DOPPLER DIRECTION FINDER RADIO DIRECTION FINDER KIT Ramsey Electronics Model No. DDF1 Get in on the fun of radio direction finding (RDF) with this super kit ! The latest in affordable Doppler direction finding equipment available in a complete kit form ..this one even includes the receiving antenna. A must for the "fox hunter" at an unheard of price! · · · · · · · · Elegant and cost effective design thanks to WA2EBY ! Featured in May / June 1999 QST Articles. Solid state antenna switching for "rock solid" performance. Convenient LED 22.5 degree bearing indicator. Audio Level indicator for trouble free operation. Adjustable damping rate, phase inversion, scan enable / disable. Complete with home brew "mag mount" antennas and cable, designed for quick set up and operation. Utilizes latest high speed CMOS technology for signal conditioning and audio processing! Complete and informative instructions guide you to a kit that works the first time, every time - enhances resale value, too ! DDF1 · 1 RAMSEY TRANSMITTER KITS · The "Cube" MicroStation Transmitter · FM25B Synthesized FM Stereo Transmitter · FM100B "Professional Quality" Stereo FM Transmitter · AM1, AM25 AM Transmitters RAMSEY RECEIVER KITS · FR1 FM Broadcast Receiver · AR1 Aircraft Band Receiver · SR1 Shortwave Receiver · AA7 Active Antenna · SC1 Shortwave Converter RAMSEY HOBBY KITS · SG7 Personal Speed Radar · SS70A Speech Scrambler · MX Series High Performance Mixer · MD3 Microwave Motion Detector · PICPRO Pic Chip Programmer · LC1 Inductance-Capacitance Meter RAMSEY AMATEUR RADIO KITS · DDF1 Doppler Direction Finder · HR Series HF All Mode Receivers · QRP Series HF CW Transmitters · CW7 CW Keyer · CPO3 Code Practice Oscillator · QRP Power Amplifiers RAMSEY MINI-KITS Many other kits are available for hobby, school, Scouts and just plain FUN. New kits are always under development. Write or call for our free Ramsey catalog. DDF1 DOPPLER RADIO DIRECTION FINDER KIT INSTRUCTION MANUAL Ramsey Electronics publication No. MDDF1 Revision 1.2 First printing: May, 1999 COPYRIGHT 1998 by Ramsey Electronics, Inc. 590 Fishers Station Drive, Victor, New York 14564. All rights reserved. No portion of this publication may be copied or duplicated without the written permission of Ramsey Electronics, Inc. Printed in the United States of America. DDF1 · 2 Ramsey Publication No. MDDF1 Price $5.00 INSTRUCTION MANUAL FOR DOPPLER RADIO DIRECTION FINDER TABLE OF CONTENTS Introduction to the DDF1 ............... 4 DDF1 Circuit Description .............. 4 Parts List ...................................... 11 DDF1 Assembly Steps ................. 14 Component Layout....................... 17 Schematic Diagram...................... 18 Initial Testing ................................ 22 Ramsey Warranty ........................ 23 RAMSEY ELECTRONICS, INC. 590 Fishers Station Drive Victor, New York 14564 Phone (585) 924-4560 Fax (585) 924-4555 www.ramseykits.com DDF1 · 3 INTRODUCTION Radio direction finding is a fascinating hobby that has been becoming more and more popular in today's portable world. More recently, Doppler "df-ing" has become the rage, with a display that gives you a direct bearing on the location of the transmitter. Pretty neat trick considering you don't need multiple separate receivers at different locations to triangulate on the mystery transmitter. DDF1 CIRCUIT DESCRIPTION The classic example of the Doppler effect is that of a car approaching a stationary observer. The car's horn sounds higher in pitch (frequency) to an observer as the car approaches. The change in frequency occurs because the motion of the car shortens the wavelength. The horn sounds lower in pitch (frequency) to the observer as the car speeds away. This occurs because the car is speeding away from the observer effectively increasing the wavelength. Fewer cycles per second, hence, lower-frequency sound. A similar effect occurs when an antenna is moved toward or away from a transmitting source. The signal received from an antenna moving toward the transmitting source appears to be at a higher frequency than that of the actual transmission. The signal received from an antenna moving away from the source of transmission appears to be lower in frequency than that of the actual transmission. Imagine a receiving antenna moving in a circular pattern as pictured in Figure 1A. Consider the antenna at position A, nearest the source of transmission. The frequency of the received signal at point A equals that of the transmitted signal because the antenna is not moving toward or away from the source of transmission. The frequency of the received signal decreases as the antenna moves from point A to point B and from point B to point C. Maximum frequency deviation occurs as the antenna passes through point B. The frequency of the received signal at point C is the same as that of the transmitted signal (no shift) because the antenna is not moving toward or away from the source of transmission. As the antenna