A Precision, Low-Cost GPS-Based Synchronization Scheme for Improved AM Reception. (2009)

This paper describes a highly accurate carrier-frequency synchronization scheme for actively, automatically locking multiple, remotely located AM broadcast transmitters to a common frequency/timing reference source such as GPS. The extremely tight frequency lock (to ~1 part in 109 or better) permits the effective elimination of audible and even sub-audible beats between the local (desired) station s carrier signal and the distant stations carriers, usually received via skywave propagation during the evening and nighttime hours. These carrier-beat components cause annoying modulations of the desired station s audio at the receiver and concurrent distortion of the audio modulation from the distant station(s) and often cause listeners to tune out due to the poor reception quality. Significant reduction or elimination of the beats and related effects will greatly enlarge the effective (interference-limited) listening area of the desired station (from 4 to 10 times as indicated in our tests) and simultaneously reduce the corresponding interference of the local transmitter to the distant stations as well. In addition, AM stereo (CQUAM) reception will be particularly improved by minimizing the phase shifts induced by co-channel interfering signals; HD will also benefit via reduction in beats from analog signals. The automatic frequency-control hardware described is inexpensive ($1-2K),more » requires no periodic recalibration, has essentially zero long-term drift, and could employ alternate wide-area frequency references of suitable accuracy, including broadcasts from WWVB, LORAN-C, and equivalent sources. The basic configuration of the GPS-disciplined oscillator which solves this problem is extremely simple. The main oscillator is a conventional high-stability quartz-crystal type. To counter long-term drifts, the oscillator is slightly adjusted to track a high-precision source of standard frequency obtained from a specialized GPS receiver (or other source), usually at 10.000 MHz. This very stable local reference frequency is then used as a clock for a standard digitally implemented frequency synthesizer, which is programmed to generate the specific (AM broadcast) transmitter carrier frequency desired. The stability of the disciplining source, typically ~ 1 part in 109 to 1011, is thus transferred to the final AM transmitter carrier output frequency. Generally, an AM radio listener during the evening and nighttime hours, and to a lesser extent in the early morning, receives undesired skywave signals from several distant stations as well as the desired local (groundwave) signal. If all of these signals are within about 0.01-0.001 Hz of each other, any resulting carrier beats will be of such long periods that the beats will be effectively suppressed by the action of the receiver s AGC circuitry and thus be unnoticeable to the listener. Many modern, synthesizer-based transmitters can directly lock to the precision disciplined 10-MHz source, while older units usually require references at either1 e, 2 e, or 4 e the final frequency. In these latter cases, the existing transmitter crystal can usually be satisfactorily pulled via injection locking. The effectiveness of the synchronization concept to reduce interference effects was demonstrated in a laboratory test setup. Many hours of careful subjective listening were conducted, with the two interfering units both precisely on-frequency with the main unit (synchronous operation) and with the two interferers at various frequency offsets, from below 1 Hz to above 10 Hz.« less