Introduction
Problem Statement
Frequency modulation is a type of signal encoding. Since its introduction and popularization throughout the first half of the twentieth century, FM has gained significant popularity and became widely used in multiple areas. Most prominently featured in the field of radio communication, mainly sound broadcasting and two-way communication, it has found application in video broadcasting and recording, sound synthesis, medicine, data transmission, both analog and digital, telemetry, military surveillance and detection systems, and, to a smaller degree, in many other fields. While being only marginally suitable for digital signal transmission, which is becoming dominant in the modern world, and demonstrating several inherent drawbacks which limit its use, frequency modulation’s potential is far from exhausted.
Thesis of Paper
While the simplicity, robustness, reliability, and relatively low cost of the technology suggest that it will remain in use in the majority of its current applications, several new and promising ways of utilizing it exists, which may be worth further research and development, and require in-depth insight.
Methodology
The method of research used in this paper is a review of secondary sources. The FM technology is a well-established concept, with most of its applications having a long and well-documented history and properly highlighted incredible scientific and technical sources. The main focus of the research will be the technical and scientific publications, as well as reliable web sources. This method will allow for a comprehensive understanding of the topic and the reliability of the presented information.
Motivation and Objectives
The main reason the review of frequency modulation is the focus of the research is the rise in popularity of the digital media, which requires the novel approach and means of operation. In this light, the analog devices and technologies seem to lose popularity among the general public. While not being of significant value, such a predisposition still has its influence on the prioritizing and progress of the implementation and utilization process. Meanwhile, the technology in question still offers interesting possibilities.
Background Information
History of the Technology
Frequency modulation is a technique of encoding information into a carrier wave. Several methods of such encoding exist, with the two most popular being changing the amplitude of the base wave, or amplitude modulation, and changing its frequency, or FM. Both of them have their strengths, and have competed in the early twentieth century, but have eventually divided the areas of application. Initially, the AM method was in broad use.
The reasons for this was the apparent obviousness of such approach (changing the amplitude was deemed a more “straightforward” approach), and the disadvantage suffered by FM in the conditions of early days of broadcasting. At the time when standards were not settled, the initial assumption was that narrowing of the bandwidth was the key to lowering the noise and interference. The amplitude-modulated signal needed a comparatively narrow channel, so it was preferred for some time.
The frequency-modulated signal, on the other hand, required a relatively broad bandwidth – in fact, the quality of the signal relied heavily on it. Several attempts to find the possible advantages of the FM failed, as the researchers all focused on what is now known as the narrow-band FM. This remained true until the work by Edwin Howard Armstrong, an electrical engineer, and inventor, who was interested in improving the AM broadcast capabilities, which were common at the time, but suffered from severe static interference. Initially failing at creating the augmentation for AM transmitters, he then switched his attention to frequency modulation.
After several years of work, he succeeded in shaping what is now known as a wide-band FM. After a series of demonstrations, it has become apparent that the frequency modulation offered unprecedented clarity and fidelity of the signal. Murray Crosby later contributed to the popularization of FM by introducing the concept of “threshold” – the level of signal strength which allowed for the unusually high signal to noise ratio. After Armstrong’s work has gained enough supporters and sufficient body of research, has confirmed the superiority of the FM signal, the technology has obtained financial support, and the equipment of the radio stations was eventually updated. This, unfortunately, coincided with the rise of television and the decline of the radio industry in general, but the FM is still in use primarily in the field of radio broadcasting industry, as well as other instances of audio transmissions.
Principles of operation
Principal Differences from AM
The frequency modulation is the superimposition of the signal onto the frequency of the carrier wave. Obviously, the amplitude of the carrier remains the same, which is the main difference from amplitude modulation. This allows for several advantages, like the amplitude-based interference and the possibility to use the amplitude limiter to further remove the errors. While the amplitude remains stable, the difference in frequencies, also known as the deviation, may vary depending on the application. In the case a high deviation is implemented, the resulting signal is deemed “wide-band” frequency modulation or WBFM. This is what gave FM the advantage over AM – the more the deviation, the better performance is possible and less noise is introduced. WBFM is used for audio broadcasting.