moves from point C to point D and from point D back to point A, the frequency of the received signal increases. Maximum frequency deviation occurs again as the antenna passes through point D. The Doppler frequency shift as a function of antenna rotation is illustrated in Figure 1B. dF= ( rfc)/c where: dF =Peak change in frequency (Doppler shift in Hertz) = Angular velocity of rotation in radians per second (2 x frequency of rotation) r = Radius of antenna rotation (meters) fc = Frequency of transmitted signal (Hertz) DDF1 · 4 c = Speed of light We can calculate how fast the antenna must rotate in order to produce a given Doppler frequency shift with the following equation fr = dF x 1879.8/R x fc where fr = The frequency of the received signal in megahertz dF= The Doppler shift in Hertz R = Radius of antenna rotation in inches fc = Carrier frequency of the received signal in megahertz As an example, let's calculate how fast the antenna must rotate in order to produce a Doppler shift of 500 Hz at 146 MHz, assuming the antenna is turning in a circle with radius 13.39 inches. R F S ig na l (fo ) F ig ure 1 D R o ta tio n C A B (A ) +f fo -f (B ) DDF1 · 5 The frequency of rotation is: fr = 500 x 1879.8/146 x 13.39 A rotation frequency of 480 Hz translates to 480 x 60 = 28,800 or almost 30,000 r/min, which pretty much rules out any ideas of mechanically rotating the antenna! Fortunately, Terrence Rogers, WA4BVY, proposed a clever method of electrically spinning the antenna that works very well. Roger's project, the DoppleScAnt, uses eight 1/4- vertical whips arranged in a circular pattern. Only one antenna at a time is electrically selected. By controlling the order in which the antennas are selected, the DoppleScAnt emulates a single 1/4 ­ whip antenna moving in a circle. A clever feature in Roger's design is the use of a digital audio filter to extract the Doppler tone from voice, PL tones and noise. The DDF1 design offers slightly improved audio filtering, 74HC-series logic circuits capable of driving the LED display directly, a wideband VHF/UHF antenna switcher and the four 1/4- mag-mount antennas. Total project cost is about one third the cost of purchasing a commercial RDF unit - and building the project is a lot more educational. HOW IT WORKS To understand the operation of the Doppler RDF circuit, see the block diagram of Figure 2. An 8 kHz clock oscillator drives a binary counter. The output of the counter performs three synchronized functions: "spin" the antenna, drive the LED display and run the digital filter. The counter output drives a 1 of 4 multiplexer that spins the antennas by sequentially selecting (turning on) one at a time in the order A,B,C,D,A, etc., at 500 times per second. The counter output also drives a 1 of 6 multiplexer used to drive the LED display in sync with the spinning antenna. The RF signal received from the spinning antenna is connected to the antenna input of a VHF or UHF FM receiver. The spinning antenna imposes a 500 Hz frequency deviation on a 146 MHz received signal. A 146 MHz FM receiver connected to the spinning antenna's RF output demodulates the 500 Hz frequency deviation and sounds like a 500 Hz tone with loudness set by the 500 Hz frequency deviation. The receiver audio, including 500 Hz Doppler tone, is processed by a series of audio filters. A high pass filter rejects PL tones and audio frequencies below the 500 Hz Doppler tone. A low-pass filter rejects all audio frequencies above the 500 Hz Doppler tone, and a very narrow bandwidth digital filter extracts only 500 HZ Doppler tone. The output of the digital filter represents the actual Doppler frequency shift DDF1 · 6 Figure 2 Block Diagram of the WA2EBY Doppler RDF System Antenna Switcher 1 of 4 Data Selector LED Compass Display Ant 8 KHz Clock Binary Counter 1 of 16 Data Selector Latch High Pass Filter AF Out Low Pass Filter Digital Filter Zero Crossing Detector Adjustable Delay R 36 Calibrate FM Receiver External Speaker shown in figure 1. - Zero crossings of the Doppler frequency shift pattern correspond to the antenna position located directly toward the source of transmission (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 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 sequenced 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 divides 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 frequency of rotation is 2 ms or 500 Hz. An FM receiver connected to the spinning 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 binary counter U7. This is done by using the binary output of counter U7 to select 1 of 16 data outputs of U11. The selected output of U11 goes low, allowing 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 receiver audio output is connected to an external speaker and audio level adjust 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 frequencies 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 divided 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 exac ...