry the majority of the power (98% of the total power). According to Carson’s Rule, or the thumb rule, 98% of the FM signal power is contained within a bandwidth that is equal to the deviation frequency plus the modulation frequency multiplied by two.
Carson’s rule: Bandwidth of FM BWFM = 2 [ Δf + fm ].
= 2 fm [ mf + 1 ]
Advantages of Frequency Modulation
The advantages of frequency modulation include the following.
- For a given transmitter power, the service areas are clearly identified.
- Less noise and interference
- The radiated power is less.
- FM has a lower power requirement than amplitude modulation.
- Between nearby stations, there is less geographic interference.
- Guard bands separate nearby FM channels.
- An improved signal-to-noise ratio, such as 25 dB, in relation to human-made intrusion
- It has the potential to use effective RF amplifiers, including signals that are modulated in frequency.
- At the transmitter’s low power phase, modulation is simple to use:
Disadvantages of Frequency Modulation
The disadvantages of frequency modulation include the following.
- High equipment cost is high
- The receiving area of the FM signal is small.
- High bandwidth
- Much more Bandwidth (as much as 20 times as much).
- For improved communication, FM antennas should be kept close together.
- The spectral efficiency of FM is lower than that of some other modulation formats:
- More complicated receiver and transmitter.
- Other modes have improved data spectral efficiency, including:
- Requires more complicated demodulator:
- FM’s spectral efficiency is inferior to that of other modulation techniques.
- Sidebands expand to infinity any side
- High data spectral efficiency and other modes
- Sidebands extend to infinity on either side
- It uses a more complicated demodulator: