Lissajous curve in analog oscilloscope

[baby_1]

Hello
we wanted to find the angle difference of two analog input signal in my university lab.as three group tested and show the result our information that we got from oscilloscope was acceptable.but we have a important question that one of the other group asked us.
here is my step to test our circuit.
1-we inject a 10khz frequency with 1VP.P amplitude to input circuit and channel 1 oscilloscope and set volt division on 1 volt
2-we find the wave form frequency amplitude with channel 2 and with volt division 5mv (the output amplitude was less than 10mv)
3-we find the angle difference in X-Y mode and with lisagor curve
4-we increase the wave frequency to 1Mhz (with step 100Khz) and write the output voltage and angle difference between channel one and two

as i said before our result was acceptable but some one asked me a strange question.if we change the volt division we got the different result( doesn't it?)
if we want to see lissajous curve is it important to set the channel 1 and 2 volt division to same value (for examlple 1Vpp)?
if yes why our result was acceptable?

Thanks

 

[BradtheRad]

I guess you mean lissajous figures?

Lissajous figures are odd beasts. A phase difference of zero produces a diagonal line of 45 degrees.

Whether you did it accidentally or deliberately, it seems to me you would have to adjust CH 1 and CH2 to a matching proportion, to obtain X/Y plots which showed similar angles onscreen.

Did you verify at least a few of your readings, by doing a direct a comparison of the sinewaves, with one displayed above the other on a dual trace?

 

[baby_1]

Thanks dear bradtherad
as i test and show to my teacher and teacher assistant person they told me that the output information are acceptable.but i don't have any time and class time to continue to check the comparison of sinewaves.
i want to know it is neccessory that we should use the same volt division value when we wanted to find the angle phase differences between input voltage? or not?

Thanks

 

[FvM]

You missed to tell how the channel 2 input voltage was generated. Apparently it's low voltage, thus the attenuator settings seems to be reasonable.

Phase difference hasn't to do with magnitude setting, isn't it?

 

[baby_1]

Dear Fvm
Thanks for your response
we connected the output of signal generator to circuit input ( A mid range pass) and channel 1 and connected the output or circuit to channel2
it confused me that why we change the volt division the lissajous figure changed?!!!because the lissajous figure that we want to find the difference angles should be the same result

 

[HTA]

The Lissajous curve is used in optical and magnetic encoder ICs with sine/cosine outputs to measure the phase and amplitude for alignment.

Enjoy your design work!

 

[BradtheRad]

it confused me that why we change the volt division the lissajous figure changed?!!!because the lissajous figure that we want to find the difference angles should be the same result

The dot traces the shape of a rectangle, if you watch for a long enough time, and if your scope leaves trails.



The upper right corner is 'in phase' or 0 deg.
The lower right corner is 180 degrees out of phase.

The idea is to adjust gain on both channels, so that the rectangle is not too thin either vertically or horizontally. Then you have an easier time measuring angles onscreen.

Nevertheless your accuracy is not going to be 100 percent. That's probably why your measurements were acceptable, as long as you got within a few degrees.

 

[FvM]

The phase difference that can be read from the lissajous curve does not change when you change the volt/div setting.