Montreal Doppler 3 DF unit

The Montreal Doppler III is an update of its predecessor - the Montreal Doppler II.  It is recommended that you read about them on their respective official pages.  Please note that this was NOT  an official page of VE2EMM - please read the disclaimer at the bottom of this page.
Important notes:

                  panel view of my Montreal Doppler III DF unit
My version of the Montreal Doppler III.  Note that this unit uses the alternate firmware for the main processor  and for the compass rose display.
Click on image for a larger version

Differences between the Doppler II and Doppler III:

The Doppler II and Doppler III units (running their respective original firmware) are very similar to each other - with the following differences:

Comments and comparisons based on using both the Doppler II and Doppler III:


Owning both the Doppler II and Doppler III units, I can say that both work very well - but the Doppler III, having the 8-capacitor "Roanoke-type" switched-capacitor filter allows it work better in the presence of weak, noisy, and/or heavily-modulated signals.  The addition of this filter is a natural evolution, I hadn't had the Doppler II too long before I'd added an additional switched capacitor filter well before the Doppler III was available.  With the turn of a potentiometer (a recommended modification) this filter can be adjusted for a quick response (e.g. little or no filtering) able to react to a brief "kerchunk" or for fairly heavy damping and a slower response (a lot of filtering) that is appropriate for very weak, noisy, or heavily-modulated signals.

Note that the potential temperature drift problem of the Doppler II is minimized by using good-quality stable capacitors in the peaking filter:  I'd found that allowing the unit to warm up for 5 minutes or so before calibrating minimized this problem.  With the clock-based switched-capacitor filter on the Doppler III temperature-related drift of the bearings has not been noticed - only a slight "amplitude" drift (that is, a slight increase in the apparent gain of IC50 when it is cold) that does not appear to have any noticeable effect on the bearing.

Compass rose display:

Perhaps the best improvement of the Doppler III over the Doppler II is the built-in LED-based compass rose display:  I'd found that the original on-screen LCD "compass rose" on the Doppler II - while clever - to be nearly unusable - mostly because it was "squashed" (not round) and difficult to interpret.  For this reason I'd built my own LED-based Pelorus (compass rose) that was driven from the serial output data to work with the Doppler IINote that my alternate firmware allows the Doppler III compass rose hardware to work even with the older Doppler II.
                  the back cover removed, this is a view inside the
                  Doppler III unit.
The "guts" of my Doppler III
Click on image for a larger version.

The unmodified Doppler III works noticeably better on weak/noisy/modulated signals than the unmodified Doppler II unit, but with the added switched-capacitor filter, I would say that the Doppler II performs equally to the Doppler III - and the LED compass rose makes the Doppler III much easier to use - particularly if operating in a mobile environment.

My version of the Doppler III:

Being one to tinker, I made some modifications to the Doppler III right off:

A few comments:

For a series of photographs visit the VE2UMS Doppler III web page (English version). There are a lot of pictures there with some good ideas!

Comment:  As of late 2006, the Doppler III circuit boards are available from FAR Circuits - see the Main ARDF page for more information.

When I originally built the Doppler III, the circuit boards (two of them) and pre-programmed pics (three of them) were available directly from Jacques.  These boards, while apparently home-made, were of reasonably good quality, being etched single-sided on 1/32" glass-epoxy circuit board.

There were only two minor difficulties that I'd encountered:  I found an open trace (on the "contrast" control line of the LCD - it caused the display to appear to be blank and non-working) and the fact that the green lacquer on the board had be be laboriously scraped from the solder pads.  While the green lacquer looked nice, it is probably unnecessary.  One builder mentioned to me that he'd simply stripped the lacquer completely off, but another mentioned that he was able to "solder through" it without difficulty.
                  back of the main circuit board of the Doppler III
View of the back side of the Montreal III circuit board.  Close inspection will reveal board mods near the upper-left corner and the bottom edge in the center where the additional components were added for the backlight control and negative supply voltage, respectively.
Click on image for a larger version.

This is not a project that should be undertaken by anyone unfamiliar with basic kit-building techniques.  Not only must the builder obtain most of the parts, but he/she must be able to install the components in their proper locations using only the parts-placement diagram on Jacque's web site and (in cases where the image is indistinct) by back-tracing the schematic diagram.

Finally, in the event that it doesn't work the very first time you power it up (mine didn't - not only did I have to locate the open trace, but I'd left an inadvertent solder bridge somewhere...) you must be prepared to do some simple troubleshooting.

It should not need to be said, but it is strongly recommended that the builder socket all integrated circuits.  This allows preliminary testing of the power supply/regulator as well as incremental testing/troubleshooting of various circuits, not to mention easy replacement of parts.

Important modifications:

Note:  The three modifications below have been incorporated into the more recent (Version 3.2) circuit boards.

Here are three modifications that I would recommend making to the Doppler III board no matter whose firmware you plan to use:

In addition to the modifications to the main circuit board, some slight changes were made to the Compass rose board as well in version 3.2:

The enclosure:

I chose a fairly small, thin enclosure in which to mount the Doppler III's circuit boards and components (a Hammond 1599KBK) and I also wanted to make sure that the various switches and knobs were also low-profile, so I used a flat pushbutton switch for the "select" button, rocker switches for the power, backlight dim/bright, and compass rose control.

Mounting the boards:

The two boards (compass rose and main board) are intended to be mounted back-to-back (see picture) but depending upon exactly how you wish to mount these boards/display in your enclosure, you can change this as needed.  The holes drilled are intended to accommodate 4-40 hardware (or its metric equivalent) and an assortment of various standoffs is very helpful in spacing/mounting these boards.

The potentiometers:

Also present (in my version) are three potentiometers:  One for menu selection, another for the volume control, and a third to adjust the "damping" of the switched-capacitor filter.  If you look closely at the "inside" picture, you'll see that these controls actually mount on pieces of glass-epoxy circuit board material offset from the front panel rather than mounting directly to the front panel.  These pots are miniature units that use 1/8" shafts (instead of the more usual 1/4" variety) and the knobs themselves are recessed, protruding through holes in the enclosure.  The result is that only about 1/4" of the knobs themselves are revealed on top, maintaining the low profile.

A view
                  showing the two stacked boards of the Doppler III
Close-up showing the back-to-back circuit boards.
Click on image for a larger version.

On the right side of the enclosure may be seen two "D"-type connectors and a flat rocker switch.  The upper connector is for the serial port in/out:  Some users choose to put a separate "in from GPS" and "out to computer" connector, but I decided to use just one connector and construct an "octopus" cable consisting of 3 connectors - one for the DF unit, one for the GPS, and another for the computer.  Unless I need the GPS input, a standard 9-pin cable may be used to connect the DF unit to a computer.

Below the "data" connector is a 15-pin high-density "D" connector of the sort typically used to connect to a computer monitor.  This connector has connections to all eight antenna drive pins plus the unregulated V+ and a V- supply (extracted from the MAX232 RS-232 driver IC) used to power the antenna array switch plus the "S-meter" input.  While not all of these pins are used for all applications, putting them on this connector makes at the time of construction makes them more easily accessible for later use.

On the left side is the power connector - a coaxial-type - but in hindsight, I would, were I to construct the unit again, hardwire a power cable:  The power cable is one item that could get separated from the unit and lost, making it unusable.

Finally, located between the two switches on the left side (but not visible on the "front panel" view) is a 3.5mm audio jack:  This allows the user to reroute the speaker audio to headphones or to an external speaker.

The audio amplifier:

Because the Doppler III uses an on-board amplifier (an LM386-4 - be absolutely certain to order the "-4" version of the LM386 - only the "-4" version is rated for operation above 12 volts!) capable of about one watt of audio output, I chose a high-quality 4-ohm speaker - in this case Jameco Electronics P/N 99995.  This speaker is rated for several watts, has a large magnet, and is very efficient:  With just the single watt output of the LM386-4 it can be very loud with minimal distortion - sometimes a requirement in a noisy car. (No, Jameco doesn't sell the "-4" version of the LM386, but Digi-Key does...)

As mentioned above, it is recommended that a 0.001 uF capacitor be connected directly across pins 2 and 3 on the backside of the circuit board to prevent any problems with RF sensitivity of the audio amplifier.

The LCD module:

The sharp-eyed observer of the pictures will notice something else:  The LCD module.  Jacques, in his board layout, made provisions to mount the LED "piggyback" to the main display board.  I, on the other hand, got the largest 2-line by 16 character backlit LCD module that DigiKey Electronics had available at the time.  Unfortunately, this LCD module had its interface connector along the bottom (the conventional location, actually) rather than along the top.  This necessitated the use of the gray interconnect cable that may be seen in the picture.  I was able to find a 13 conductor pre-assembled cable on a defunct VCR - but this left me one wire short of the 14 conductors needed.  No problem:  The Doppler III uses the "4-bit transfer mode to send data to the LCD and thus only 10 wires were actually needed, so some of the extra were used to make up the deficit.

The switches:

To maintain a "low profile" I chose to use "snap-in" switches for the power, backlight, menu select, and compass rose control.  Each of these switches require that a fairly precise square hole be made in the enclosure, but this is easily done using the "drill 'n file" method in the plastic enclosure:  With a little bit of care, a suitable mounting hole can be made.

Not everyone stocks the variety of switches required, but I was able to find the necessary switches in the Mouser Electronics Catalog:

SPST/SPDT rocker switch:  CW Industries,  model:  GRS-4012-1600  (used for power on/off and backlight on/off)
Center-off SPDT momentary:  CW Industries, model:  GRS-4013C-0001  (used for mode control of the compass rose)
SPST momentary pushbutton:  Mountain Switch, model:  103-1211  (used for the "menu" button)

Details of the modification to add a "damping" control to the Doppler III.

Power supply fusing:

Another important feature is overcurrent protection on the power supply leads.  While fuses are suggested on the schematic, I used 0.6 amp self-resetting fuses (such as those sold by Jameco Electronics - click here for a datasheet) instead along with a reverse-biased diode across the power supply leads (placed after the fusing) for polarity protection.  It is strongly recommended that you put your chosen means of overcurrent protection in both the positive and the negative leads.


Damping control:

Once you have gotten the DF unit completely operational, a highly-recommended and useful feature to add is an adjustable "damping" control.  This replaces R55, a fixed 680k resistor, with a 1 meg potentiometer (an "audio taper" if is preferred, if available,) a 47k resistor, and a capacitor of "approximately" 180 pf - more on the "approximately" part in a moment.

It is recommended (but not required) that the metal body of the potentiometer be grounded:  At the "1 meg" end of the rotation (highest amount of damping) it is possible that a small amount of noise or some "hand capacity" effects may slightly skew the reading when the metal shaft is being touched if the potentiometer is not grounded and a metal knob is used.

When wiring the damping control, I used some "miniature 2-conductor plus shield" (with the shield grounded) wire extricated from a junked VCR.  If you do not have such wire, three conductors twisted tightly (or braided together)  will work just as well.  In this case, one wire would connect to each side of the (removed) R55 location and the third wire (or shield) should go to ground.

The 180pf capacitor is used to counteract various other reactances found in the circuit:  Without it, there would be a 10-15 degree shift in bearing when the control was rotated from minimum to maximum damping.  While 180 pf is a good "starting" value, it may not be the exact value required.  To determine the precise value, tune in an unmodulated, clean, multipath-free, stable signal and observe the bearing at all rotations of this potentiometer:  If it moves more than a degree or so, increase or decrease the capacitance slightly (by 5-10pf or so) until this variation is nulled out.  One possibility is to (initially) use a variable capacitor (5-50pf, for example) and use a fixed 150pf capacitor:  Adjust the variable until the phase variation is nulled out and either measure the variable/fixed capacitor combination and replace it with fixed capacitor (or combination of fixed capacitors) or just leave the variable in place.

Schematics showing the components added for the "Adaptive Filter" modifications - see text.
Schematic of adaptive filter

An "Adaptive" filter  (Version 7A and newer of the alternate firmware only):

One advantage of the filtering used in the Doppler III, if one has added the Damping Control is that it may be adjusted to provide a very fast response or a slow response.  A fast response (low damping or "Q") is often desirable when the signals being monitored are of fairly good quality while a slow response (high damping or "Q") provides better filtering of noise and modulation and can be helpful weak or noisy signals.

Occasionally, one may wish to use the "slow" filter but encounters difficulty when trying to determine the bearing of signals that last only a short time, such as a "kerchunk."  If a slow response is used, the "Q" of the filter may be such that it does not register the signal before it is gone.  Another problem is when a short-duration signal such as this immediately follows another signal:  In this case, the filter will still "contain" the bearing from the previous signal and may not be able to read a correct bearing until enough time has passed for the "old" bearing to be purged from the filter.

This "adaptive" filter option causes a "fast" mode to always be automatically selected when the signal goes away, reverting back to the selected response time shortly after a signal is detected.  In this way, the "old" signal is purged from the filter immediately and when the new signal appears, the filter is "charged" up for a brief moment (between 50 and 100 milliseconds) at the "fast" rate before the "slow" rate is re-enabled once again.

With this feature, it is more likely that even very brief transmissions (especially if they closely follow other transmssion) can be analyzed - even if the damping/Q of the switched-capacitor filter is set to a fairly high value (e.g. a "slow" filter.)  For more information, see the section on Adaptive Filtering in the New Firmware manual as well as this brief description of adaptive filtering.

This modification involves the isolating of pin 5 of the main processor on the Doppler II to disconnect it from the +5 volt supply.  Once this is done, the firmware detects that this modication is present and when the "Average Clear" indicator is active (see the "Adaptive Filtering" section in the New Firmware manual) this pin will be set at +5 volts.  This signal may be applied to a 4066 analog gate that, when the 5 volt signal is present, "shorts out" the "damping" control in the schematic above.  Note that once this is done, it is likely that the value of 180 pF capacitor will have to be readjusted to accommodate the added capacitance of the 4066.  When wiring in the 4066, be sure that the 47k resistor shown in the above diagram is still in series with the circuit when the 4066's gate is closed!

