Today I took some examples of the aliasing, the first one is a Tektronix TDS3034A, 300 MHz, 4-channel, 2.5 Gs/s DSO. I used a 10 MHz sine wave input. I don't know if that qualifies as "modern scope". But as you'll see, it does have alias problems.
First, a quick look at 20 ns/div horizontal setting:
increase horizontal to 10µs/div:
increase horizontal to 40µs/div:
Now the "bar" starts to break up, let's bump the time/div to 4 ms/div:
Wait a minute, measurement says now that our 10 MHz signal has turned into 57 Hz one! What if you wanted to measure some kind of an amplitude modulation from 10 MHz carrier? The amplitude and waveshape are quite correct. The only artifact seems to be that the scope seems not to trigger on the sinewave even if the settings are appropriate. Let's do a single sweep:
Perfectly nice "57 Hz" sine
Now, let's try more complex waveforms. First, a 10 MHz square wave @ 20ns/div:
And then with 4 ms/div single sweep:
Quite amazing undersampling from Tek, even the very small reflection bumps get nicely undersampled
And how about 1 MHz sinc-function, certainly that can't be undersampled? Think again.
Like I said previously I and my colleagues have been stumbled on this effect for several occasions, and it really sometimes takes a while to realize what is wrong. Next, an Agilent MSO6034A in next post...
Regards,
Janne