Important information about the Doppler II:

• This is not an official page of VE2EMM.
  
• The "Doppler II" has been obsoleted by Jacques, but the boards are still available from FAR circuits and alternate firmware is still available.  Please note that the original web page is no longer online, but:
• Here is a .PDF representation of most of the original "Montreal Doppler II" web page.
  
• Please be aware that Jacques may or may not be able to provide support for this project and your understanding on this matter is appreciated.  Feel free to ask me (via the link at the bottom of this page) if you have any questions.
• For additional information about the "Doppler II" see the "Doppler II Alternate Firmware" page.
• The "Alternate Firmware" for the Doppler II is still available - go here for more information on pre-programmed chips
  

Abstract:

Jacques Brodeur, VE2EMM, has designed a microprocessor-based "doppler" type DF unit described at VE2EMM's Montreal Doppler II page that drives the antenna switching circuit, updates the display, and does signal processing to determine the bearing.  This unit works along the same principles that the so-called "Roanoake" DF units do - but the addition of a microprocessor (to do the generation of switching signals and processing of the received audio) adds additional flexibility.  For a bit of information about the newer Montreal III Doppler unit, go to the "Doppler 3" page.   This page is not maintained by Jacques - see the disclaimer below.

This page describes some of the technical details of the VE2EMM Doppler DF unit, as well as a few more topics such as:

• Constructing the unit -  A few salient details that might prove to be important when you are building it.
• Operating notes -  If you didn't read the documentation too carefully, some of the info here might remind you - and maybe cover a bit more ground.
• Backlit display -  An illuminated display is very handy when trying to use this at night...
• Modifications of the Doppler II for use with the Alternate Firmware - The alternate firmware offers added functionality, and this page details the modifications to take advantage of it.

Constructing the Montreal Doppler II:

Getting parts

While this unit is not available as a complete kit, a complete parts list is available on the Doppler II page and the key components are available from Far Circuits as a "partial kit" that includes three ICs (the programmed microprocessor, a low-voltage rail-to-rail op amp, and the RS-232 interface IC) as well as a 20 MHz crystal, the circuit board for the DF controller and a circuit board for the PIN diode antenna array switch.

This is not a "kit" for a novice builder:  If you plan to undertake construction of this project you should either have good soldering skills as well as plenty of experience with this sort of thing - or a lot of help from those who do!

Construction isn't particularly difficult for the experienced builder:  The use of the schematic, parts location map, and parts list makes it pretty easy to populate the board, but there are a few things that should be kept in mind when component selection is made:

• Pay close attention to the types of capacitors used for C5, C6, C7 and C8.  These capacitors are the main frequency determining components in IC2, a 500 Hz bandpass filter.  Do not even think about using any ceramic or electrolytic capacitors for C6 and C8 (1 uf) as they simply are not suitable in terms of temperature stability.  For capacitors C5 and C7 one may use NPO ceramic if you have them, but polystyrene or polyester are preferred for these units, as well as for C6 and C8.  The key here is temperature stability!  If you skimp here, the unit will drift terms of bearing accuracy and usability over temperature.  I had on-hand some 0.47 uf high-stability polyester capacitors and used them in pairs, resulting in 0.94 uf of total capacitance for C6 and C8 while I used 470 pf polystyrene units for C5 and C7.
• Pay close attention to the "Construction Hints" found in the Operation and Construction text on the Doppler II page - particularly the part labeled "Adjusting the Peaking Filters."  You really must do this step to get optimum results.  In my case, I ended up paralleling 470k resistors across R4 and R5 to "dial in" the frequency precisely - but the precise values needed will vary with each construction.

A front panel view of an as-constructed VE2EMM Doppler-II unit
Click on the image for a larger version

Pictures on VE2EMM's page show the DF unit housed in a food tin, but that should be considered only to be a "serving suggestion."  Actually, about any sort of enclosure will do.  The only (minor) concern about a nonmetallic enclosure is that having to do with RFI - either RF getting into the unit (not likely to be too much of a problem, as a local transmitter would probably overload the DF receiver anyway, invalidating readings) or RFI from the unit, causing interference to received signals.  Actually, I have not noticed a tendency for problems of either sort.  Anyway, if there is a problem with RFI it will more likely be a result of RF being conducted along a wire or cable connected to the unit rather than from direct radiation from the circuit board and related components.

