A digital phase meter has been designed to satisfy stringent requirements for measuring differences between phases of radiofrequency (RF) subcarrier signals modulated onto laser beams involved in the operation of a planned space-borne gravitational- wave-detecting heterodyne laser interferometer. The capabilities of this system could also be used in diverse terrestrial applications that involve measurement of signal phases, including metrology, navigation, and communications. The capabilities of this system include:
- Accuracy to within a millionth of a cycle (greater than the accuracy of any prior phase meter);
- High dynamic range of phase (limited only by numerical capabilities of a supporting computer);
- Wide input frequency range ( 10 kHz to 20 MHz),
- Maintenance of accuracy even when the frequency of a signal slews by as much as 700 kHz/s within this range; and
- The ability to distinguish and measure multiple tones on the same carrier with undiminished accuracy.
In the operation of this system, two RF modulated laser beams interfere on a photodiode; their frequency difference results in a RF heterodyne signal. The RF signal output by the photodiode is digitized at 40 Mega-samples/s, then fed into a field-programmable gate array (FPGA). The digitized heterodyne signal is split and multiplied by two adjustable local oscillators (LOs), which separates the signal into in-phase (I) cosine and a quadrature phase (Q) sine components. During stable operation at a small phase difference between the RF and LO signals, the Q multiplier output includes a low-frequency component proportional to the phase difference. This phase-difference output is used to update the LO frequency (and, thereby, the LO phase) to keep the local oscillators locked to the incoming signal. The tracking loop updates the LO frequency every 0.1 ms.
While this makes for a stable digital phase-locked loop, to achieve the desired micro-cycle phase accuracy, the residual phase-tracking error is combined with the LO phase. The residual phase difference is computed as the arctangent of the ratio between the Q and I signals, filtered appropriately, and added to the phase used to update the local oscillators.
For transmission to ground, the phase measurements are decimated from 40 MHz to the frequency-and-phase-update rate of 10 kHz, then again to a final output rate of 100 Hz. To suppress aliasing of noise in the signal band from 1 mHz to 1 Hz in the decimation from 40 MHz to 10 kHz, an antialiasing filter is applied prior to decimation. After the reconstruction of phase at 10 kHz and prior to the final decimation to 100 Hz, another finite-impulse-response anti-aliasing decimation filter is applied. The characteristics of the antialiasing filters are tailored to the phase noise of the input signal.
This work was done by Daniel Shaddock, Brent Ware, Peter Halverson, and Robert Spero of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
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4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-41927, volume and number of this NASA Tech Briefs issue, and the page number.