BJT Zero-Crossing Detector for 40kHz Ultrasonic Signal - Distortion & Receiver Issues

Question

General Information About the Project

I am trying to design a zero-crossing detector circuit for convert the sinusoidal signal from an ultrasonic transducer to a square wave. Here is the image of ultrasonic transducer:

To trigger the transmitter transducer I generate 40kHz square wave with SG3525. Square wave is 12Vpp and duty cycle is 49%. Oscilloscope output shown below:

When I drive the transmitter with this pwm wave, I can read the signals from receiver sensor. Regardless of the location of the transmitter, the receiver always receives a signal at a frequency of 40kHz. What changes is the amplitude value and phase shift. Here is the receiver signal outputs below:

These measurements performed at different times, so frequency values of transmitter and receiver shown not equal. Even if their frequency values don't look compatible, they are compatible.

Zero-Crossing Detector

I want to convert the receiver's sinusoidal signal into a square wave to analyse the phase-shift between signals of transmitter and receiver. I used a Schmitt Trigger circuit, but it created a phase shift and could not successfully convert the sine wave into a square wave. Then, I decided to build the circuit with BJTs. The circuit that I built, and the output shown below:

^{simulate this circuit – Schematic created using CircuitLab}

Experiment

I tested the circuit shown above on a breadboard and analyse the circuit with an oscilloscope. My test steps are as follows:

1. I test the circuit with function generator with different voltages and frequencies.
2. I test the circuit with the receiver's output signal.

1st Experiment with Function Generator

When I analyse the circuit, I applied the voltage values from 1V to 10V and frequency value from 1kHz to 40kHz. When I apply a frequency value above around 5kHz, the square wave produced by the circuit begins to distort. Here are the images:

for 10Vpp and 1kHz:

for 10Vpp and 7kHz:

for 6Vpp and 40kHz:

2nd Experiment with Receiver

I tested the signals of receiver to convert them into square waves. To trigger the receiver, I used the SG3525 PWM circuit and transmitter. Bu it does not work on the circuit. I read sinusoidal 40kHZ 3Vpp wave from receiver. Then I connect the signal into the circuit to examine the conversion, but it did not work.

Results

When I feed the circuit with a sine wave using the Function Generator, I can receive a square wave at low frequencies. But when I increase the frequency (about 2-3 kHz and above) the square wave starts to distort. When I give the signals from the receiver to the relevant circuit, I cannot get any output. Not even a distorted signal occurs.

Question

I want to convert the sinusoidal wave coming from the receiver into a square wave, compare it with the wave I created in the transmitter, and obtain the phase shift by looking at this. However, the circuit I built to convert the sinusoidal wave into a square wave is malfunctioning at high frequencies. At the same time, this circuit does not even process the signals coming from the Receiver.

If this method I have applied is a logical method to see the phase shift, why can't I process the receiver's signal? If you know a more logical way to see the phase shift, I would be happy if you share it with me.

Best regards

Answer 1

Assuming a noiseless, perfectly sinusoidal, ideal voltage source, a \$12\:\text{V}\$ power supply rail, and that you only need one pulse or edge per full \$360^\circ\$ cycle consider the following experiment:

^{simulate this circuit – Schematic created using CircuitLab}

(For the following tests please ignore the negative-going edge at OUT. You don't care about that. It's not important.)

Here, I've taken seriously your order-of-magnitude variation in the input voltage peak. And I'm trying to find a circuit that will allow you to handle it, while at the same time attempting to achieve other important goals. Hopefully, the above circuit provides some useful thoughts in that direction.

Set \$R_3\$ and \$R_{11}\$ to their midpoints and apply a \$50\:\Omega\$ signal generator output set to \$10\:\text{V}\$ peak sine source to IN. Adjust \$R_{11}\$ until you get a sharp positive-going edge at OUT that is exactly aligned with the zero-cross of your input source.

Now switch over to a \$1\:\text{V}\$ peak sine source in your signal generator (same output impedance) to IN. Adjust \$R_3\$ until you get a sharp positive-going edge at OUT that is exactly aligned with the zero-cross of your input source.

At this point you have calibrated the Schmitt trigger and I think it will work to provide a positive-going edge at the zero-crossing point, once per \$360^\circ\$ cycle regardless of the magnitude of your receiver signal.

Note: Don't use a solderless breadboard. (Or, use it, but worry.) There are random order-of-magnitude of \$10\:\text{pF}\$ capacitances floating about all over such breadboards. Large enough to be a pain. Small enough to possibly not matter... sometimes. Best to avoid them, if possible. Use construction techniques such as manhattan, rat’s nest, dead bug, live bug, etc. I prefer to avoid solderless breadboards when I'm uncertain about trace capacitances. As glen_geek also points out, scope probes also can matter. And when you are trying to evaluate Schmitt trigger circuits it is better to avoid confusion than to embrace it. So mitigate as much as you can.

(Finally, I've specifically avoided specifying the BJTs because the above design should accommodate a variety of small-signal devices.)

Here's a simulation using LTspice stepping the peak voltage from $1:\text{V}\$ to \$10:\text{V}$.

Pretty close fit.

Best wishes, kkrgzz...

Answer 2

Instead of using a zero-crossing detector, you can convert a sine wave to a square wave directly using a comparator.

^{simulate this circuit – Schematic created using CircuitLab}