My "copy" of the 'EMM box is built as pictured, put in a plastic enclosure.  While it would have been ideal to place the buttons below the display (to more-closely match the items on the various menus) I miscalculated exactly how large the LCD display was (I got the largest 16 character by 2-line display that I could find) and had to settle for the arrangement pictured.  The upper switch on the right-hand side is the power on/off while the lower switch selects Bright-Dim-Off for the LCD's LED backlight. More on the backlight later...  Even though the unit is in a plastic box, I have observed no RFI problems.
 

A rear panel view of the same VE2EMM Doppler-II unit
Click on the image for a larger version

The other picture shows the rear panel, along with the various controls, cables, and connectors.  These are:  Speaker volume control (lower left), input level adjust (upper left) and headphone jack (next to the input level control.)  Next to this is the "damping control" for an added switched-capacitor filter (more on this later.)  The gray cable is the audio input cable, the red/black cable is for power, and the connector on the left is a high-density 15 pin "D" connector for the antenna controller (wired to allow the use of up to 8 antennas) and the right-hand connector is a 9 pin "D" connector for the serial data.

Construction difficulties

Having had quite a bit of construction experience, I had no problem assembling the parts and constructing the unit.  Some fellow amateurs are constructing their own copies of this unit, so I'll ask them about their experiences when they are done...

The only problem that I experienced appeared to be related to misprogramming of the processor:  When set for continuous serial data output of the bearing, it no longer provided any data that actually resembled the bearing:  Semi-random bearings were spewed out on the LCD and on the serial port.  Jacques was helpful in my getting a programmed part and the problem was resolved.  (This appears to have been a singular incident rather that an actual design problem, so don't worry...)

Since building the unit, Jacques has been kind enough to send me a copy of the source code.  Originally written to use the Hi-Tech C-compiler, I have since modified it to use the CCS PICC compiler and added some minor features and fixed some minor bugs.

More recently, very significant rewrites and modifications have been done to provide more features and operating flexibility.  Information on this new firmware, go to the Updated firmware for the Montreal Doppler II - page.  Firmware with the same added functionality/enhancement is also available for the newer Montreal Doppler III hardware.

The antenna array

Discussion of antenna arrays usable with this system may be found on the Doppler Antenna page at this web site.

Operating the DF unit:

Note:  This descrption refers to VE2EMM's original firmware.  For information about the newer firmware, go here.

If you have used "doppler "type DF units before (e.g. the "Roanoake" types) that directly indicate bearing, operation will be a generally familiar experience:  The unit will need to be calibrated, in terms of bearing.  In a vehicle, this is typically done by setting "0" degrees (due North) to be straight ahead of the vehicle while a unit mounted at a fixed location is typically configured to display bearings relative to True North.

Being microprocessor-controlled, a variety of configurations may be easily set up.  A few of these include:

• Calibration of the bearing readout.  It is very easy to set "straight ahead" as 0 degrees - very useful for vehicular work.  Manual calibration (to set any arbitrary offset) may also be done.
• The number of antennas may be selected.  This unit supports 4, 6 or 8 antennas.
• The switching polarity may be selected.  The "active" antenna may be selected using a logic "high" (5 volts) or a logic "low" (0 volts.)  The required setting will depend on how the antenna switching diodes are driven.
• The "spin" direction may be selected as either CW or CCW.  Some people number their antennas clockwise when viewed from above, and others when viewed from below.  This selectability facilitates either scenario.
• A facility is provided to test each antenna individually.  This makes it easy to verify that the PIN diode switches are working and/or to help identify each coax.
• Serial data output is available.  The current bearing - and signal quality - is output in the (so-called) "Agrelo" format (ala the "DF Jr." [tm].)  This format is:  "%bbb/q<cr><lf>" where "bbb" is a 3-digit bearing and "q" is the signal quality.  A baud rate of 2400 or 4800 baud may be selected (with the possible selection of 9600 baud on newer software.)  This bearing data may sent continuously (at up to approx. 10 bearings/sec.) for logging or compass rose displaying or in timed intervals - for APRS packet use.
• This unit will also accept serial input from a GPS receiver.  This may be done to facilitate the use of both GPS and DF information using a single serial port on a computer or with APRS.
• A "minimum quality" display threshold may be selected.  Signals of quality lower than this will not update the display.
• An "S-Meter" reading may also be displayed.  This takes a voltage that goes from 0 to between 2 and 5 volts for a numerical "S-meter" display.
• Configuration for settings for up to 3 radios/installations may be stored in nonvolatile memory.  These settings include bearing calibration, number of antennas, their polarity and spin direction and make it relatively easy to move the unit between, say, two vehicles and their radios and a house-mounted system.
• Readings may be "averaged" to improve the stability of the displayed reading.  Internally, one reading is taken for every 24 "revolutions" of the antenna array, and since this unit "rotates" the antenna 500 times per second, 20.83 (500/24) bearings are available each second.  The display may be updated this quickly (i.e. no averaging) or 2, 4, 8, 16, 32 or 64 averages may be taken between updates.  This averaging is very useful for removing "jitter" from the derived bearing - a condition often encountered due to modulation of the carrier itself and/or signal degradation due to noise, mobile flutter, and/or multipath.

• Unlike the traditional "Roanoake" DF box, this unit uses a 16 character by 2 line LCD display for showing the DF information as well as configuration data.  This includes a sort of simulation of a "36 segment" compass rose showing the direction of the bearing.  While a clever attempt, this display is not as useful as one might hope - particularly if one is attempting to use this unit in a vehicle:  It is very small and might be difficult for some people to see.  It is also not at all circular, so it is more difficult to translate the displayed bearing to the direction in which to drive or for an observer to look.  While the bearing is shown numerically, this is also not as intuitive as the "traditional" compass rose in its usage.  To this end, an external compass rose may be "driven" via the serial port - more on this later.  Jacques has addressed this issue in his more recent version of the Doppler DF unit, the "Montreal III."  More on info on this may be found at his web site.
• While the LCD-based faux compass rose shows the last valid reading, the 3-digit bearing display does not (it shows "BAD" - a status also indicated by a "0" for the signal quality indicator.)  If this signal has gone away before you happened to glance at the reading, you will have to "guesstimate" based on the faux compass rose display.  Having a serial port on this unit, however, make is perfectly practical to log the readings.
• While not really a complaint, it would have been nice if the "averaging" of the reading were a "sliding" average rather than simply taking a block of N readings ("N" being the number of averages) and then spitting out the result (and resulting in a much slower display update rate.)  Of course, to have a "sliding average" requires having enough memory to store N readings - as well as a bit of extra computational horsepower - so it is perfectly understandable that this was not done.

• In real life, one would rarely operate using "1" average (that is, no averaging - 10 readings/sec.)  First of all, this update rate is a bit too fast to be truly useful to a human operator and bearings may be severely affected by modulation or multipath on the received carrier.  Experience shows that an averaging of 4, or 8 readings (correlating to 2.5 or 1.3 readings per second respectively) "stabilizes" readings quite well while still maintaining a comfortably fast update rate.
• Volume setting appears to be critical.  One should set the volume so that the display shows a "signal level" of about 90 on "average" signals.  An occasional "OL" (overload) indication may result - but this doesn't appear to affect accuracy.  What can affect accuracy is a dramatic drop in audio level - and this seems to cause a slight shift in bearing. (This is another issue that Jacques is working on addressing in the Montreal III doppler.)  The workaround is to set the volume at a given level and not change it.
• The schematic suggests a rheostat to set the speaker level.  The way this is typically set, when turned all of the way up the volume of the speaker mounted in the DF unit's enclosure will be at the loudest expected level, with reduction done with the rheostat.  Chances are, this will overdrive the DF unit, so the use of another potentiometer is suggested in order to independently set the drive level to the input of IC2.  A 1k potentiometer is suggested for this, as this would still be low compared to the input impedance of the first section of IC2.  This is one of the controls visible in the "rear panel" picture.

Note:  See the links at the top of this page for info about the Switched Capacitor filter, a Pelorus, and a Comb Filter.
 

