Advertisement
Figure 2 Block Diagram of the WA2EBY Doppler RDF System
Antenna
Switcher
1 of 4 Data
LED Compass Display
Selector
1 of 16 Data
Ant
8 KHz Clock
Binary Counter
Selector
Latch
High Pass
Low Pass
Zero Crossing
Adjustable
Digital Filter
Filter
Filter
Detector
Delay
AF Out
R 36
Calibrate
FM Receiver
External
Speaker
shown in figure 1. - Zero crossings of the Doppler frequency shift pattern cor-
respond to the antenna position located directly toward the source of trans-
mission (position A) or directly opposite the source of transmission (position
C). The zero-crossing signal passes through an adjustable delay before it
latches the direction indicating LED. The adjustable delay is used to calibrate
the LED direction indicator with the actual direction of the transmission.
CIRCUIT DESCRIPTION
Take a look at the schematic of the WA2EBY Doppler RDF on page 18. The
heart of the system is an 8 kHz clock oscillator built around a 555 timer, U4,
configured as an astable multivibrator. C26, R27, and R28, R29 determine
the multivibrator's oscillation frequency. R27 and R28 are series connected
to allow fine tuning the oscillation frequency to 8 kHz. It is important that the
clock frequency be exactly 8 kHz; I recommend that it be adjusted to
+/-250 Hz of that frequency for reasons that I'll discuss shortly. The 8 kHz
output of U4 provides the clock for 4 bit binary counter U7. The 3 bit binary
coded decimal (BCD) output of U7 is used to operate three synchronized
functions.
DDF1 • 7
Hide quick links:
Advertisement
