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 • 2
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 • 3
DOPPLER RADIO
DIRECTION FINDER
Ramsey Publication No. MDDF1
Price $5.00
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
INSTRUCTION MANUAL FOR
DDF1 • 4
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= (
rf
c
)/c
where:
dF =Peak change in frequency (Doppler shift in Hertz)
= Angular velocity of rotation in radians per second (2 x frequency of ro-
tation)
r = Radius of antenna rotation (meters)
f
c
= Frequency of transmitted signal (Hertz)
DDF1 • 5
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 f
c
where
fr = The frequency of the received signal in megahertz
dF= The Doppler shift in Hertz
R = Radius of antenna rotation in inches
f
c
= 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.
RF Signal (fo)
A
B
C
D
Rotation
f o
+ f
- f
(B)
(A)
Figure 1
DDF1 • 6
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 pro-
ject, 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 sin-
gle 1/4 –
whip antenna moving in a circle. A clever feature in Roger's de-
sign 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 an-
tenna 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 • 7
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.
8 KHz Clock Binary Counter
Digit al Fil ter
Low Pass
Filter
High Pass
Filter
Zero Crossi ng
Dete ctor
Adjustabl e
Del ay
1 of 16 Data
Sele ctor
LED Compass Display
1 of 4 Data
Select or
FM Receiver
Latch
Antenna
Switcher
External
Speaker
Ant
AF Out
R 36
Cali brate
Figure 2 Block Diagram of the WA2EBY Doppler RDF System
DDF1 • 8
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 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 • 9
truly an elegant feature of the Doppler RDF design. Using the same clock oscillator to spin the antenna and clock the digital filter ensures the Doppler
tone produced by the spinning process is precisely the center frequency of
the digital filter. Even if the clock oscillator frequency drifts, the Doppler tone
drifts accordingly, but the center frequency of the digital filter follows it precisely because the same clock runs it. Excessive drift in the 8 kHz clock
should be avoided, however, because the analog high and low pass filters
that precede the digital filter have fixed passband centers of 500 Hz. A drift
of +250 Hz on the 8 kHz clock corresponds to +62.5 Hz (250/4) drift in the
Doppler tone produced. This value is acceptable because of the relatively
low Q of the analog bandpass filter.
Digital filter Q is calculated by dividing the filter's center frequency by its
bandwidth (Q=f/BW) or 500 Hz/4 Hz=125. It's very difficult to realize such a
high Q filter with active or passive analog filters and even more difficult to
maintain a precise center frequency. The slightest change in temperature or
component tolerance would easily de-Q or detune such filters from the desired 500 Hz Doppler tone frequency. The digital filter makes the high Q possible and does so without the need for precision tolerance components. By
varying damping pot R19, the response time of the digital filter is changed.
This digital filter damping helps average rapid Doppler tone changes caused
by multipath reflected signals, noise or high audio peaks associated with
speech. A digitized representation of the Doppler tone is provided at the digital filter output. A two pole Sallen Key low pass filter built around U2B filters
out the digital steps in the waveform providing a near sinusoidal output corresponding to the Doppler shift illustrated in Figure 1B. The zero crossings of
this signal indicate exactly when the Doppler effect is zero. Zero crossings
are detected by U2C and used to fire a monostable multivibrator (U6) built
around a 555 timer. U6's output latches the red LED in the display corresponding to the direction of transmission with respect to the green center
LED, D16. Calibration between the actual source of transmission and the red
direction indicating LED latched in the display is easily accomplished by
changing the delay between the Doppler tone zero crossing (firing of U6) and
the generation of the latch pulse to U11. C23, R36 and R37 determine this
delay. Increasing or decreasing the delay is achieved by increasing or decreasing the value of the calibrate potentiometer R36.
Low Signal Level and Audio Overload Indicators
Two useful modifications included in this design are the low signal level lockout and audio overload indicators. U2D continuously monitors the amplitude
of the Doppler tone at the input to the zero crossing detector. U2D’s output
goes low whenever the Doppler tone amplitude drops below 0.11 V peak.
This is done by referencing the negative input of U2D to 2.39 V, 0.11 V below the nominal 2.5 VDC reference level output of U2B by means of voltage
DDF1 • 10
divider, R21 and R22. U2D's output remains high when the Doppler tone is
present and above 0.11 V peak. C9 discharges via D2 whenever U2D goes
low, causing U3's output (pin 7) to go high, turning on Q2 via R24 and illuminating low signal level LED, D4. D4 remains on until the Doppler tone returns
with amplitude above 0.11 V peak and C9 recharges via R23. The LED display remains locked by disabling U11's strobe input whenever the Doppler
tone is too low for an accurate bearing. This is done by pulling pin 1 of U11
low via D5 when Q2 is turned on.
