Testing

 

 

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Prototype Testing

The picture below illustrates our testing setup for the prototype transmitter.  We used a digital power supply for our 1.4V source.  To view the output of the circuit, we used a x10 probe and an oscilloscope.  Using the oscilloscope, we measured the frequency and amplitude of waveforms at various nodes in the circuit.  In addition, we used a digital multimeter to measure DC voltage and current values in the circuit to compare with values measured on our final PCB.

Figure 1.  Prototype testing setup.

A battery-operated handheld AM/FM radio was used to receive the transmitter signal.  In order to locate the correct frequency of transmission, the radio was held close to the microphone while the frequency of the radio was varied.  When the transmission frequency was found, a high-pitched squeal was produced by the radio due to feedback.  The radio was then moved further away from the transmitter to eliminate the feedback, and the frequency was fine-tuned to receive a clear speech signal.

The performance of the prototype was very impressive.  Testing revealed a clear, line-of-sight transmission distance of approximately 10 feet with no antenna.  Within this distance, the transmitted speech was very clear and intelligible.

An antenna was not used because we found that the circuit operated much better without one.  The circuit operates in the 88-108 MHz range and thus requires an antenna length of approximately 3 feet.  This was much too large for our application.  Therefore, we removed the antenna and found that the circuit alone provided enough signal power to transmit clearly within 10 feet.

 

PCB Testing

Testing of our final PCBs was very similar to testing of our prototype.  However, PCB testing was much more difficult due to the extremely small size of the board (7.8 x 11.6 mm).  Small probes were required to make contact with the nodes within the circuit.  Our initial problem was finding a tool small enough to adjust the variable capacitor.  After trying to create our own tool by modifying several small screwdrivers, we found and purchased a special tool made by the variable capacitor manufacturer.  This made adjusting the capacitor much easier.

Initial testing of our first PCB with a probe and oscilloscope revealed that it was not oscillating at all.  Rather there were pure DC voltage values at every node in the circuit.  After several weeks of troubleshooting and speaking with various professors, we found two major problems with the layout of parts on the bottom side of the board. 

The first major problem that we discovered was that the bottom two and top two resistors were mounted backwards on the bottom side of the board.  This was simply fixed by providing the correct view of the board layout to the technician who mounted the parts on the PCB. 

Second, when designing the board layout, the transistor pads were not switched to account for the transistor being mounted on the bottom side of the board.  Thus, the base and emitter pads on the PCB were switched.  This mistake was corrected by our excellent circuit technician who was able to mount the transistors upside down on the board.

After fixing these problems, the PCB finally produced an output signal.  However, due to the extremely small size of the circuit, it did not provide enough inherent transmitting power to effectively transmit the signal.  Therefore, an antenna was needed to receive the signal with the radio.

 

 

Team AudioVibe - University of Idaho 2004-2005

Questions or concerns: audiovibe@uidaho.edu