Doppler Rdf

Posted on Feb 5, 2014

The power supply is just a 7812 voltage regulator followed by an op-amp set to 6V to create a half way supply voltage for the audio op-amp stages. A 12V regulator seems to be too high for a battery fed application. These 4 outputs are all followed by a op-amp set up as an integrator. and the R/C circuitry around these op-amps ensure a very soft transition to the next

Doppler Rdf
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antenna element. A network of two resistors feeding the + inputs of the op-amps is used to bias the op-amps; the voltage on the + inputs is determining the output waveforms and therefore the resulting transition from one to the next antenna. This is quite important for normal operation; in existing designs very small input signals will not give a stable reading, and signals that are too big will clip in the last filter stages resulting in serious bearing deviations. Therefore, the automatic gain control in the PA8W Doppler is a valuable addition. That means that a constant audio signal exactly on frequency will hardly be affected by the capacitors that will already have been charged to the right voltage over some time. Only parts in the audio that deviate from average will be reduced extremely because the capacitors will have to be charged or discharged to the new value. All kinds of parameters, such as group delay in your receiver and phase shift in the RDF low pass filter stages will shift this point away from the truth, The output of the NAND poort should be connected via the 3M3 resistor to the base of the dimmer transistor to provide some schmitt-trigger action for a clean "freeze" of the last bearing. As soon as the AGC voltage is dropping below the "freezing point" the collector of the dimmer transistor will go up and the output of the NAND will go down, preventing the 40174 to be retriggered. A simple way to adjust...

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