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latency of cheap DSO

Hi everyone,

I've been using one of those cheap digital storage oscilloscopes (Protek DSO-2090) that plugs into your computer's USB port. I think maybe I'm beginning to see one of the downsides of this type of device. Either that or my circuit is really messed up--that's what I'd like to find out!

If I measure two signals around 7mhz using the two channels I have available, does the phase angle between the two signals that I see on the screen represent what's actually going on in my circuit? Or just some variable latency in the connection between the scope and my PC?

With the differential amplifier I'm testing right now I can see that the phase angle between the two outputs is around 180 degrees on average, but it varies wildly--so much so that at some moments the two outputs are nearly in phase. I'm not sure if that is what's really happening in the circuit.

Thanks a lot!

Kyle

Reply to
Kyle Cronan
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How hard is it to look at the same signal with both channels so you know any observed phase difference between the signals isn't actually what's going on in your circuit?

Reply to
nospam

to measure analog circuits, you have better use an analog scope than a digital one ... I'm afraid you will need a far better digital scope to measure efficiently a 7 MHz signal ...

try to borrow an analog scope even cheap and measure your signals.

just to see ...

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Jean-Yves.
Reply to
Jean-Yves

No. Of course not. What a digital scope is showing you is the value of a channel at the time of sampling.

You can only expect to see phase differences as soon as the time delay involved exceeds 2 sampling intervals.

Just to make the picture more pleasant to the eye, it depicts a line in stead of a series of dots. (Even if you select the dot-display it will likely show you the dots on a fixed grid only, not accounting for the sampletime or any time difference caused by sequential sampling. Rule of thumb: As soon as the scope's pricetag exceeds that of your car you can expect otherwise.)

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Kind regards,
Gerard Bok
Reply to
Gerard Bok

Ugh, why I didn't think to just hook both channels up to one signal... :) It does the same thing.

Thanks for the explanations. I'll see if I can pick up an analog scope used. Gerard, your rule of thumb frightens me very much!

Kyle

Reply to
Kyle Cronan

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