latency of cheap DSO

[Kyle Cronan]

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

 

[nospam]

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?

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?

--

 

[Jean-Yves]

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

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 ...

 

[Gerard Bok]

I've been using one of those cheap digital storage oscilloscopes
(Protek DSO-2090) that plugs into your computer's USB port.

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?

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.)

 

[Kyle Cronan]

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