With the DM6350 chip and the PCB in hand, the testing procedures started. First the bonding wires from the die to the IC package as well as the power supply vias on the PCB were verified. Then an extensive verification on the bias voltages of the circuit was completed. Finally the functional tests were done, initially with a signal generator generating the input signals and later with the actual transducers.

Grounding the inputs, the DC level at the output of the LNA, the mixer, the filter, and the comparator were measured and analyzed. The DC level at the end of the LNA is set by a DC feedback, in order to match the currents of the two current sources. As long as all the transistors on the current sources were in the proper operating region, the DC tracked the external DC voltage. For the mixer, the DC voltage depends on the current source at the tail of the Gilbert cell and the value of the resistance. Pushing a current of 20uA at each branch and placing 40KOhms resistors a common mode voltage of 1V is expected at the output. This same voltage, in addition to a systematic offset by the OpAmp, should also be the output of the filter. Both results were confirmed on the test bench. The output of the comparator is 5V in the absence of a signal.

The next step was to analyze the waveforms at the end of each of the blocks with the presence of an input signal. Applying a 1mVpp 40KHz sinusoids modulated with a much lower frequency square wave the behavior of each step of the receiver could be observed.

Fig. 14 - Waveforms probed with the signal generator as input to the distance sensor

Fig. 15 - Same waveforms of the Fig.13 with a reduced time scale

In the figure above, the first signal in yellow is the output of the LNA. With a gain of 55dB, it is an amplified version of the input. The green and pink signal are the differential outputs of the mixer; note in figure 15 that the frequency has doubled and the DC offset is present. At the bottom, in blue, is the output of the filter.

Finally the transducers were connected to test the complete system. The signal remains the same (yellow for the LNA, green for the Mixer, blue for the Filter), but now the pink signal represents the output of the comparator.

Fig. 16 - Waveforms probed of the complete system. Highlighted is the Tdead of 1.32ms

Fig. 17 - Waveforms probed of the complete system. Highlighted is the Echo time (Techo) of 5.24ms, corresponding to a Measured distance of 35 inches

The very first burst observed at the LNA happens almost exactly when the TX transducer is activated. This is expected due to the proximity of the transducers and the mechanical coupling that they have. While the system is still detecting this first burst (while the pink signal still down), the sensor cannot respond to anything else. This creates a dead time (Tdead) that is disregarded by the microcontroller, limiting the minimal distance that the sensor can measure. This Tdead depends on how much of the TX transducer signal is coupled into the RX transducer and on the threshold of the comparator. Small threshold values will result in larger dead times. From the measurements this time can vary from 1ms to 1.5ms, leading to minimum distance (Dmin) from 8 to 12 inches. The maximum distance measured (Dmax) depends on the threshold value of the comparator, but, when the threshold is too low, random noise at the output can trigger false measurements. The maximum stable distance measured was 200 inches.

Note that there is a tradeoff in the threshold selection, a large threshold value reduces the Dmin, and while a small threshold value will increase the range of the sensor. But we know when each of the options is more suitable. At the vicinity of the first burst sent by the TX transducer, it would be better to have a large threshold value. But as time passes without an response, weaker signals are expected, hence we should lower the threshold. A possible future improvement for this design is to use an adaptive threshold value.

Fig. 18 - Resulting waveforms with a far object. Techo =20.54ms and the measured distance is 137.5 inches

Fig. 19 - Plot of the Measured Techo Vs Distance

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