REFERENCE SYSTEMS FOR LARGE DISTRIBUTED APPLICATIONS
Large frequency reference systems present unique challenges to the system designer. Large reference systems can involve hundreds of drops and can cover large factory areas involving tens of thousands of square feet and multiple buildings. Each drop will likely require accuracy and low noise. Many factory installations require redundancy. Having the frequency reference system fail would stop product testing and shipments - an unacceptable situation for most locations. The integrity of the system is essential. You must know that each drop is functional. The system must be robust and survive routine abuse such as faults and transients. The system must be self-monitoring to detect a degradation of performance that might render the system unavailable. Large distances between the reference and the source create problems. Fiber optics offer a solution to long distances and noise immunity.
Designing the system can be broken down into three elements - the master reference, distribution amplifiers and monitoring.
- GNSS locking is essential. Whether you end up selecting an atomic reference or OCXO, GNSS locking brings unmatched long-term stability cost effectively.
- Atomic source: Due consideration should be given to an atomic source to be certain the reference system has sufficient holdover stability during periods of GNSS loss. The loss could be weather related or antennas might be compromised. An atomic reference will assure the best short-term stability that will meet most applications.
- Redundancy: Atomic and OCXO references have become extremely reliable. Physics packages routinely last more than ten years. But despite the quality of the design, a failure is possible and the lead-time for atomic references can be excessive. Redundancy for a demanding manufacturing application is not a luxury but rather a must for most applications.
- Monitoring: Somehow you must know if the reference is right. Further, you must know that the reference has been compromised and long-term statistical measurements are essential. These measurements must be independent and operate from a separate time base.
- Dual power sources: A loss of primary power cannot be ignored. Select a reference that operates from AC power but consider having a DC back-up. Frequency references consume very little power, so a battery back-up is generally a small fraction of the overall system cost.
- The amplifiers must contribute minimal noise to the master reference.
- High channel count is desirable. Each time the signal is split or amplified, the reference is degraded. Multi-channel amplifiers reduce the number of times the signal must be amplified and split.
- Channel monitoring to the individual channel allows the system designer to detect when a channel has been faulted or cabling has been degraded.
- Redundancy: The down aside of multi-channel amplifiers is that a failure takes out many drops. Amplifiers with built-in monitoring and automatic switch over prevents a failure on one channel from taking a significant number of drops off line.
- Dual- power sourcing from AC and DC.
Cabling, Lock-in Amplifers
- Improperly shielded reference lines that run hundreds of feet across an electrically noisy factory can degrade a reference to the point of being unacceptable.
- Do not use inexpensive cable in reference systems. Cable is less expensive than distribution amplifiers. Best practices for shielding and cabling with minimal losses is critical.
- For extremely long cable runs or environments that are very noisy, consider a fiber optic link. You will lose some phase noise performance, but ideally less than a standard cable run in an extreme environment.
- Consider lock-in amplifiers to recover a reference that has been compromised. The lock-in amplifier will restore your phase noise and maintain synchronization to the master reference.
Fiber Optic Channels
Frequently a sine wave is not the most desirable format for a reference. In some applications, a very high slew rate is required and a square wave is needed.
Very often copper coaxial cable is not the ideal transport means for a reference. In an electromagnetically active environment, copper coaxial cable can pick-up noise and degrade the reference. Fiber optic cable is basically immune to electromagnetic interference.
The NF0100 was designed to easily convert a reference signal to a more compatible format:
Sine wave to a square wave
Square wave to a sine wave
Sine wave to a fiber optic signal- 650,1300 nm -single and multi-mode
Fiber optic to a sine or square wave
Pulsed waveform (such as PPS) to fiber optic
Pulsed fiber optic back to electrical pulse
The NF0100-LN converts from copper to fiber and from fiber back to copper with minimal phase noise degradation