Figure 3 shows a block diagram of the Doppler D/F, which consists of two assemblies : the display unit houses most of the electronics, and resides in the vehicle passenger compartment, where it can be observed by the T - hunters. The antenna unit contains a few chips and the switching diodes for the antennas. It is mounted beneath the antenna ground plane.

Figures 4 and 5 show the schematic diagram of the D/F. The "vertical" schematic is provided to simplify hardcopy printout from a web browser. A "horizontal" version of the main board schematic is available [ HERE ]. Most of the components are common enough to be obtained from a local Radio Shack, but a few will require a better source. The parts list provides part numbers from various "catalogue" parts suppliers. None of the parts are very unique or specialised, so they should be fairly easy to find.

Antenna performance can be improved by substituting better parts, but the "better" parts are not as common as the ones listed on the schematic... see the section titled "ANTENNA" for more information.

The enclosure used by the author is manufactured by SerPac ( available from Digi-Key and Allied ) but any enclosure will do. The mounting holes on the PC board artwork will mate perfectly with SerPac enclosure p/n S-132, ( for the display ) and SerPac p/n S-131, for the antenna unit. The SerPac enclosures were chosen mostly because they were available, stylish, and made of plastic... holes for the indicator LED s and various cables can be easily "drilled"... with a soldering iron.

The following "MOD" was added in June of 2000, but the PC artwork was not changed to accomodate it, and it must be installed "by hand". This mod improves the behavior of the ( NE555 ) CALIBRATION 1-shot, by disabling it's trigger source while it is producing an output pulse. The 555 does some "strange things" if it is triggered more than once during an output pulse, which causes multiple ( different ) LED displays.

This mod prevents that, and allows the full range of the CALIB trimpot to be used. ( = 150 % of display scale )


The chip used for the switching filter ( U2 ) is not really intended for this kind of service, but it works well, in most cases. Normally, a small portion of the control signal on pins 9, 10 and 11 "leaks" into the signal circuits, causing the filter to produce a small DF signal at the output, even when no signal is applied to the DF. this signal is usually small enough to be ignored. In a few cases, ( depending on the particular chip ) this leakage is more severe, and can interfere with the proper operation of the DF.

The leakage signal can be reduced by a factor of 10, by increasing the values of capacitor C2 through C9 to 1.0 uF, instead of 0.1 uF. To maintain the same filter bandwidth, the values of resistors R2 and R3 must be reduced to 100K from 1.0 megohm. The values of C2 through C9 can be increased even further, if R2 and R3 are proportionately reduced. If you do this, use capacitors of fairly good quality... avoid low-grade tantalum capacitors. Ordinary aluminum electrolytic capacitor work OK, but inexpensive ones suffer from wild variations of capacitance, due to their wide tolerance ratings. The exact value is not important, ( if you adjust R3 and R4 to compensate ) but ALL the capacitors should have the same value, within 10 or 20 percent. ( to avoid large DF bearing errors )

A few folks have commented about the ground connections for these capacitors. The schematic shows all capacitors running to +5V, but capacitors C6 and C8 actually run to ground, on the circuit board. For the filter to work, all the capacitors must be connected to a suitable "AC ground". When I created the artwork for the board, some of the capacitors were close to a "convenient" ground connection, ( C6 and C8 ) and the others were not. There was a convenient "+5V" supply connection nearby for the other capacitors, so I used that for "AC ground", instead of a "true" ground connection.

The +5V supply is "AC grounded" by capacitor C22, at the voltage regulator. The +5V supply is actually a better ground for the switching filter... If any of the filter capacitors "leaks", they will leak to a "zero signal" level, if they are tied to +5V. If capacitors C6 or C8 "leaks", they will generate a "false" DF signal. If you use "polarized" ( i.e. "electrolytic" ) capacitors, be sure to observe proper polarity ( for C6 and C8 ) when installing them. The other capacitors are "biased" at +5V and "grounded" to +5V, so there is no DC "bias" voltage ACROSS them. Electrolytic capacitors can operate normally with a small amount of reverse voltage applied to them... as a "rule of thumb", they can operate normally with a reverse voltage equal to 5% of their rated voltage value. This can happen for all capacitors, ( except C6 and C8 ) because they have no DC bias on them, and the signals voltages can drive these capacitors ( slightly ) into reverse polarity.

The signals here are small enough to fit that definition, but ( FYI ) you can make your own "non-polarized" capacitors, simply by connecting two ( polarized ) capacitors together "in series", if they point in opposite directions. Personally, I've never seen problems with this circuit that required the use of non-polarized capacitors.