A2: ZK25 Group0n2-FM applications


Frequency modulation (FM) is a form of modulation which conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. This form of modulation is commonly used in the FM broadcast band.

In telecommunications and signal processing, frequency modulation (FM) conveys information over a carrier wave by varying its instantaneous frequency. This is in contrast with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant. In analog applications, the difference between the instantaneous and the base frequency of the carrier is directly proportional to the instantaneous value of the input signal amplitude. Digital data can be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequency-shift keying.

A signal modifies the frequency of a carrier in FM.


Edwin Howard Armstrong (1890–1954) was an American electrical engineer who invented wideband frequency modulation (FM) radio. He patented the regenerative circuit in 1914, the superheterodyne receiver in 1918 and the super-regenerative circuit in 1922. He presented his paper: "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation", which first described FM radio, before the New York section of the Institute of Radio Engineers on November 6, 1935. The paper was published in 1936.

As the name implies, wideband FM (WFM) requires a wider signal bandwidth than amplitude modulation by an equivalent modulating signal, but this also makes the signal more robust against noise and interference. Frequency modulation is also more robust against simple signal amplitude fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radio transmission: hence the term "FM radio" (although for many years the BBC called it "VHF radio", because commercial FM broadcasting uses a well-known part of the VHF band—the FM broadcast band).

FM receivers employ a special detector for FM signals and exhibit a phenomenon called capture effect, where the tuner is able to clearly receive the stronger of two stations being broadcast on the same frequency. Problematically however, frequency drift or lack of selectivity may cause one station or signal to be suddenly overtaken by another on an adjacent channel. Frequency drift typically constituted a problem on very old or inexpensive receivers, while inadequate selectivity may plague any tuner.

An FM signal can also be used to carry a stereo signal: see FM stereo. However, this is done by using multiplexing and demultiplexing before and after the FM process. The rest of this article ignores the stereo multiplexing and demultiplexing process used in "stereo FM", and concentrates on the FM modulation and demodulation process, which is identical in stereo and mono processes.

A high-efficiency radio-frequency switching amplifier can be used to transmit FM signals (and other constant-amplitude signals). For a given signal strength (measured at the receiver antenna), switching amplifiers use less battery power and typically cost less than a linear amplifier. This gives FM another advantage over other modulation schemes that require linear amplifiers, such as AM and QAM.
FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech. Normal (analog) TV sound is also broadcast using FM. A narrow band form is used for voice communications in commercial and amateur radio settings. In broadcast services, where audio fidelity is important, wideband FM is generally used. In two-way radio, narrowband FM (NBFM) is used to conserve bandwidth for land mobile radio stations, marine mobile, and many other radio services.

FM Multiplex with RDS on dScope
The ability to generate an FM Multiplex (MPX) for stereo FM radio reception testing is made possible by the extended audio bandwidth of the dScope Series III audio analyzer. This application note is an introduction to the process of creating an FM multiplex that enables the dScope to test FM stereo radio reception with the simple addition of an FM exciter. The inclusion of an RDS (Radio Data System) subcarrier allows the dScope to facilitate the testing of simple RDS functionality.


Prism Sound's dScope III audio analyzer has long been a test and measurement tool of choice with car audio manufacturers. In addition to the already feature-rich software within dScope for testing analogue and digital audio devices, this FM Stereo multiplex test script has been designed specifically to extend the spread of tests that can be conducted with one piece of test equipment, cutting the cost for production testing. The script enables test engineers to generate stereo FM signals and to send RDS data via an FM exciter to the device under test. The audio performance can be verified by the dScope automatically. The Visual Basic script generates a multi-tone stimulus encoded as an FM stereo multiplex (MPX) signal, enabling ultra-fast simultaneous measurement of signal level, distortion, signal-to-noise-ratio, frequency response and crosstalk at the output of the receiver under test.
In addition, an RDS sub-carrier can be included in the MPX, carrying basic information such as 'Program Service Name' for RDS decoder testing. The RDS reception can be checked on the devices RDS display.