Implementation

A block diagram of our implementation is shown in the lower left hand corner of the summary poster. The signal received is filtered, fed through a pre-amp, filtered again, and fed into the phase comparator. The locally generated signal is generated by a crystal-PLL-VCO combination and fed into the phase comparator (the transmitter is similar except that a power-amplifier is included after the VCO. The transmitter also has a microcontroller to control the PLL (not pictured)).

The phase comparator consists of cascaded logarithmic amplifiers (to turn the signals into a square waves, thus eliminating amplitude dependence and creating a zero-crossing based phase measurement), the outputs of which are fed into an XOR-gate. Analog circuitry measures the length of the pulses emitted by the XOR gate, and a representative voltage is produced. This voltage is fed into the A2D of a microcontroller, which produces a distance and does any needed software filtering. The resulting numbers are fed through a serial port interface to a host computer. A parallel port interface is provided to program the microcontroller.

The circuit schematics are available below. We use discrete components designed for all of the components except the PLL's loop filter (which is a relatively low-frequency RC filter, so discrete components are not needed).

The receiver consists of these blocks:

The transmitter consists of these blocks:

Data sheets:

Parts List/Digikey Part #'s:

Layout is critical. Layout is still in progress, using the following guiding principles (in order of priority)

  1. Certain traces need to be specific lengths (noted on schematic)
  2. Keep all high freq. traces as short as possible (and not forked)
  3. Keep VCC capacitors as close to the corresponding package as possible.
  4. Keep all ground plane vias as close to the package as possible (ground plane fills unused space on the bottom of the board)
  5. Keep all traces as short as possible
  6. Route digital signals exclusively on the bottom layer and analog signals excluisvely on the top layer

Progress

The following things have yet to be implemented:

© 2002 Peter Schmidt, All rights reserved.