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Three Synchronized Functions
The first function derived from binary counter U7 is antenna array spinning.
This is accomplished by using the two most significant bits of U7 to run 1 of 4
multiplexer U8. The selected output of U8 (active low) is inverted by buffer
U12. The buffered output of U12 (active high) supplies current sufficient to
turn on the antenna to which it is connected. (The details of how this is done
will be covered later.) Buffer outputs U12A, U12B, U12C and U12D are se-
quenced in order. The corresponding buffer selects antennas A,B,C,D,A,B,
etc. Driving multiplexer U8 with the two most significant bits of counter U7 di-
vides the 8 kHz clock by four, so each antenna is turned on for 0.5 ms. One
complete spin of the antenna requires 0.5 ms x 4 = 2.0 ms, thus the fre-
quency of rotation is 2 ms or 500 Hz. An FM receiver connected to the spin-
ning antenna's RF output has a 500 Hz tone imposed on the received signal.
Sequencing the 16 LED display is the second synchronized function from bi-
nary counter U7. This is done by using the binary output of counter U7 to se-
lect 1 of 16 data outputs of U11. The selected output of U11 goes low, allow-
ing current to flow from the +5 V supply through current limiting resistor R51,
green center LED D16, and direction indicating red LED's D17 through D32.
Each antenna remains turned on as the LED display sequences through four
direction indicating LED's, then switches to the next antenna. Each direction
indicating LED represents a heading change of 22.5 degrees.
The third synchronized function is operating the digital filter responsible for
extracting the Doppler tone. The 500 Hz Doppler tone present on the re-
ceiver audio output is connected to an external speaker and audio level ad-
just potentiometer R50. The signal is filtered by a two-pole Sallen Key high
pass filter built around op amp U1A. It filters out PL tones and audio frequen-
cies below the 500 Hz Doppler tone. Next, a four-pole Sallen-Key low pass
filter using U1B and U1C band limits audio frequencies above the 500 Hz
Doppler tone. The band limited signal is then applied to the input of a digital
filter consisting of analog multiplexer U5, R18, R19 and C10 through C17.
(Readers interested in the detailed operation and analysis of this fascinating
digital filter are encouraged to review QEX magazine for June 1999)
The Digital Filter
Using the three most significant bits of U7 to drive the digital filter divides the
8 kHz clock by the two, making the digital filter code rate 4 kHz. The center
frequency of the digital filter is determined solely by the clock frequency di-
vided by the order of the filter. This is an 8th order filter, which makes the
center frequency of the filter 4 kHz/8 =500 Hz. This is the exact frequency at
which the antenna spins, hence, the same frequency of the Doppler tone
produced on the receiver audio connected to the spinning antenna. This is
DDF1 • 8
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