| 600-GHz Electronically Tunable Vector Measurement System |
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| Jun 01 2007 | |
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The RF excitation signal is radiated and made to pass through or reflect from an object of interest, and the response signal is mixed with the aforementioned subharmonic LO signal to generate a down-converted signal [denoted an intermediatefrequency (IF) signal in the radio art]. The RF and LO synthesizers are controlled from a laptop computer, which adjusts their frequencies to keep the IF constant at 450 MHz. Thus, the RF- and LO-synthesizer outputs differ in frequency by 450/36 = 12.5 MHz. The phase and amplitude measurements are performed indirectly, on signals derived from the IF signal, rather than directly on the RF response signal. For the purpose of generating a phase reference signal, portions of the outputs of the RF and LO synthesizers are mixed, yielding a 12.5-MHZ signal, which is then multiplied ×36 in frequency to obtain another 450- MHz signal. This 450-MHz signal cannot be used directly as the phase reference signal because the outputs of the inexpensive microwave synthesizers have such poor phase-noise characteristics that this signal and the 450-MHz IF signal are indistinguishable from the accompanying phase noise. Therefore, it is necessary to perform further processing as described next. In particular, it is necessary to further reference both the 450-MHz IF and the 450-MHz reference signal to a stable source before detection. This involves down-converting the raw 450-MHz reference signal by 12.79 MHz by use of a 12.79- MHz fundamental crystal oscillator, a mixer, and a band-pass filter. The resultant 437.21-MHz signal is then mixed with the 450-MHz IF signal. Inasmuch as the phase noises of the 437.21-MHz signal and the 450-MHz IF signal are totally correlated, mixing these two signals cancels out that noise, leaving a 12.79-MHz signal that has the same amplitude and phase characteristics (minus the synthesizer noise) as does the 450-MHz IF signal. It should be noted that any 450-MHz signal passing through the 437.21-MHz bandpass filter would be down-converted along with the 437.21-MHz signal, resulting in cross-talk and loss of dynamic range. It is therefore essential that the 437.21-MHz band-pass filter have extremely high rejection at 450 MHz. The 12.79-MHz signals in the response and reference channels are converted to a frequency of ≈66 kHz in a tracking downconverter, then detected by a lock-in amplifier that functions as a variable-bandwidth magnitude and phase receiver. The bandwidth and gain are controlled by a laptop computer. The vector DC outputs of the lock-in amplifier are read by an analog data-acquisition card in the computer, wherein these readings are converted to polar format. At maximum detection bandwidth, real-time acquisition speeds of >3,000 points per second are possible. This work was done by Robert Dengler, Frank Maiwald, and Peter Siegel 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 Refer to NPO-43394, volume and number of this NASA Tech Briefs issue, and the page number. This Brief includes a Technical Support Package (TSP).
600-GHz Electronically Tunable Vector Measurement System (reference NPO-43394) is currently available for download from the TSP library. Login first to download.
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