In the process of connecting widely distributed antennas into a coherent array, it is necessary to synchronize the timing of signals at the various locations. This can be accomplished by distributing a common reference signal from a central source, usually over optical fiber. A high-frequency (RF or microwave) tone is a good choice for the reference. One difficulty is that the effective length of the optical fiber changes with temperature and mechanical stress, leading to phase instability in the received tone. This innovation provides a new way to stabilize the phase of the received tone, in spite of variations in the electrical length of the fiber.
Stabilization is accomplished by two-way transmission in which part of the received signal is returned to the transmitting end over an identical fiber. The returned signal is detected and used to close an electrical servo loop whose effect is to keep constant the phase of the tone at the receiving end.
The technique is useful in large arrays of Earth-based antennas used for space communication or radio astronomy. It is also useful in any situation where precise timing information must be transferred over distances for which optical fiber transmission is appropriate (~10 m to 30 km). It has been used in a demonstration uplink array as part of a technology development for the Deep Space Network.
In the past, other techniques have been used for a similar purpose, but they involve either manipulation of the optical fibers, or they measure and record the phase variation rather than correcting it immediately via a closed-loop servo. The optical methods are generally slow, so they cannot correct rapid variations. The open-loop methods are less accurate, and they are not useful in situations where real-time correction is needed. The method described here is fast, accurate, and inexpensive to implement. A method similar in principle to this one has been reported earlier, but the new configuration is different and permits variable transmitted frequency and higher correction speed. Related round-trip stabilization methods have been used for signal transmission in coaxial cable and in waveguide
The new method is illustrated by the block diagram in Figure 1. The signal to be transmitted is at frequency f0. It is generated by mixing an input at f0 + f1 (from the first master synthesizer) with a voltage-controlled crystal oscillator (VCXO) at nominal frequency f1 (77.76 MHz in this implementation). It is then used to modulate a diode laser coupled to the outgoing fiber. At the far end of the fiber, the signal is optically split and half of its power is converted back to electrical form in a photodetector, producing the desired output. The other half is returned to the stabilization system over a nominally-identical fiber in the same cable. The returned signal is converted to electrical form, then downconverted in frequency by mixing with the same VCXO, and the result is compared in phase against an input from the second master synthesizer at f0 – f1. The phase difference is used to drive the frequency of the VCXO via filter H(f), forming a phase-locked loop. If the input signals are considered to have phase zero, it is easy to show that the loop will drive the phase θ of the VCXO so that 2θ = (2L)mod2π, where L is the electrical length of the line in each direction, causing the phase of the signal at the turn-around point to remain constant as L is changed. The actual phase there has a two-fold ambiguity; it could be either 0 or π. (These expressions neglect delays in the electronics, but those cancel if they are equal in the two directions. If they are unequal but stable, the output phase remains stable even though the electrical length of the fiber varies.) The important assumption is that the variation in fiber electrical length is the same in the two directions.