# What's the phase relation between the sine wave and the sync output from a function generator?

I'm using a function generator to generate a sine wave, and am trying to use the sync (TTL) output to flash an LED at the beginning of each wave pulse. However, when I compare the sin wave and the sync pulse on a function generator, I notice that while both have the same frequency, there's a phase difference between the two.

In particular, for a 2 V peak-to-peak signal, I would expect the square pulse from the sync to go from low to high when the sine wave is at minimum (0 volts, or ) or maximum amplitude (+- 1 volt) but instead the sync wave goes from low-to-high when the sine wave hits around 0.4 V (or in general, around 40% of the amplitude).

This number seems arbitrary to me, but I've checked two function generators and both seem to trigger around this point. It's also worth noting that when I generate a square wave, it overlaps with the trigger pulse exactly.

Why does the TTL pulse trigger at this point, rather than at the minimum or maximum? Additionally, do most function generators have a way to change the phase of the sync pulse?

• Do you have the manual for the function generator? If so, it's likely there is a specification that addresses your question. With the cheap units I have around, I've never expected a specific phase relationship. If I saw a difference, I didn't worry about it. It's possible that with more expensive units, they've added features which include a method to adjust the phase relationship. I just haven't owned one of those.
– jonk
Jul 10, 2019 at 18:19
• It is very likely specific to the brand/model of function generator. What one are you using? It is also much more probable that if you have phase control, it would be for the main sine signal output; you could adjust that to get the location you expect. Again, that will depend on the model though. Jul 10, 2019 at 18:19

It's also worth noting that when I generate a square wave, it overlaps with the trigger pulse exactly.

This may be a clue. It would be logical to use the same squarewave signal for both.

The better question might be, "How is the sinewave generated from the square?" If this is done by look-up table, for example, then the trigger point could be any point on the waveform and a non zero-crossing choice would be strange.

I had a look for a random signal generator manual and foun the TTi TGA1240 Series 40 MHz Arbitrary Waveform Generator which states:

Waveform sync

A sync marker phase coincident with the MAIN OUT waveform of that channel. For standard waveforms, (sine, cosine, haversines, square, triangle, sinx/x and ramp), the sync marker is a squarewave with a 1:1 duty cycle with the rising edge at the 0° phase point and the falling edge at the 180° phase point.

At least this one works as you expect.