You won't need to worry about frequency folding if you make your shift frequency lie outside the bandwidth of your received signal, i.e. Too large of a signal bandwidth would impede the op-amp's performance. We would rather band-limit the incoming ultrasonic signal to fall within the window we designed for.Ī second consideration is wanting to band-limit the signal before it goes through the op-amp gain stage, since op-amps have a fixed gain-bandwidth product. For instance, if the incoming signal was a mix of 34kHz and 36kHz, and the shift frequency was 35kHz, we wouldn’t want to produce double the amplitude of a 1kHz audio output. The main concern is the folding of negative frequencies back into the (positive) audible range. This turned out to be redundant with our particular receiver’s transfer function (which was far narrower), but we kept it as a preventative measure in case we found a receiver a larger bandwidth, one extending beyond our passband. We added a band-pass filter (BPF) after the receiver in case the received signal landed outside the assumed range of 35kHz to 45kHz. Check the max input voltage your multiplier can handle, and make sure that won't be exceeded when the receiver is at its minimum distance from the ultrasonic source. The gain for the configuration above is set by (R1+R2)/R2. Simply measure the voltage of the signal without the gain stage, and set the gain accordingly to provide a signal with the strength of a few volts. The gain stage should be designed last out of these three steps. See the following section if you're interested in more detail. The bandpass filter does two things: it attenuates noise outside of our expected frequency band, and its high-pass portion prevents negative frequencies from occurring. Set a feedback loop with no resistance from the output to the inverting input of the op-amp. Simply connect one end of the transducer inducer to the non-inverting input of the op-amp. Otherwise current must come from the sound itself via the ultrasound receiver, and that current is insufficient. Voltage Buffer after Receiver Supplies current. Several stages are needed between the ultrasonic receiver and the multiplier: a voltage buffer, a band-pass filter, and a gain stage. Upon building the entire circuit and realizing the bandwidth of the receiver was far narrower than expected, we made the shifting frequency higher to produce a lower, more pleasant center frequency of the audio output. The center frequency would land at 5kHz, which was a pleasant-enough audible sound. Consequently, our shifting signal needed to be 35kHz to shift the lower bound of the incoming signal to 0Hz, and the upper bound to 10kHz (still within the audible range). We assumed a receiver bandwidth of 10kHz maximum, which meant that our incoming frequencies would be between 35kHz and 45kHz. We planned for an incoming ultrasonic signal centered around 40kHz, since that is by far the most common center frequency of affordable ultrasonic receivers. For more information, see the technique of heterodyning. The only difference is that we will also see the harmonics A and B in our output as well. In reality, our incoming ultrasonic signal will be the composition of several frequencies, but the principle outlined above still holds. We can measure A, and we can choose B in order to place cos(A-B) in the audio range. Think of cos(A) be our incoming signal from the ultrasonic receiver, and think of cos(B) be a local oscillation we create. Our ears cannot distinguish a 90 degree phase shift of sound wave. Do not worry that we are now using cosine instead of sine, cosine is just a sine wave shifted 90 degrees, sin(wt) = cos(wt +90). the product of two signals at different frequencies produces the sum of two signals: one at the sum of the incoming frequencies, and one at the difference of the incoming frequencies. To achieve a frequency shift of our incoming ultrasonic signal we took advantage of the following trig identity:Ĭos(A)*cos(B) = ½, i.e. An introduction to the theory is given here, and more detail for each part of the circuit is provided in later steps. The block diagram above shows the high-level view of our circuit.
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