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By the end of the year 2011 I started a new project; developing the PA8W Doppler Radio
Direction Finder.
I named it the "PA8W Doppler" not because my design is
exceptionally original or exclusive.
On the contrary; wherever possible I used available knowledge found
on the internet.
However, the PA8W Doppler does have a few features that I missed in
existing designs.
For example, it has an automatic gain control to make it highly
tolerant to input level variations.
It also features an antenna test mode, in which all antennas are
selected in a slow sequence.
And last but not least, I optimized the necessary antenna array using
modelling software.
So, despite its simplicity, the "PA8W doppler" really
has a few advantages over existing designs,
resulting in surprisingly good accuracy even on weak signals.
As soon as a FM signal is strong enough just to be readable,
the RDF is providing a pretty stable and accurate bearing.
The RDF also produces steady bearings on signals with strong
modulation.
Its performance turns out to be much better than expected.
Some new ideas and experiments led to the development of
the Second Edition RDF, which has been extensively tested and running fine
as well.
And right now the development of Version #3 is almost completed.
Initial Design Goals:
Simple design, easy to reproduce, using very common components,
mainly based on the buffered C-Mos 4000B logic series.
Single sided PCB's only.
No software necessary, no PIC's. Lots of people still don't like
handling bits and bytes...
It should however have an output available to feed data into a
computer, because there's very nice software freely available for
presentation in computerized maps.
Very good performance, avoiding the pitfalls of earlier
designs.
Autonomous operation from 12VDC, to support use in cars or all kinds of
simple temporary setups.
It should work with practically all FM receivers from say 120MHz up
to 450MHz.
It should come with optimized antenna designs for base-operation and
car-operation.
Features of Version 2:
32 LED pelorus display (compass rose) plus a +5,6 degrees LED.
Drives an array of 4 antenna elements.
Wide band capability: 25MHz to 500MHz using appropriate antenna
arrays.
120MHz to 170MHz coverage using a 145MHz array.
Very good signal conditioning using a high grade digital filter with
extremely high, variable Q.
Automatic gain control.
On board loudspeaker, since a lot of small receivers ony offer a
headphones output, switching off the internal speaker when used.
Wide calibration range, to compensate
for all possible antenna configurations and receiver
properties.
Antenna check mode, selects the four elements in a slow sequence,
and the display shows the selected antenna element.
Soft control with extensive overlap instead of hard switching of the antenna elements,
since hard
switching produces a lot of noise and unwanted mixing products.
Hardware accuracy: better than 5 degrees
How It Works:
First, let's get to understand what Doppler shift is.
Everybody knows of the pitch change of a fast moving object, like a passing car.
The motor or horn sound of a car at constant speed seems to be
pitched up as long as it approaches, and it pitches down as soon as it
has passed.
This is called Doppler shift.
Only at one short, particular moment in time, when the car is closest to you,
the pitch is correct, it has no doppler shift.
Let's call this moment the trigger moment.
The same doppler effect would occur if the car would be static and
you would be running by at very high speed.
Or imagine you would take a microphone and swing it around at its
cord.
As long as it swings towards the car, the microphone will pick up a
higher pitched tone of that car.
In the opposite part of the swing, it will move away from the car,
pitching down the sound of the car.
There's only two points in the circle, where the pitch is correct,
one point going from pitched too low to too high (the closest point), and the other
point will be found going from pitched too high to too low.
A Doppler Radio Direction Finder does exactly the same thing; it
uses a circular arrangement of antenna elements which are activated
one after the other in a very fast sequence, simulating one antenna
element being swung around in a circle at very high speed.
This means that the received carrier of the tracked radio signal
will be moved up in frequency as long as the antenna element swings
towards the transmitter, and shortly after that the
frequency will apparently be lowered below the true frequency due to
the antenna element moving away again.
The antenna array is rotated electronically at around 500
revolutions per second, so a FM
receiver connected to this electronically rotated antenna will
produce a 500Hz audio tone.
The zero crossings of that tone (plus or minus some phase shift etc. etc.
) will mark the points where the doppler shift is zero.
These points are easy to detect electronically, and after the
necessary compensation for phase shift mentioned above, one of these
points is used as a trigger to "freeze" the LED display at
the correct point in the circle.
You may imagine that the cirular LED pelorus display copies the circular
movement of the antenna array and only the LED that is exactly at
the trigger point is allowed to light.
Controls:
The PA8W Doppler has the following controls:
Input sensitivity, to adjust to the actual receiver output
Speaker on/off
Calibration, to make the RDF point into the right direction
Phase switch, to enhance calibration with 180 degrees shift
Filter-Q, to set the signal conditioner for fast or slow response
Antenna check switch, selects
the four elements in a slow sequence, and the display shows the
selected antenna element
(In this mode the 500Hz tone is absent so you
can listen to the modulation of the tracked transmitter, and check
for equal performance of all 4 antenna elements. I added this test
mode mainly for developing purposes of the antenna arrays, but
I kept it in since it presents an easy way to make sure all antenna
elements are ok.)
The simplified Block Diagram:
The above block diagram is easy to understand:
Top-left we have the 4-element vertical dipole array, meant for
non-mobile applications.
(for mobile applications there's a similar arrangement of 4 magnet
mount whip antennas)
The antenna array is "rotated" by the antenna multiplexer,
which activates only one element at the time.
