Two airborne spectrum analyzers were developed for acquiring dynamic data, characterized by frequencies up to 25 kHz, from a hypersonic-crossflow-transition experiment aboard the air-launched Pegasus® space booster rocket (see Figure 1). Real-time transmission of time histories of the dynamic data via a pulse-code modulation (PCM) telemetry encoder would have required sample rates of at least 50,000 s -1. The telemetry bandwidth necessary to support rates as high as this was not available; however, conversion of the data to the frequency domain aboard the rocket would make it possible to trade frequency resolution for a reduction in telemetry bandwidth. The two airborne spectrum analyzers implement this type of conversion. One spectrum analyzer is based on a swept-tuned receiver; the other is based on digital signal processing technology. The remainder of this article describes the swept-tuned spectrum analyzer.
Spectrum analyzers are typically used for radio-frequency measurements. Because the measurement bandwidth in the hypersonic-crossflow-transition experiment extended only slightly beyond the audio range, commercially available analog function modules (rather than radio-frequency components) were used in constructing the swept-tuned spectrum analyzer. They included an analog multiplier instead of a mixer, a root-mean-square-to-direct-current (rms-to-dc) converter as an envelope detector, and a digital sine-wave generator combined with a voltage-to-frequency converter for a local oscillator (LO). In lieu of a conventional display, the amplitude and frequency outputs of the analyzer were sampled by a PCM encoder. Then the display was obtained on the ground (where the calibrations were applied) by plotting the amplitude readings against the frequency readings.
The lower part of Figure 2 shows two vibration spectra obtained during one flight. The dashed line represents the vibration level while the rocket was held by the launching aircraft prior to launch; the solid line represents the vibration level at a high angle of attack after ignition of the rocket motor.
Calibration of the spectrum analyzer was done by Philip J. Hamory of Dryden Flight Research Center. Design was done by John K. Diamond of Langley Research Center. Analysis of data was done by Arild Bertelrud of Analytical Services and Materials, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category. DRC-00-12