Owon 'scope not showing expected waveform

[cricalix]

Hello folks.

I'm tackling a small project (I think) to convert the signal from the back of a Yanmar tachometer into something that I can safely feed to an ESP32, and then convert it to a NMEA0183 sentence. When I attached my HDS1021M-N to the back of the tach, I got a good indicator that the waveform is in the low kHz range, but there are some nasty voltage spikes that might exceed the tolerances of the ESP32, and I don't want to let the magic smoke out.

To that end, I built the attached circuit (courtesy of my father checking one of his radio newsletters, because none of the ones I could find on various websites that claimed to be about circuits worked) to give myself a basic sine wave. This circuit works just fine, converting the 6V input to a ~350mV wave at ~1 kHz. The precise voltage and frequency aren't too critical for my testing, so long as they're within the bounds of what the tach spits out.

My next goal was "rectify it to simulate how I can clip out the spikes from the tach". To that end, I placed a single 1N4148 diode after the cap leading to Vout.

What I'm not undestanding (and my electronics knowledge is rusty, because I last used it about 20 years ago in college) is why I'm not seeing a waveform that shows a more pronounced clipping of the wave (either the +ve side or the -ve side). The 'scope is showing a wave that's very close to the original wave, with a slightly more rounded appearance on the "clipped" side of 0.

Dad sent me a photo of his cheap scope hooked up to the same circuit, and he's getting a waveform that's much flatter than mine.

On the assumption that I haven't messed up the circuit somehow (and we agree on the unrectified waveform appearance and values), am I experiencing a limitation of this 'scope?

[cricalix]

This is the data from Owon's scope software for Windows.

[cricalix]

And this is the waveform after passing through the diode. You can see how the +ve curve is nowhere near "flat". I'm not expecting a perfect flat line, but I am expecting to see a flatter line than the hump that's showing up.

[cricalix]

And this is what Dad's scope has showed on his build of the same circuit, with a 1N4148 diode in place. This is what I'm expecting my scope to show.

[EHT]

What is the spec of the tach output? Have you measured this with your scope?
You should make the output of the sig-gen circuit approximately the same as the tach or more. Let’s say that is 5V TTL and you want to feed it into 3v3 logic level for the ESP32. You can most safely convert that using an NPN transistor with C fed from 3v3. If you are concerned about high voltage spikes you can place a resistor in circuit from the tach output and a pair of clamping diodes to VCC and GND.
The reasons you are probably not seeing what you expect in your setup are:

• the input impedance of both scopes should be 1M, probably the reverse leakage current of the 1N4148 is enough to drive the signal. I guess the cheap handheld one is maybe a lower impedance. Fix by connecting a resistor from this output to GND, say 10k-ish
• the magnitude of your test pulse is 0.3V, less than the fwd voltage drop of the diode (0.6-0.7V).  Amplify the output of your oscillator using another transistor, or borrow/buy a commercial signal generator.

[cricalix]

No spec available (the tach is 20 years old, integrated into the control panel of the Yanmar engine), but I've measured that both the voltage and the frequency change as the revs move from ~800 rpm to ~2500 rpm (about double the RPM as frequency, voltage spikes get anywhere up to 7V).

I /think/ a resistor to ground didn't do anything useful, but I can re-test that.

Protecting the ESP32 is why I started simple, with a circuit that spits out about 1kHz to be somewhat like the tach, and then tried to put the diodes in the output path to ensure that I could get a clipped signal that would let me bench-test the ESP32 setup before wiring it to the tach.

I'll give the amplification thing a try, and see how it goes.

Thank you.

[EHT]

OK, actually what might be best is:
- capacitor in line from the tacho, as you had
- clamping diodes after that to limit the the spikes to 3V3 + 0.6V and 0V - 0.6V as protection
- feed this into a Schmitt Trigger IC (very common cheap part)

This would convert the variable voltage AC into a logic level squarewave