DDF1 • 28
ms (8 kHz). Use a frequency counter to verify the clock frequency is 8 kHz
+/- 250 Hz if you desire. Getting the clock frequency to exactly 8 kHz is not
necessary for proper operation, however, the values of R27 and R28 can be
changed to adjust the clock frequency. Verify that closing switch S2 disables
the clock. Open S2 (scan stop) and verify the operation of BCD counter U7
by connecting the oscilloscope sequentially to pins 14,13,12, and 11. The
signal frequency on these outputs should be approximately 4, 2, 1 and 0.5
kHz, respectively. Verify the presence of a square wave signal on pins 2,4,6
and 8 of buffer U12.
Signal Level Indicators
The following test uses an audio signal generator to simulate the presence of
the Doppler tone. Disconnect the speaker from audio level control potenti-
ometer R50 to prevent loading the signal generator. Connect an audio signal
generator to the receiver audio input terminal. Set the generator to apply a
500 Hz sine wave with amplitude 1V P-P. Rotate R50 until the audio over-
load LED D3 illuminates. Then, adjust R50 until low signal level LED D4
lights. Adjust R50 so that LEDs D3 and D4 are off, set calibrate control R36
to the center of its range and adjust damping control R19 for minimum damp-
ing (fully CCW).
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