The Deviation
The standard deviation for the very high-frequency radio stations, operating between 88.5 and 108 MHz is ±75 kHz, meaning it can go 75kHz up and down the frequency spectrum. The wideband transmissions are usually given as much as 200 kHz of the spectrum. The narrow-band transmissions, used for two-way communication among other things, have a much smaller deviation of about ±3 kHz. While providing a substantially lower quality of audio, the decreased deviation allows for better spectrum efficiency.
Modulation Methods
The process of modulation can be achieved id different ways, which are commonly referred to as direct and indirect modulation. The distinction is based on the fact that in the former case the signal is modulated “directly” while the latter implies the “synthesized” FM. The direct method is achieved by using a voltage-controlled oscillator (VCO). The information is fed into a VCO, and the modulated signal is generated as a result. The indirect method comprises several ways of synthesizing a modulated signal. One of the most common has an intermediate link in the form of the phase-modulated signal. Then, the resulting signal is processed by a crystal-controlled oscillator to generate a narrow-band FM signal.
This signal is then passed through a frequency multiplier to convert it to a wideband FM. Another approach, known as Armstrong method, produces a synthesized frequency modulated signal by first generating an AM with no carrier, also known as a double-sideband suppressed-carrier transmission, or DSB-SC, then inserting the 90° carrier wave to obtain a phase modulation. The multiplier is then applied to the resulting signal, effectively turning it into a frequency-modulated signal of sufficient deviation, while the frequency translation allows the placement of the signal at the desired carrier if necessary.
The indirect method of modulating a signal has several advantages – the produced signal is generally of higher quality, as the correct mean carrier frequency can be maintained. At the same time, the direct method is deemed as a simpler and less expensive alternative. However, the complexity of the system, as well as the cost of equipment, was an issue mainly in the earlier days of the technology, and since the 1950s has mostly been irrelevant. Thus, the indirect methods are preferred as their cost-efficiency is surpassing that of direct VCO modulation.
Capabilities and Perspectives
As already mentioned before, the independence of amplitude modulation allows for a clearer signal by applying a limiter. If the required amplitude is known, the equipment can be tuned to exclude the artifacts that exceed a certain limit (assuming they are interference-based), providing the additional clarity [1]. The most obvious method for doing this is making the receiver frequency-sensitive but amplitude-insensitive. For example, using a high gain intermediate frequency amplifier will result in the signal running into limiting after being amplified significantly, and the excessive amplitudes are cut out.
Another augmentation method that is commonly used in FM broadcasting to decrease noise is pre-emphasis and de-emphasis. Specifically, this technique targets the noise that results from the highest audio frequencies of the source signal. Such noise is obviously proportional to the audio frequency, so one of the ways to decrease it is to boost the peak frequencies before the transmission and reduce them accordingly on the receiving end. The former process is known as pre-emphasis while the latter – as de-emphasis.
The effect is achieved by using the capacitor-resistor network. While passing through the CR filter, all the cut-out frequencies, determined by the time constant, are boosted by 6 dB per octave and fall back by the same amount when received. The time constant almost universally recognized is 50 µs, with 75 µs in the U.S. and South Korea [2]. While not yielding the ultimate difference, this simple augmentation allows for an even clearer signal in comparison to already inferior AM.
Finally, wideband FM is capable of transmitting a multi-level signal thanks to the method of multiplexing. The multiplexing process consists of mathematically encoding several channels of data into a single broad channel, transmitting it in a single bulk, and then spitting it back into individual channels once received by applying the same calculations. In the case of the radio signal, this is used to broadcast the stereo sound. The transmission consists of the dual-channel audio, where the left and right channels are combined, and the difference between the two.
This was done to leave the mono speakers fully compatible with the signal. In the case of the receiver equipped with just one speaker, the combined audio from both channels is played, and the remaining difference is ignored. In case the receiver is stereo-compatible, the received difference is added to the sum to obtain the data for the left channel and subtracted from the same value to produce the sound for the right one. The main part, containing the sum of both channels, occupies the range of 30 Hz to 15 kHz, while the difference used to calculate the separate channels is amplitude modulated onto a double-sideband suppressed-carrier (DSB-SC) signal [3].
The multiplexed signal has a lower signal-to-noise ratio and is generally more susceptible to interference, including the multipath distortion, so the receivers are usually equipped with the option to switch between the mono and the stereo reception in case the latter is of unacceptable quality. Furthermore, the pre-emphasis technique mentioned above is applied to both channels before muxing to make channel separation more precise, and de-emphasis is accordingly applied after the reception.
