MIL-STD-469B APPENDIX A
detector conversion efficiency must be known or obtained through supplemental calibrations. If attenuators are necessary for reducing the signal level at the crystal detector input, the least attenuation that prevents detector overload and maintains the level within the linear range characteristic of the detector should be used.
40.3.3.5 A typical block diagram of the test setup for pulse width and risetime measurements is shown on figure 5. When the crystal detector and oscilloscope are used for recovering the RF pulse the following precautions should be observed:
a. The characteristic impedance of all interconnecting cables should match the impedance of the devices being connected.
b. The test oscilloscope input should be terminated to match the detector output and interconnecting cable impedance.
c. The bandwidth of the crystal detector system should be greater than 3/t r, where tr is the transmitter pulse rise time, with the transmitter fundamental as the mid-bandwidth frequency. The oscilloscope bandwidth should be greater than 3/t r. A measurement objective for both the
crystal detector system and test oscilloscope should be a bandwidth of approximately 10/t r.
40.3.3.6 The linearity of the crystal detector or test oscilloscope assembly should be checked to ensure adequate definition of the amplitude points where the level is 10 to 90 percent of the nominal flat top level. To perform this check, increase the attenuation inserted, in selected steps, and note the oscilloscope response obtained after each step. The data from the rise time tests should provide definition of voltage points which
are one-tenth of the nominal flat top level (see figure 4). The dynamic range of the test set up should be at least 26 dB (1/20 voltage) below the nominal flat top level.
40.3.4 Procedure. The block diagram of the equipment setup should be as shown on figure 5.
40.3.4.1 Pulse width measurements: Set the test oscilloscope for a direct current (dc) input and adjust the synchronization (trigger threshold) for a stable single pulse display. Set the oscilloscope sweep rate to the value that displays a complete pulse and covers as much calibrated horizontal scale as possible. The pulse envelope at the baseline should cover at least 1/3 of the horizontal scale. Set the oscilloscope vertical sensitivity to obtain a pulse height which covers more than 1/3 of the calibrated vertical range to obtain good readability, then photograph the display.
40.3.4.2 Rise time measurement: Increase the sweep rate of the oscilloscope time base to display the rise time (10 to 90 percent point) over at least 1/3 of the oscilloscope horizontal width. Photograph the display and note oscilloscope control setting.
40.3.4.3 Pulse repetition frequency (PRF) measurements: Slow the test oscilloscope sweep rate until two pulses are seen on the display. Note the time (T), in seconds, between corresponding points on the two pulses. The radar PRF should be calculated as PRF = 1/T. For radars having more than one interpulse period, each different interpulse time should be measured.
40.3.5 Sample calculation.
40.3.5.1 The data in the rise time photographs should be compared to the rise time capability of the test oscilloscope. The rise time of the test oscilloscope (T a) should be less than 1/3 times the indicated rise time shown in the photograph (T i) i.e.:
Ta < 1/3 Ti.
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