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Reviews Categories | Direction Finding equipment | Montreal Doppler III by VE2EMM Help

Reviews Summary for Montreal Doppler III by VE2EMM
Reviews: 1 Average rating: 5.0/5 MSRP: $(missing—add MSRP)
Description: Computer-based "Doppler"-type DF unit for FM recievers.
Product is in production.
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You can write your own review of the Montreal Doppler III by VE2EMM.

KA7OEI Rating: 5/5 Jan 19, 2006 13:21 Send this review to a friend
Works really well!  Time owned: more than 12 months
This is the 3rd DF unit by Jacques, VE2EMM, that uses the PIC family of microprocessor to help analyze the signal and display it. The original "Doppler I" was simply a "software" implementation of the traditional "Roanoke" hardware, but the Doppler II and the Doppler III use DSP techniques to analyze the received signal and produce the bearings.

This version is an improvement over the previous Doppler II: It has both an LED-based compass rose display to provide an intuitively-obvious representation of the bearing as well as a 2-line LCD display to show the bearing numerically as well as step through menu and configuration settings, and it has the well-proven "Roanoke-type" 8-capacitor switched-capacitor bandpass filter to provide optimal filtering. (The Doppler II had only an LCD-based representation of the compass rose - which was more difficult to interpret - and it did not have the 8-capacitor "Roanoke-type" filter, relying instead on "bearing integration" in software.)

An improvement on the Doppler I and II is that all bandpass filters are switched-capacitor type and based on a crystal reference: This means that there is negligible phase drift with temperature - a potential problem with the Doppler I and II units that relied on the inherent stability of the bandpass filters' frequency determining components (namely the capacitors.)

The firmware provides adjustable levels of filtering to "smooth" the bearings that are recieved - something that is helpful if you are experiencing a lot of multipath or heavy modulation and/or noise on the signal is "spreading" the bearings a bit.

Also nice is the ability to interface a GPS receiver and computer with it: Using a single serial port, one can send both bearing (in "Agrelo" format) and GPS location data to a program such as APRS or GPSS, providing a moving-map display with "live" bearings being displayed.

Another very helpful feature is that this unit will, on the fly, calculate the "quality" of the bearing being received and it allows for a threshold to be set below which bearings will not be displayed and "clutter" the display. While this cannot detect all bad bearings (one's own experience and skill is most helpful in that!) it does help weed many of those bearings that might be utterly meaningless and confusing.

One modification that I would strongly recommend would be the replacement of the fixed resistor (R55) with a 1 meg pot in series with a 47k resistor, with a 180 pf capacitor in parallel with the two: This provides a continuously adjustable "damping" control to allow both slow, well-filtered response to weaker, noisy signals as well as a very fast response to "cleaner" and/or short-duration signals.

Also owning the older Doppler I and II units, I have since added additional features of my own to the software of all three of the units to provide even greater flexibility and further improve performance.

As with any type of RDF activity, it takes practice and more practice to become proficient in the use and interpretation of the data one obtains. This and any type of RDF equipment is prone to multipath and reflections and it is ONLY through experience that one learns to recognize and deal with these situations!

This unit is available as a "pseudo-kit" - that is, as bare boards and/or pre-programmed microprocessors: The circuit board patterns are on the website so you can either "burn" your own board or, if you are ambitious, lay out your own version. It will be up to the builder to acquire most of the parts and put them in the enclosure, as well as provide an antenna switching unit.

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