The VE2EMM "Doppler II" DF
A few notes/comments/modifications, etc...

New firmware is now available for the Montreal Doppler II and Doppler III DF
(click here for details!)

Important information about the Doppler II:


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:

For even more information about the Doppler II and Doppler III units (at this and other sites) go to the Unofficial VE2EMM Doppler Information page.

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:

Putting it in a box
The front panel of the as-constructed DF unit
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.
The back panel of the as-constructed Montreal Doppler II.
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:

As good as it is, there are a few minor drawbacks that must be worked around: A few operational details Modifications and accessories:

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 how the LED backlighting is wired.
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:

In the case of the unit pictured, a green LED backlight was chosen for the display.  This has the obvious advantage that no inverter is required (as would be required for Electroluminscent) and the further advantage that the display brightness may be easily adjusted - simply by selecting the current.

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:

For these reasons (and a few others that I could come up with) I chose to put series resistors on the PIC's output and use an external antenna drive circuit.  Fortunately, the circuit board has been designed to accommodate these resistors and values from 1k to 10k should work well - depending on how your external circuit is driven.  Having these resistors will allow the PIC to tolerate momentary faults to (even to V+) on these connections as well as providing very effective RFI suppression (the typical ferrite bead would only have 100-200 ohms of "resistance" at VHF, while the 1k resistor will have, well, 1k of resistance...)

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.

Return to the KA7OEI ARDF Page.

This page updated on 20070214

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

Since 12/2010: