Rapid-Burst Receivers

Description

The burst receivers (BR) were 32-channel, stepped-frequency (25 kHz to 13.1 MHz) receivers, which obtained one sample at each frequency every 7.68 s. One receiver (BR-1) was connected to the upper V-antenna and one receiver (BR-2) was connected to the lower V-antenna. A third burst receiver was connected to the dipole antenna, but it failed 1 week into the flight and no significant data resulted. The RF voltage at the feed point of each half of the V-antenna was sampled by a wideband, high-impedance preamplifier, and the preamplifier outputs were combined in a balun transformer and fed to the burst receiver. Each burst receiver was composed of a pair of redundant IF amplifiers and detectors, which shared a common set of crystal-controlled local oscillators and mixers. Only one IF strip was powered on at a given time; the other was used as a backup system. Low-pass filters at the input of the burst receiver prevented strong signals at the 21.4 MHz intermediate frequency from entering the IF strip. Each receiver had a crystal-controlled IF bandwidth of 20 kHz and a post detection integration time constant of 6 ms. A thermistor located in each burst receiver provided a measurement of the ambient temperature of the receiver, and this information was included in the housekeeping data telemetered every 19.7 min. Also, the normal antenna signal measurement sequence was interrupted for 1.28 min every 19.7 min, and calibration noise source signals were injected into each burst receiver to provide a check of their long-term gain stability. The total dynamic range of the burst receivers was approximately 60 dB and was divided into two 30-dB ranges by logic circuitry in the detector electronics. The limit of the input signal level resolution that was due to telemetry quantization step size was about 0.3 dB. Saturation level signals at the preamplifier input often resulted in the generation of intermodulation products in the RF amplifiers, which then appeared as wideband signals in the telemetered data. This problem was most acute when intense kilometer wavelength emissions from the terrestrial magnetosphere were observed at frequencies in the 200 to 300 kHz range. BR-1 was less susceptible to intermodulation problems than BR-2 by 6 to 10 dB. Because of a failure in the local oscillator circuitry in BR-1, channels 4 (55 kHz) and 12 (210 kHz) did not provide usable data. During periods when a portion of each orbit was in the lunar shadow, cyclic variations in thermal gradients across the V-antenna booms resulted in scissor-mode oscillations of the booms, which did not occur when the spacecraft was in 100% sunlight. This effect had a period of approximately 50 min (the scissor-mode period) and was most pronounced on the upper V-antenna during the first and fifth lunar shadow period and on the lower V-antenna during the second and third lunar shadow periods.