Audio overload indicator D3 tells you that audio clipping of the Doppler tone
is occurring. Clipping results if the signal level from the digital filter is too high
and can produce an erroneous bearing indication. The output of U1D goes
low whenever the output of the digital filter drops below 0.6 V as the amplitude of the Doppler tone approaches the 0V supply rail. C7 discharges via
D1 and causes the output of U3C to go high, turning on Q1 via R16 and illuminating audio overload LED D3. We elected not to lock the LED display on
audio overload; doing so, however, only requires connecting a diode between the collector of Q1 and pin 1 of U11, similar to the low level lock out
function.
Phase Correction
If the audio output of the Doppler RDF FM receiver is incorrectly phased, S3,
phase invert, can fix that. (If phasing is incorrect, LED direction indications
are 180 degrees opposite that of the actual signal source.) Moving S3 to the
opposite position corrects the problem by letting U2C sense the trailing edge.
This is particularly useful when switching between different receivers. S2 disables the 8 kHz clock to disable the antenna spinning. This helps when
you're trying to listen to the received signal. Presence of the Doppler tone in
the received audio makes it difficult to understand what is being said, especially with weak signals.
Power Supply
Power is delivered via on/off switch S1. D6 provides supply voltage reverse
polarity protection by limiting the reverse voltage to 0.7 V. U10 provides a
regulated 5 VDC to all digital ICs. C29 through C33 are bypass filters. U10's
5 VDC output is dropped 2.5 V by resistive divider R43 and R45. Noninverting voltage follower U3B buffers the 2.5 V source to provide a virtual
ground reference for all analog filters and the digital filter. Using a virtual
ground 2.5 V above circuit ground allows op amps to process analog signals
without the need of a negative power supply voltage. Analog voltages swing
from near 0 V to near +5 V with the virtual ground level right in the middle,
2.5 V.
DDF1 • 11
DDF1 PARTS LIST
Sort and “check off” the components in the boxes provided. It’s also helpful
to sort the parts into separate containers (egg cartons do nicely) to avoid
confusion while assembling the kit. Leave the IC’s on their foil holder until
ready for installation.
RESISTORS AND POTENTIOMETERS
2 47 ohm (yellow-violet-black) [R42,51]
2 330 ohm (orange-orange-brown) [R17,25]
4 470 ohm (yellow-violet-brown) [R46,47,48,49]
1 3.3K ohm (orange-orange-red) [R14]
7 10K ohm (brown-black-orange) [R13,16,22,24,27,37,39]
1 18K ohm (brown-gray-orange) [R28]
2 22K ohm (red-red-orange) [R8,32]
1 27K ohm (red-violet-orange) [R4]
18 33K ohm (orange-orange-orange)
[R1,2,3,5,6,7,9,10,11,20,26,30,31,34,35,38,43,45]
1 56K ohm (green-blue-orange) [R12]
2 68K ohm (blue-gray-orange) [R29,33]
1 100K ohm (brown-black-yellow) [R18]
3 220K ohm (red-red-yellow) [R15,21,23]
1 PC mount 10K trimmer potentiometer (103) [R50]
2 PC mount 500K trimmer potentiometer (504) [R19,36]
CAPACITORS AND INDUCTORS
11 1000 pF disc capacitors (labeled .001 or 102) [DDF1 board
C22,24,26][ANTINT-1 board C1,2,3,4,5,6,7,8]
1 4700 pF disc capacitor (labeled .0047 or 472) [C23]
10 .01uF disc capacitors (labeled .01 or 103 or 10nF)
[C1,2,3,4,5,6,9,18,19,38
15 .1uF disc capacitors (labeled .1 or 104)
[C7,10,11,12,13,14,15,16,17,21,31,51,52,53,54]
1 .47 uF electrolytic capacitor (labeled .47) [C20]
3 1 uF electrolytic capacitor (labeled 1uF) [C8,25,32]
1 10 uF electrolytic capacitor (labeled 10uF) [C33]
2 100 uF electrolytic capacitors (labeled 100uF) [C29,30]
8 1.2 uH inductor (brown-red-gold) (ANTINT-1 board [L1,2,3,4],
ANTMTG-1 board, 1 ea.)
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