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There is an increasing need to generate high spectral purity microwave signals at arbitrary, user-defined frequencies and phases to:

  • increase the resolution in Doppler and multi-static radar:
  • improve the performance of high spectral efficiency advanced communication systems;
  • increase the precision and accuracy in microwave spectroscopy; and
  • improve time and frequency metrology of microwave atomic clocks and oscillators. 

The current state-of-the-art in agile electronic frequency synthesis relies on a radio or microwave frequency source (such as quartz crystal) as the basis of the synthesis chain.  This limits the spectral purity and frequency stability of the resulting synthesized signal.  This signal generator replaces the traditional microwave reference with an optically derived array of equally spaced radio/microwave frequencies by photodetecting a train of optical pulses.  Such frequency lines have demonstrated spectral purity and stability orders of magnitude greater than traditional electronic sources, but do not exhibit the broadband frequency agility of traditional electronic synthesizers. 

This invention adds frequency agility to the optically derived microwave signals while maintaining the high spectral purity and stability.  This is done by using one of the frequency tones to clock a direct digital synthesizer (DDS) to generate any frequency within approximately half the clock frequency.  The frequency tuning range is then extended by mixing the output of the DDS with any other of the photonically generated microwave tones.  When the frequency output of the DDS covers the line spacing of the optical derived microwave tones, any frequency within the photodetection bandwidth may be generated with spectral purity orders of magnitude better than the current state of the art for electronic frequency synthesizers.

The figure below illustrates the signal generator that includes optical pulse source (4) to produce optical pulses (6) to illuminate photosensitive element (8) (e.g., a photodetector such as a photodiode) that is provided to generate electrical signal (10) that includes an optical pulse repetition rate FR and harmonics. Electrical signal (10) is communicated from photosensitive element (8) is transmitted to frequency selector (16) that isolates a selected harmonic HF, which can be selected by frequency selector controller (24). Frequency selector (16) produces a frequency that is transmitted to frequency converter (22) as harmonic HF2 and to DDS (18) as harmonic HF1. Here, frequency selector (16) splits a power of harmonic HF included in electrical signal (10) and communicates harmonic HF1 to DDS  (18) at clock source (20) of DDS (18). In an embodiment, harmonic HF1 received by clock source (20) is not a same harmonic that is transmitted to frequency converter (22) as harmonic HF2.

In a certain embodiment, harmonic HF1 sent to DDS (18) from frequency selector (16) is a same frequency as harmonic HF2 that is transmitted to frequency converter (22). First output Out1 produced by DDS (18) is received by frequency converter (22), and the frequency of harmonic HF2 is shifted by the frequency of Out1 from DDS (18). Second output Out2 is an output frequency of frequency converter (22) and is increased or decreased relative to harmonic HF2 by the frequency of first output Out1 from DDS (18), as selected. Second output Out2 produced by frequency converter (22) can be filtered, e.g., by tunable filter (32), or an adjustable gain can be applied to second output Out2 from frequency converter (22) by adjustable gain amplifier (34).



A signal generator includes an optical pulse source to provide a plurality of optical pulses; a photosensitive element configured to receive optical pulses and to produce an electrical signal from optical pulses, electrical signal including a spectrum that includes a plurality of discrete frequencies that occur at a repetition rate corresponding to that of the optical pulses or a harmonic thereof; a frequency selector to receive the electrical signal from the photosensitive element, to select dynamically the harmonic from the electrical signal and to communicate the dynamically selected harmonic; a direct digital synthesizer (DDS) to receive the harmonic of the electrical signal from the frequency selector and to produce a first output; and a frequency converter to receive the harmonic from the frequency selector and the first output from the DDS, wherein the frequency converter shifts a frequency of the harmonic by an amount substantially equal to a frequency of the first output from the DDS to produce a second output.

Scott Diddams, Franklyn Quinlan, Tara Fortier, Antoine Rolland
Patent Number: 
Technology Type(s): 
Electron Physics, Electronics, Optical Frequency Measurements, Optical Technology, Micro- and Optoelectronics
Internal Laboratory Ref #: 
Patent Issue Date: 
August 14, 2018
Lab Representatives
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