Demodulation Techniques
In order for the information encoded by frequency modulation to be read, or utilized in any way, it needs to first be extracted from the FM signal. The receiver thus needs to apply the process of demodulation. Several attempts exist to achieve demodulation through different circuits, and all of them have their advantages. The first concern, however, is the question of converting the frequency deviations into voltage variations.
The perfect scenario for this is a linear voltage-to-frequency dependence. In practice, of course, such linearity is not achievable, as the bandwidths of any system are finite, so instead the limited bandwidth is accounted for, and all the response to frequencies outside the threshold falls considerably and goes in decline within short range. The frequencies within the limit form the approximation of a linear dependency and are converted to voltage signals that are later transmitted to the speakers or other output equipment. The most common threshold for a VHF radio broadcast, for example, is 1 MHz with the typical signal being around 200 kHz in width.
The most widely used types of demodulation circuits are Foster-Seeley FM detector, ratio detector, slope FM detector, phase-locked loop FM demodulator, coincidence FM demodulator, and quadrature FM demodulator. Other types exist but are either obsolete or of very limited use in the modern world.
The simplest method of demodulation is tuning a circuit to a frequency slightly offset from the base carrier signal. This method is utilized by the slope detection method, as the signal moves up and down the “slope” of the tuned circuit. This leads to the amplitude variations as well as already existing frequency ones. Once the amplitude varieties are obtained, they can be demodulated using a simple diode circuit, a simple and reliable technology. The downside of this approach is the introduction of the amplitude interference, which, again, should be cut out to avoid noise. Furthermore, the output is not linear as the filter curvature factors in. This method also relies on the less powerful signal.
The advantage is the already mentioned simplicity, as the circuitry is most basic and only the AM detector is required. Nevertheless, such setup is only used to a limited degree nowadays, as its cost-effectiveness ratio is below acceptable in most implications. A much better result in terms of quality can be achieved by using the ratio discriminator, or detector. It requires more complex circuitry, including the wound inductors. The principal difference is the use of the transformer, which directs the detected signal through three windings.
One of them is up-tuned and tightly coupled with the primary one, which results in the phasing between the two pairs of windings being the same. The detected difference between the voltages of these couplings is subtracted from one side of the secondary winding and adds to the other, thus producing the output modulation. The unwanted RF signal that possibly appears in the process is filtered out by the four capacitors. The resulting signal is much more reliable and of higher fidelity than that of the slope detection method as it is fairly linear and does not introduce the amplitude-based interference, and is assembled relatively easily. However, the hardware required is relatively expensive to manufacture and has been since its introduction surpassed by cheaper alternatives of equal quality.
Another disadvantage is its reliance on discreet components while many products nowadays tend to incorporate the integrated circuits. Still, the radio detector sees limited use today in applications where these criteria can be ignored. The same could be said of the Foster-Seeley detector, which uses a very similar principle, also incorporates the transformer, and operates by matching two signals that are out of phase at 90 degrees and transforming the imbalances into voltages. A similar setup implies the mirrored disadvantages, such as the high cost of discreet components and the lack of easy integration.
This circuitry is rarely used today but remains a reliable and high-fidelity method despite the existence of better alternatives. Finally, the most recent development in the field is the integrated circuitry. When the technology allowed the manufacture of the demodulator capable of operating on a relatively high frequency of an FM receiver, the discreet-based technology became of lower demand. The most widely used application of integrated technology is the Phase-locked loop FM demodulator (PLLFM). As it incorporates the integrated VCO in its design, the PLL-based technology exhibits linearity that is enough for a high-fidelity signal, and, if the frequency deviation filter is correctly applied to include the wide FM signal, the resulting quality is sufficient for any modern hi-fi system relying on the radio transmission.
The cost of the PLL integrated circuit is also much lower than that of making use of wound inductors, so the overwhelming majority of the equipment manufactured nowadays relies on it. The alternative to PLL is the quadrature detector, a relatively simple construction which can be used with the integrated circuit. Essentially it combines two phase-shifted signals in a mixer, which produces the voltages based on the phase offset. The quadrature detector offers good linearity, little to no noise, is fairly simple to assemble and tune, but may require a set procedure. Besides, its construction incorporates a coil, which may not be acceptable in some designs. Otherwise, it is a demodulator of choice in many receivers today. [4]
Advantages and Applications
The main difference from the amplitude modulation method is the tremendous difference in the quality of the signal thanks to the reduced interference and the high signal-to-noise ratio. The reason for that is the fact that the majority of the interference is amplitude-based. For this reason, the FM signal is almost exclusively used in all the broadcasting that deals with analog sound.
