Hello all,
I've been working off of TI "Ultrasonic Distance Measurement With the MSP430" application report (http://www.ti.com/lit/an/slaa136a/slaa136a.pdf) and I found that I was having a bit of difficulty achieving similar results to those shown in the report. The problem that I have been having is with the voltage driver configuration that uses the the 6 CD4049 inverters to double the voltage swing of the the MCU signal (set to a 9Vpp from the voltage rail) to 18 Vpp.
I've seen this configuration in other ultrasonic range finder designs and in those design notes this inverter configuration has been referred to as a push-pull amplifier which somehow doubles the voltage swing by combining two signals that are 180 degrees out of phase. All other instances of this push-pull set up that I have seen involve the use of transformers and an AC input signal to achieve this signal amplification, which leaves me confused as to how this setup would work for a strictly DC MCU produced square wave signal. I was wondering if anyone here could explain how this particular configuration works and how the result shown in figure 2 of the application report achieved? How does the combination of two 180deg out-of-phase DC signals result in a amplified signal with double the voltage swing?
I recently built a similar circuit using two different buffer configurations for the transmit signal drive section. I originally used the CD4049UB to create two inverted 9Vpp signals that were then sent through two sets of two CD4503B buffers in parallel. The resulting signals were still 9Vpp square waves with presumably "adequate" current for driving the transducer. The problem was that even with these two signals going in to the two pins of the transducer, the resulting signal (which was still 40KHz) was not nearly strong enough to be picked up by a separate receive transducer more that a foot away (with the receive transducer directly facing the transmitter). Although the report doesn't state any expected distances for this device, I feel that this result is far below the potential capabilities of this range finding circuit.
I built the circuit again but with the configuration shown in the report (using all 6 inverters on the chip) and I was still getting the same result as before but with what appeared to be an even weaker transmit signal ( I was unable to pick up anything more that 7 or so inches away). I was going to attribute these results to potential hardware failures, however, since I don't quite how this form of push-pull amplification works I can't really say that my theory is correct.
If anyone can provide some insight in to what should probably be a very simple range finding circuit configuration then that would be very helpful.
Thanks
