Methods of Transmission: FM or Frequency Modulation
Understanding Frequency Modulation (FM): A Comprehensive Guide
Frequency Modulation (FM) is a key technology in the world of radio communications, offering a unique way to modify a carrier signal. In this blog post, we’ll dive into the fundamentals of FM, explore its various applications, and compare it to other modulation techniques. Want to learn more about modulation? Check out the first post in our Methods of Transmission series and get back to the basics of sending information over the radio waves.
How does FM work, and what are its applications?
It turns out there are two ways to modify a radio signal, commonly called a carrier. In a previous post, we talked about modifying the level or amplitude of the carrier, which is known as AM. The other way is to change the frequency of the carrier, known as FM or frequency modulation.
In its simplest form, the frequency of the transmitter is controlled by the modulating signal. For example, as the voltage of the modulating signal rises, the transmitter’s frequency increases. As the voltage falls, the frequency decreases. A common variation of FM is PM or phase modulation. As the voltage of the modulating signal increases, the phase of the transmitted signal is advanced (a momentary frequency increase). As the voltage falls, the phase is retarded (a momentary frequency decrease).
What are the advantages and disadvantages of FM?
The most significant advantage of FM is its immunity to changes in signal levels, which makes it less susceptible to noise and interference and eliminates the need for gain control circuitry found in AM receivers designed to compensate for varying signal levels. FM also exhibits something called “capture effect.” If two signals are on the same frequency, and one is stronger than the other by a certain amount, the stronger signal “wins,” and the other is suppressed. In this way, a distant FM transmitter will not interfere with a local station, a definite advantage for FM broadcasting.
These advantages come at a cost: The amount of spectrum occupied by an FM signal is much larger than that of an AM signal. For example, an AM signal with the same audio bandwidth as a modern FM station would occupy about 36 kHz of spectrum space. The same signal from an FM station would occupy over 200 kHz, more than five times the signal bandwidth!
Would a “single sideband” version of FM cut the occupied spectrum in half, like it does with AM?
Nice try, but no cigar! In an AM signal, the sidebands are identical mirror images. In FM, the sidebands are not mirrored, and if you try to remove one of them, the signal information is destroyed.
Is there a way to send data using FM?
Just as data can be sent using various forms of amplitude modulation, there are multiple ways to send data using FM. Frequency shift keying, or FSK, is the simplest method commonly used in various applications, such as paging systems. In an FSK system, a zero is represented by one frequency, and a different frequency represents one that shifts a specific amount from the first frequency (hence the frequency “shift” keying). Multiple frequencies can be used, with the different frequencies representing more than one bit at a time.
Other variations include phase shift keying, where the data is encoded via changes in the phase of the carrier, as well as hybrid forms, such as quadrature amplitude modulation (QAM), where amplitude and phase shifts are combined to send many bits of data in each transmitted symbol. The downside of these systems is that they are more complex to implement, and they don’t perform as well under adverse noise conditions as systems that do not send so many bits per symbol.
There are as many modulation systems as there are inventors to dream them up. Deciding what type of modulation to choose can be a complex and daunting task. Need help deciding what works best for your application? Let us help!