Frequency response with a TEK MDO3000

[onesixright]

As a side note, we actually do a sweep on the InfiniiVision 3000T and 4000 X-Series scopes very similar to what a standard FRA or VNA would do. We built it for power supply testing - namely control loops response and PSRR. We plot both gain and phase up to 20 MHz (the bw limit of our "WaveGen"). Johnnie Hancock did a video on it recently, too: http://bit.ly/2dfGaLY

You probably aware your hijacking this thread, like this, right? 

[Keysight DanielBogdanoff]

Sorry, I'm not trying to derail the conversation. I rarely push things like this, but I thought it was relevant to the discussion in terms of the validity of using a scope and function generator for frequency response measurements.

Thanks for bearing with me!

[analogRF]

<snipped>
One thing to note, the spectrum analyzer in the MDO3000 is not the same as an FFT on a scope. The difference here is that a normal FFT on a scope channel is just using the scope's standard analog input, whereas the MDO3000's spectrum analyzer uses a separate RF signal path and provides a much more user-friendly interface than a scope FFT.

To add to this - the MDO's spectrum analyzer features some additional differences compared to a simple FFT on a scope channel.  The SA input does not have to support DC operation, and does not have to include DC offset adjustment capability. These omissions make it easier to optimize the RF path in terms of noise and distortion.  Also, the use of DDC (digital down conversion) helps with the process gain afforded by the discrete transform.  The transform used is a chirp-z transform, which removes the 'power of 2' vector size limitations of an FFT, thus giving the user more flexibility in setting a desired RBW.  Additional processing is employed (using dither) to reduce the distortion products that result from the DNL (differential non linearity) of the DAC.  These contribute to giving the SA similar performance (DANL, SFDR, etc.) to many entry level conventional spectrum analyzers.  So, it really is a bit more that a simple scope channel FFT.

I understand this is a very very old thread but I had a question about MDO3000 and when I searched eevblog for a possible answer this is the thread that got me interested. so here it is
MDO3000 (up to 3054) only have 2.5GS/s sampling rate yet their SA can go up to 3GHz. Since this SA is not a swept frequency SA
and therefore it is just based on sampling and FFT, how is that possible? Even if they combine (interleave) samplers of two channels it will still be 5GS/s not enough for 3GHz SA.
I assume a downconversion occurs somewhere but what is the LO of that? and when that downconversion kicks in?

by the way, does anybody know if I use the SA and scope channels simultaneously, what happens to the scope channel sampling rates? are they affected?


EDIT: NEVER MIND. I was too quick to post a question before reading all the posts  . I just saw Allen had explained the RF input is always sampled at 10GS/s and downconversion to baseband is in digital domain. Now everything make sense.