Zero Crossing Detection of ~ 400 kHz Signal with MCU

Question

I want to measure the frequency of a signal using the digital input pins of the NUCLEO-F767ZI. The signal is sinusoidal with an amplitude of 5 V and a frequency ranging from 100 kHz to around 400 kHz.

1.) First I thought about simply feeding the pure analog signal to the input pin that is 5 V tolerable. I thought about using a serial diode for protection against the negative half cycle and using the internal pull down resistor of the MCU. Then I could generate an interrupt whenever the sinusoidal signal is high enough for the GPIO to recognize it as logical HIGH.

^{simulate this circuit – Schematic created using CircuitLab}

2.) After a bit of research on StackExchange, I also found configurations using opto-isolators: Detecting Zero Crossing of Mains (Sine and Square Wave) The advantage is that it would output a sharp rising edge easily recognizable for the digital input pin, rather than the limited slope steepness of a sine wave.

3.) Since the signal does not have a dangerously high voltage, I could also skip the isolation and use a simple BJT or MOSFET instead. This would also output a sharp rising edge.

^{simulate this circuit}

Which of the above options would you recommend? And above all: I hope that the parasitic capacitances of the semiconductor devices do not have any effect below 500 kHz, is that right? Or do you have a different and better approach?

Best regards and thanks in advance!

Answer 1

If you are dealing with an analogue signal and trying to convert it to a suitable square wave for frequency measuring you have to consider the effects of noise and implement some form of hysteresis so that at the threshold point (where the circuit arbitrates between 0 and 1) there isn't oscillation of the digital output.

The above picture taken from here and it hints at using a schmitt trigger like the one below (I have used this circuit several times): -

It works from 3 volt supplies or 5 volt supplies. The line-in capacitor is to remove any DC component of the input. The capacitor on the inverting input filters the signal so that what appears at that input is Vcc/2. The picture comes from here Turning the output of an opamp into a square wave.

Answer 2

You could use a comparator (or any opamp basically).
Since all you need to do is limit the frequency and convert the arbitrary wave into a square wave for the schmitt triggered timer capture input pin.

The most basic approach would be this absolute comparator based on a fixed reference, without any hysteresis.

^{simulate this circuit – Schematic created using CircuitLab}

You could even use an on-chip analog comparator, if you can connect the input capture event to the comparator signal internally.

There will be caveats with this approach, but you didn't provide much details about the signal source and it's (common mode) levels.

Answer 3

You could try zero crossing detector.

^{simulate this circuit – Schematic created using CircuitLab}

For most of the time (positive half cycle) Vin >> 0.7V hence the Q1 is in saturation and the output is low (GPIO). As input signal "approaching" zero the Q1 turns-OFF and the output voltage goes "high". And it stays high until the input signals turn-ON Q2 and Q3 (negative half cycle). And this will happen around -0.6V volts. So, for the remaining part of a negative half cycle, the output is low (Q2 and Q3 Turn-ON).

But to be honest I never try it at such a "high" frequency.

Answer 4

A very simple and low cost (< $1) solution that also has the benefit of electrical isolation is to use an AC opto copuler like the TLP184SE shown below. These type of couplers have a "current transfer ratio" that specifies how much of the input LED current is converted into output transistor current. Just use the correct size resistor to set the LED current and you get a full wave rectified output at the other side. No extra power supply is necessary. You decide the output voltage by the pull up voltage used on the output resistor.

The LED coupling also provides some "edgification" of the signal because they do not conduct until around 1.2V, so there is zero output when the input is between -1.2V and +1.2V. You will of course have to divide the frequency that you measured by 2 because you are now counting both halves of the pulse.

You should also take a look at the STM General Purpose Timer Cookbook, which shows how to use the internal timer/counter logic to process external signals without CPU resources.