From the antenna array, a coax runs to our FM-receiver. The audio
output of the receiver is fed into a series of audio filters.
Next, a zero crossing detector marks the point of no phase-shift,
and, after a calibration pulse shift section, this triggers a data
latch.
This data latch then copies the current state of the address lines, and
presents it to the LED driver, which lights the corresponding LED in
the pelorus display.
Of course, the entire process is timed by the central clock +
address counter.
Considerations:
One could think of a LED pelorus display with 64 or even more LED's to get
better display resolution.
Knowing that a 4 antenna array is capable of offering a maximum accuracy of
around 5 degrees, it would make sense to use 64 LED's giving the
pelorus a resolution of 5,6 degrees.
However, this would make the RDF much more complex, and I promised to
keep it simple, remember?
So I designed a 32 LED pelorus plus a 5,6 degrees indicator, giving
5,6 degrees resolution with only 33 LED's.
And if you really want more then a hookup to a
computer will give all the features you could want, even a direct plot
of the transmitters direction on an electronic road map.
Two or more RDF stations can exchange this information over internet
to get a cross bearing of the tracked signal!
You have to realize that the simple 4 element array does not exactly
simulate a smooth rotating antenna.
It will produce 4 phase jumps every cycle, or even only two jumps
per cycle, worst case, when two elements are at equal distance from
the transmitter.
Obviously this is pretty hard for the RDF to analyse with some
degree of accuracy.
A 8 element array already does a better job, and professional
systems often work with even 16 or more antenna elements.
As for practical reasons we will have to stick to 4 or 8 elements
max (in my Version3 doppler),
our RDF will need to have an incredibly good signal
conditioner, and that's where the digital filter and additional
low-pass filters step in:
They integrate the incoming audio, they don't allow for fast signal
jumps to pass, but they average everything into a sinus-like
waveform.
To achieve this the applied digital filter has an extremely high Q.
It will have a bandwidth of just a few Hertz.
The extremely narrow bandwidth will force about any signal shape
into a sinusoidal shape, and it
also effectively cuts off modulation on the tracked signal, which
would otherwise blur the display severely.
As far as I know this is the only type of filter this narrow, that
is not depending on very high accuracy components.
Its centre frequency is determined by the clock frequency only.
In this design, the same clock is used to rotate the antenna, and if
the clock would deviate, the filter would track that deviation
automatically.
No crystal controlled precision necessary. This is why this type of filter is the eureka part of the design!
This signal smoothening process actually fills-in the gaps between the
jumps from one antenna element to the next, giving the RDF a highly
improved accuracy.
And in fact the softened activation of the antenna elements plus the
considerable overlap on every transition from one element to the next also
add to that process.
The soft switching has an even bigger advantage: It reduces the level
of spurious in the radio band so your reception of the weaker
signals will improve by a big amount.
Developing the PA8W Doppler RDF, I experimented quite a bit with a
variable timing and overlap for the antenna elements to find the
best spot, so I assume we have achieved pretty much the maximum performance
one can squeeze out of this simple antenna configuration.
However, we should realize that particularly a fixed (base) RDF
accuracy will
suffer from objects in its vicinity, which may introduce bearing
deviations much larger that the intrinsic accuracy of this RDF.
A mobile RDF has the same problem, but since it changes location
constantly, the changing deviations will be averaged into a much
smaller bearing error.
Illustrations:
The following screenshots will illustrate the excellent signal
conditioning in the heart of the RDF:
First of all the red wave shows an audio signal right out of
the receiver.
It is a bit noisy because it was a rather weak carrier.
But since it is a clean carrier without modulation, this signal is
easy to process by the RDF.
A modulated signal would be total chaos to the eye, and still the
RDF is able to pick out what's important.
This is achieved mainly by the digital filter, of which the output
is shown in yellow.
It resists to all rapid changes in the signal, so what remains is
information that is constant over some period of time, like the
doppler information.
After the digital filter two low pass filter stages are responsible
for filtering out everything above the doppler frequency.
So the output is only the base frequency, an nice 500Hz sinusoid
shown in blue,
very well suited for a precise phase measurement.
Note there's a considerable phase shift between the yellow and the
blue signal, due to the low pass filters.
This is of no meaning as long as it is a constant factor, since the
calibration will compensate for this automatically.
Accuracy measurement:
I did perform some measurements with the base array on a rotatable
glassfiber mast.
Using a constant rotation speed and a solid carrier on the RDF, I
timed the intervals between LED transitions of the 16 LED pelorus.
(version 1!)
I took 6 measurements and averaged their results.
Then, I looked at the biggest deviation from average, this turned
out to be 5,6 degrees,
which is in accordance with what has been stated by professionals
about 4-antenna doppler RDF's.
Note that these measurements are done using the new soft switch
driver with 1/8 cycle antenna overlap.
As I mentioned earlier, I really did not expect to achieve such good
results and accuracy of the RDF.
I will investigate some additional ideas, mainly about the type of
array control, but I guess there's hardly any room left for
improvement.
Especially keeping in mind that bearing errors introduced by
multipath signals may massively exceed the errors left in the RDF
system itself.
Please don't hesitate to email me if you have any comments or
questions.
73's
Wil.
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