Maintaining fixed phase differences between microwave signals generated at remote sites

Accurate phase comparison of signals received at two or more widely separated points becomes exceedingly difficult at distances greater than a few hundred wavelengths. A relatively simple solution has been developed using closed-loop compensation of transmission medium irregularities to maintain fixed phase relationships between remote sites. In principle, the output,
K_{1}
, of a local oscillator at one of the stations is locked in phase to the received signal,
K_{s}
, and a reference signal
K_{r}
;
(K_{1} = K_{s} - K_{r})
.
K_{1}
is also transmitted to a central site (becoming
K_{1}'
), where another oscillator output,
K_{2}
, is locked to it
(K_{2} = K_{1}')
.
K_{2}
is transmitted back to the receiving site (becoming
K_{2}'
) and, mixed with
K_{1}
, generates the previously mentioned reference signal (
K_{2}' - K_{1} = K_{r}
). The phase of the signal at the central site,
K_{2}
, then bears a known phase relationship with the received signal,
K_{s}
. Several such systems using a common central site thus provide a means for transmitting phase coherent signals from many remote sites to one central site. Conversely, the system can be used to transmit phase coherent signals to several remote sites from one central site. The scheme has obvious advantages in both receiving and transmitting (or both) systems which require effective antenna apertures of thousands of wavelengths. The signal transfer takes place at carrier frequency and does not require additional low frequency data links. A mathematical analysis of the system is given, including sources of error and parameters limiting the ultimate accuracy. This accuracy is determined by the number of wavelengths between sites and the information bandwidth required. With proper design, the phase-lock loops incorporated in the system will introduce negligible phase errors for reasonably stable carrier frequencies. A multivibrator phase detector is particularly suited to the method and will provide a coherent system with a noise bandwidth determined by the "post-detection" bandwidth. Design criteria are derived for given system tolerances. A sample design problem is solved for a 75,000-wavelength path, in the 2200-mc band. Experimental results show a phase uncertainty of much less than one electrical degree under these conditions. The system is composed of conventional circuitry and can be easily adapted to any frequency range.