THE CIRCUIT
The figure below shows the block diagram of the portable Doppler, which is constructed on two PC boards. ( ANTENNA and DETECTOR boards ) Audio from the reciever is terminated into a 10 Ohm, 1/2 Watt resistor and buffered by a unity gain amplifier, which is DC biased at mid - supply. From here, it is fed into a Switched Capacitor Filter, ( SCF ) which also serves as a synchronous detector, producing DC voltages in two capacitors which are of equal amplitude, but opposite polarity. ( referenced to mid - supply ) These DC signals are differentially amplified by another buffer amplifier, and the resulting DC "error" voltage varies above or below mid - supply, depending on which antenna is closer to the signal source.



From here, the error voltage is fed to a pair of op-amps, connected to serve as voltage comparators, ( "threshold detectors" in the block diagram ) and the outputs of these comparators are used to open / close CMOS switches that enable / disable two separate ( 20 db ) audio attenuators... one in each earphone audio channel. The two audio channels are then amplified with op-amps and buffered with discrete transistors to provide a drive level suitable for 8 - ohm stereo headphones.

A 555 oscillator serves as a 1000 Hz clock source, which drives a D - type flip flop ( divide by 2 ) to achieve a fully - symmetric 500 Hz switching clock, with two complementary outputs. These two outputs are used to drive the SCF / detector, and are also buffered with discrete transistors to provide drive signals for the antenna switching diodes.

The figures below show the schematics of the unit. Note that all inputs and outputs of the detector board are filtered with small RF chokes, and RF bypass capacitors. Much greater attention was paid to EMI / RFI "hygiene" in this unit, ( compared to the mobile unit ) because there is a far greater chance that this unit will be used in a "high radiation" environment... directly adjacent to a transmitting antenna. For identical reasons, the PC artwork employ "boundary etching" methods, to maximize the area of the ground plane, and reduce sensitivity to RFI.



The PC board for the detector fits into a SerPac enclosure model A-27-9V, which has a compartment for a 9 volt battery, and a small panel suitable for mounting the various jacks and connectors. It is small enough to be easily carried on the belt of a hunter. This enclosure is plastic, and ( therefore ) offers no EMI shielding, but various "tricks" can be employed to achieve this, ( described in the next section ) or some other type of enclosure can be used.

CMOS circuits were employed for battery conservation, and to eliminate the need for a voltage regulator. Each CMOS switch is "closed" when its control input equals +9V, and "open" when control = GND. The audio attenuation is set by the values of R11 / R12 ( for one channel ) and R13 / R14, for the other channel.

The audio output amplifiers use complementary transistors to provide enough drive capacity for 8 ohm headphones. Note that the feedback signal for these ( unity gain ) amplifiers is taken from the transistor emitters, rather than from the output of the op-amp... this eliminates the non - linear behavior of the base - emitter junctions, so no loss of audio fidelity will occur. It also means that the transistors used are not really important... they do not have to be a "symmetric pair" of transistors... any NPN / PNP "pair" should work equally well.



The PC board for the antenna switching unit is much smaller, and designed for mounting in a SerPac enclosure, model C-4. Again, this enclosure is plastic and offers no RF shielding, but some "tricks" can be used to achieve this, ( e-mail me ) or another type of enclosure can be used. The prototype unit did not appear to suffer any adverse effects due to this lack of shielding.

In any event, the ANTENNA unit should be mounted directly on the headset... I used tie wraps to secure mine to the top/center of the headband, equidistant from each antenna. The co-ax cables to each antenna should be of equal length, within one inch.


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