RDL's® ACM technology is the only time-proven, industry accepted method of controlling Synchronous AM Noise (the elusive FM transmission bandwidth variations that aggravate multipath artifacts in listeners' receivers). Hundreds of top performing FMs in every major U.S. market turn to ACM technology for maximum bandwidth performance. Every transmitter, new or old, can easily produce poor bandwidth, or can be made to perform as well as any signal in the market. Every FM station needs ACM technology to provide the accurate AM measurment and continuous monitoring needed for optimum transmitter tuning and performance.
The ACM-3 is a test instrument which monitors the amplitude component present on frequency modulated carriers. The AM component is representative of bandwidth characteristics in the transmission system. The technology of significant synchronous AM noise monitoring was pioneered by Radio Design Labs, the originator of AM noise monitors for FM radio and television aural transmission.
In theory, FM carriers are of a constant amplitude over the range of frequencies they swing above and below carrier frequency. In practice, however, the amplitude is compromised by a variety of factors. These amplitude modulations of the carrier comprise AM noise. Two general varieties of AM noise are considered: 1] Noise induced by power supplies and blower vibrations (noise not synchronized to the applied modulation); 2] AM Noise resulting from frequency modulation of the carrier (noise synchronized to the applied modulation, hence "synchronous" AM noise). This noise is produced by passband amplitude and phase non-linearities in the rf transmission system. Consequently interstage matching is a significant contributor to AM noise as is the phase delay inherent in high gain amplifier stages. These factors combine to produce an operating passband for the transmission system.
As the carrier frequency shifts with modulation, the carrier frequency (or actually resultant sidebands) at a given instant will fall on some point of the skirts of a tuned circuit. If the bandwidth of the tuned circuit is sufficient, the amplitude of the FM carrier will remain constant, or so nearly constant that the amplitude variation is not significant. If there is any roll-off to the passband of the tuned circuit, the carrier amplitude will change with shifting frequency resulting in amplitude modulation. The FM transmission system considered as a whole will exhibit different passband characteristics above the carrier frequency as it does below that frequency. Therefore the sidebands, both upper and lower, generated by a single cycle of modulation will be unequally attenuated. The resulting disparity produces a difference in carrier amplitude which can be demodulated in an FM receiver. The phase relationship between this demodulated signal and the recovered frequency modulation produces an audible mix which, in stereo systems, is then further demodulated into left and right. It is easily seen how these effects will degrade both stereo and audio receiver performance. The incoming amplitude modulations are often further affected by automatic gain control circuits in the receiver.