Schematic of the backlight circuit control

Display backlight:

The DF unit is designed to work with a standard 16 character by 2 line LCD and an obvious nicety is to select an LCD with a backlight.  There are several options available for backlights:

• None at all.  This is fine if you never plan to use the unit in the dark or subdued light - unless you have a flashlight, that is...
• Electroluminscent.  This type of lighting is very efficient, but it requires an inverter to generate the required voltage (approx. 100 volts at 100-400 Hz or so.)  Unless one already has a surplus unit on-hand, the inverter is often as expensive as the display itself.  This type of lighting also has a relatively short lifetime - only a few thousand hours until the brightness dims to 50%.
• LEDs.  Integrated within the background of the display, an array of LEDs provides backlighting.  Typically, a parallel array of two green LEDs in series is used, requiring a voltage of 4.2 volts (nominal) at a maximum current of between 100 and 200 mA.  A color of green or yellow is highly recommended even though these colors are not as "snazzy" as a red or blue because the human eye is not "designed" to resolve small details at either end of the color spectrum, and the looking at a display illuminated with red or blue can cause excessive eye fatigue.

One suggested scheme is shown in the schematic.  In this example, the "dim" setting is implemented by putting the LED array in series (the switch in the center position) with the input of the 7805 regulator on the VE2EMM DF board.  With the typical current consumption of the DF unit being on the order of 50-60 mA or so, the LED is operating nowhere near its 200 mA maximum continuous current rating.  Additionally, the current to operate the backlight in this mode is "free" as the power operated the LED would otherwise be dissipated as heat by the voltage regulator.

The only caveat to method of powering the backlight this that the power supply voltage must be at least 4.2 volts (or so) higher than the minimum voltage at which the 7805 will regulate (about 7 volts) so one needs more than 11.2 volts to maintain voltage regulation when operating the backlight this way - an effect of the 4.2 volt drop across the LED array.  Another effect of this operation is that the brightness of LED backlight may fluctuate very slightly according to the load of the unit itself (this effect being mitigated somewhat by the 220 uf capacitor.)   This load can vary slightly, depending on the operation of the RS-232 port and how the drive to the antenna unit is provided.

There is also the "bright" mode.  This is accomplished by putting a 100 ohm 1 watt resistor to ground, increasing the current through the LED backlight.  The "off" mode is implemented simply by shorting out the LED backlight.  (If you feel nervous about shorting out the capacitor with the switch, you may put a low-value  from 1 to 4.7 ohms - resistor in series with its center terminal.)

Antenna drive circuit:

If you look at the schematic for the DF unit and read the text at the VE2EMM web site, you'll see references to using the output of the PIC to directly drive the PIN diodes.  This is perfectly practical, as the PIC is capable of sinking or sourcing 20 mA or more - plenty to drive a diode.  Although this "Direct drive" works, it makes me a bit nervous:

• With direct drive, one connects the PIC's output pins directly to the real world.  In terms of RFI, this is just asking for trouble, as the PIC (or any processor) can easily generate RF noise clear into the VHF/UHF spectrum.  This is why ferrite beads are recommended with this ("direct drive") approach.
• There is little protection offered to the PIC:  With the "direct connect" method, one relies on the PIC's own protection circuitry to protect against static discharges, etc.  While this circuitry is effective against the casual minor "zap" it will do nothing to protect against potential damage from nearby lightning strikes or the accidental shorting of an antenna drive output to, say, the +12 volt supply.  If you do accidentally make the latter mistake, you have just bought yourself another PIC...

There is one disadvantage of doing it this way, though:  The amount of current available from the PIC's 5 volt output through the 1k resistor is not likely to drive the switching diodes properly and an external driver circuit will likely be required - but that isn't really too much of a problem.

Note:  Neither the author or UARC officially endorse any vendors mentioned above.  The level and satisfaction of performance of any of the above circuits is largely based on the skill and experience of the operator.  Your mileage may vary.

Note:  This page (and other pages on this site) are not "official" pages of VE2EMM and he cannot reasonably be expected to answer questions about everything that is covered here.  These pages are simply set up to aid those who have built or might build the described equipment.

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

This page updated on 20070214

Yes, the plural of "pelorus" is "peloruses" - rather than "pelori."  Look it up if you don't believe me!