A document discusses a component of a laser metrology system designed to measure displacements along the line of sight with precision on the order of a tenth the diameter of an atom. This component, the phasemeter, measures the relative phase of two electrical signals and transfers that information to a computer.
Because the metrology system measures the differences between two optical paths, the phasemeter has two inputs, called measure and reference. The reference signal is nominally a perfect square wave with a 50- percent duty cycle (though only rising edges are used). As the metrology system detects motion, the difference between the reference and measure signal phases is proportional to the displacement of the motion. The phasemeter, therefore, counts the elapsed time between rising edges in the two signals, and converts the time into an estimate of phase delay.
The hardware consists of a circuit board that plugs into a COTS (commercial, off-the-shelf) Spartan-III FPGA (field-programmable gate array) evaluation board. It has two BNC inputs, (reference and measure), a CMOS logic chip to buffer the inputs, and an Ethernet jack for transmitting reduceddata to a PC. Two extra BNC connectors can be attached for future expandability, such as external synchronization. Each phasemeter handles one metrology channel. A bank of six phasemeters (and two zero-crossing detector cards) with an Ethernet switch can monitor the rigid body motion of an object.
This device is smaller and cheaper than existing zero-crossing phasemeters. Also, because it uses Ethernet for communication with a computer, instead of a VME bridge, it is much easier to use. The phasemeter is a key part of the Precision Deployable Apertures and Structures strategic R&D effort to design large, deployable, segmented space telescopes.
This work was done by Shanti Rao of Caltech for NASA’s Jet Propulsion Laboratory. NPO-45504
This Brief includes a Technical Support Package (TSP).
FPGA-Based Networked Phasemeter for a Heterodyne Interferometer
(reference NPO-45504) is currently available for download from the TSP library.
Don't have an account?
Overview
The document outlines the development of an FPGA-based networked phasemeter designed for use in a heterodyne interferometer, as part of NASA's efforts to enhance the precision of astronomical instruments like telescopes and interferometers. The primary challenge addressed is the need for extremely accurate distance measurements, which are essential for advanced astronomical observations.
To meet this challenge, the team has created a laser metrology system capable of measuring displacements with precision on the order of a tenth the diameter of an atom. A key component of this system is the phasemeter, which measures the relative phase of two electrical signals and relays this information to a computer. The phasemeter operates with two inputs: a Reference signal, which is a perfect square wave, and a Measure signal. By detecting motion and calculating the phase difference between these signals, the phasemeter can estimate the displacement accurately.
The hardware consists of a circuit board that connects to a Spartan-III FPGA evaluation board, featuring two BNC inputs for the Reference and Measure signals, a CMOS logic chip for buffering, and an Ethernet jack for data transmission to a PC. The design allows for future expandability with additional BNC connectors for synchronization purposes. Each phasemeter is capable of handling one metrology channel, and a bank of six phasemeters can monitor the rigid body motion of an object. The estimated hardware cost is approximately $150 per channel, making it a cost-effective solution compared to existing technologies.
The novelty of this phasemeter lies in its compact size and affordability, utilizing commercially available FPGA boards instead of custom-made circuit boards, which significantly reduces costs. The use of Ethernet for communication simplifies the setup compared to traditional VME bridges.
The document also mentions that further details about the software component of the invention can be found in related NASA Technical Reports (NTRs 45505 and 45575). For additional inquiries, contact information for the Jet Propulsion Laboratory is provided.
Overall, this document highlights a significant advancement in metrology technology, promising to enhance the capabilities of astronomical instruments through improved precision and cost-effectiveness.