Advantages over Other Methods
Despite requiring a formidable width of the signal, it surpasses AM by a wide margin in noise resilience. Furthermore, several applications of FM allow for some noise present, so even narrow-band FM may be acceptable. Furthermore, the frequency-based nature of the signal implies independence from signal strength: as long as the signal can be caught by the receiver, its quality remains unchanged. Another advantage is the ability to use non-linear amplifiers. The AM and other amplitude-based methods require linear amplification, which is less efficient while the FM does not have such restriction. Besides, the independence of linear amplifiers makes it possible to apply the modulation to a low-power stage of the transmitter.
Drawbacks
The FM also has certain disadvantages, like the distance of the broadcast, which is sufficiently lower than that of AM, but this has been effectively circumvented, and even made advantage of: the stations can control their broadcast ranges and the level of frequency pollution. Another disadvantage is the relative complexity of the demodulators, covered extensively in the previous chapter. While more complicated than their AM counterparts, they still yield a better result, so the bottom line cost efficiency is higher.
Current Applications of the Technology
Besides radio broadcasting, which was extensively covered in the paper, FM has many other applications in the modern world. Most of them are restricted to the domain of analog transmissions, but digital information can also be transmitted using FM, although other standards exist that tackle the task more effectively. The FM is also known to have somewhat limited spectral efficiency for digital data transmission. Thus, except for radio broadcasting, FM is most commonly used in two-way communication systems, like those installed in taxi cabs or the portable handheld transceivers.
The non-stationary nature of the signal of such mobile devices requires a high level of fidelity that cannot be achieved by using AM signal. The FM is also used in recording the audio information on magnetic tapes for both audio and video format. In the case of the videotape, the FM is used to store the audio track in VHS and other VCR systems. It is also suitable for storing the luminance information for a video signal due to the nature of the information, as luminance requires a wide frequency range.
Other applications of the FM include the telemetry and monitoring. Several methods of analyzing seismic activity and geologic reconnaissance utilize the frequency-modulated signal. The continuous-wave (CW) radars operate on FM principles, which gives them an advantage over pulsed radars, which cannot receive the signal while transmitting and thus fail to detect close-range targets [5]. The monitoring newborns for seizures via EEG also uses the FM signal.
Possible Future Application
The possible novel applications of FM is utilizing its principles in hearing devices. Among the drawbacks of a hearing, the device is the so-called “cocktail party effect” – the inability to hear among the sounds of a similar volume. It appears that the human anatomy accounts for this, and is able to apply listening selectively while the hearing devices do not have such a feature. Meanwhile, the FM exhibits what is known as the capture effect, in which the stronger signal overtakes the nearby weaker one when they are close enough on a frequency scale. While the concept is in its earliest stage [6], it is possible that this feature of FM can also serve as an effective solution for the currently existing problem.
Conclusion
The frequency modulation is a well-established technology with a strong background. Throughout the twentieth century, it has found a way into various parts of our everyday life. The current shift of prerogatives toward digital media implies that it is already diminishing in demand. Nevertheless, the multitude of applications it is currently used for, the low cost, simplicity, as well as some promising novel ways of utilizing it assure us that FM is still a sound choice and has at least some future potential.
References
- H. Silver and M. Wilson, The ARRL handbook for radio communications. Newington: American Radio Relay League, 2010.
- D. Withers, Radio spectrum management: management of the spectrum and regulation of radio services. London: The Institution of Electrical Engineers, 1999.
- U. Bakshi, Analog communication. Pune: Technical Publications Pune, 2009.
- I. Poole, Newnes Guide to radio and communications technology. Oxford: Elsevier, 2003.
- B. Boashash, Time-frequency signal analysis and processing – a comprehensive reference. Oxford: Elsevier, 2003.
- S. M. Schimmel, “Theory of modulation frequency analysis and modulation filtering, with applications to hearing devices,” Ph.D. dissertation, Dept. Elect. Eng., Univ. of Washington, Seattle, WA, 2007.