The schematics to the right show how this adaptive filter may be implemented.  For the analog gate, either a standard 4066 or a 74HC4066 may be used, powered from the +5 volt supply, and it is up to the user to decide whether to wire up just a single gate, or all four of the 4066's gates:  The resistance of the 4066 is not significant in this case, so a single gate will suffice.  It is strongly recommended that one tie all unused pins to either +5 volt supply or ground to keep the pins from floating at undefined voltages.  For the ultimate in simplicity and small size, it is also possible to use the Toshiba TS4S66 - a surface-mount chip that consists of a single 4066 gate.

Selection of the proper capacitor value:

One important aspect of this modification is the addition of the parallel capacitor.  As in the case of the damping control, a bearing shift will occur with different values of resistance.  Without the capacitor, the user would observe a 10-15 degree bearing shift immediately after acquisition of the signal that would, after a second or two, disappear.  The reason for this is that when the 4066 is closed, the phase shift across the filter is different that that which would occur with the 4066 open and the full value of R50 (or the damping control) in place.  To compensate for this, the parallel capacitor should be added.

The precise value of this parallel capacitor should be determined experimentally, but it will likely be in the range of 150 to 180pF.  If you have installed the damping control, determining the precise value is easy:  Just follow the procedure described above in the section describing the damping control.  If you have NOT installed the damping control, do the following:

Note that this adaptive filter can also be added to a Doppler II that has had added to it a switched-capacitor filter.

Bright/Dim backlight control:

The LCD module that I chose uses green LEDs in series-parallel resulting in a voltage drop of approximately 4.2 volts.  As with my implementation of the Doppler II backlight, I decided to operate the LED using "free" current by putting in series with the Doppler III's logic supply.

A slight complication arose, however:  The LM386 audio amplifier and the compass rose display.

I did not want to run the current from either the audio amplifier or the compass rose display through the LCD's backlight:  The backlight's brightness would vary wildly with the speaker audio variations (not to mention the lowered voltage to the LM386 reducing it's capable power output) and a potentially similar effect with a varying load current by the compass rose.
Details showing the modifications for a "Dim/Bright" backlight switch
Click on image for a larger version.

The solution was simple board modification:

The supply current for VR20 - the 78L05 voltage regulator - flows through the backlight of the LCD first:  Having removed the load of the compass rose and audio amplifier causes the current through this backlight to be somewhere between 30 and 50 mA - a nice value for a fairly "dim" backlight.  The obvious advantage to this method is that the backlight draw no extra current at all!  For a "bright" mode, an additional resistor (a 120 ohm, 2 watt unit) is connected from the "low" side of the LED backlight to ground via an SPST switch, causing more current to flow.  I could have used an SPDT switch and added an "off" mode (shorting out the backlight) but I found that I'd never used this feature on a similar backlight system that I'd wired on my Doppler II unit.

The caveat to this method of powering the backlight is that there is a (nominal) 4.2 volt drop through the backlight.  In order for the 78L05 regulators to do their job properly, they need at least 1.75 volts higher than this (almost 7 volts) and this means that a minimum of 11 volts (I'd measured 10.75 volts) or so is required for proper operation.  If this amount of voltage drop bothers you, consider using a low-dropout voltage regulator - such as the LM2930 or LM2936 for VR20.  (The separate regulator is used for the compass rose, this need not be a low-dropout regulator as it is not in series with the LCD's backlight.)

Note:  While the Doppler II's rotation rate is 500.8 Hz, the antenna rotation rate of the Doppler III (using the original firmware by VE2EMM) is approximately 499.3 Hz.  If a frequency-sensitive application is envisioned (such as the comb filter) then this must be taken into account.  Note that my rewrite of the Doppler II/Doppler III firmware operates at 500.8 Hz in both cases.

AVAILABILITY of the "Alternate Firmware":


The code for the alternate firmware was originally based on that of the original Montreal Doppler II DF unit by the late Jacques Brodeur, VE2EMM, and full credit is given to him for this fine work.  Because the additional modifications are my own and were not done with his involvement!

Although good faith efforts have been made to make certain that the operation of the hardware/firmware is as described, it is possible that "undocumented features" (bugs) may be present:  It is through testing, use, and feedback from the users that projects such as this may be improved, and the user is asked to be understanding of this fact.  This firmware is strictly intended only for non-commercial amateur-radio use and any other use is in violation of applicable laws.

Do you have any questions on this or other DF-related topics?  Go here.

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