Products > Test Equipment
Two Tone Test with Scope and SA
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mawyatt:

--- Quote from: bicycleguy on August 02, 2020, 05:02:18 am ---Since you seem to be comparing FFT implementations wouldn't it be better to ask for something that can be compared like:
What are the closest 2 tones that can be separated with 3dB of dip between peeks and what's the highest and lowest frequency they can be done at.
Unfortunately the limits are set by the weakest link in the tone generator or the scope.

With Siglent SDG2042x and Keysight DSOX2014A, both enhanced.

Example: 119.99MHz and 120MHz high freq. limited by SDG and resolution by DSO FFT


Example 10 Hz and 10.05 Hz  and low, resolution limited by taking too much time to scan  :=\

--- End quote ---

In my case I was interested in the linearity performance of the scope front end and ADC. Also looking at the AWG output with a SA provides a view into how good the AWG signal is. The 2 tone tests was a simple means to show such and why I asked. If you are interested in the limits of the scopes FFT capability, then other tests like you've suggested might be considered.

Honestly I'm impressed by all the scopes behavior, even the low cost "entry' level devices seem to be performing better than I expected.

Now that I've retired (chip design) my plan is to purchase a scope and AWG, then a modest SA for starting a home lab. Have a good initial start with a couple repaired Tek 2465 analog scopes, a couple repaired 34401A DVMs and a new Keysight 34465A DVM (superb DVM).

I realize a scope can't compete with a SA regarding linearity & dynamic range, thus including the SA in the desired equipment list.

Anyway, this has created lots of responses and I wish to thank everyone for jumping in on the fun :)

BTW the two images show exactly the same screen capture??

Best,
bicycleguy:

--- Quote from: mawyatt on August 02, 2020, 12:54:18 pm ---BTW the two images show exactly the same screen capture??

Best,

--- End quote ---

I was modifying the post.  Maybe try reloading the page.
gf:

--- Quote from: PeDre on August 01, 2020, 02:07:56 pm ---And here the Rigol DS1104Z and MSO2302A. I haven't managed to get better settings. The DS1104Z FFT uses the memory, not the screen as source.

--- End quote ---

If you can't increase the number of FFT points, then you can alternatively reduce the sampling rate in order to improve the resolution (i.e. choose a timebase which happens to lead to a lower sampling rate). For 10MHz sine waves you don't need a sampling rate of 1GSa/s, but (say) 25 MSa/s should basically suffice. Reducing the sampling rate from 1 GSa/s to 25 MSa/s is supposed to improve the FFT frequency resolution by a factor of 40 then (if the number of FFT points remains the same).


--- Quote from: bicycleguy on August 02, 2020, 03:53:55 pm ---I was modifying the post. Maybe try reloading the page.

--- End quote ---

Unfortunately still the same. The two preview icons look indeed different, but the large images are still the same. Maybe a bug in the forum software :-// I have noticed this effect a couple of times at other posts, too.

EDIT: The image in this posting https://www.eevblog.com/forum/testgear/two-tone-test-with-scope-and-sa/msg3168886/#msg3168886 seems to be broken as well.

_Wim_:

--- Quote from: mawyatt on July 31, 2020, 09:40:36 pm ---
--- Quote from: jjoonathan on July 31, 2020, 08:59:16 pm ---Here's a SDG6022X into a RTO1024 oscilloscope and FSP spectrum analyzer.
(Attachment Link)
(Attachment Link)

--- End quote ---

This is superb performance, what you would expect from a 16 bit ADC based scope!! However it's well out of the price range of these other scopes, and I see it's been replaced with the RTO2000 starting at only $14,500!!

Best,

--- End quote ---

I think the RTO1024 is an 8-bit scope, but increases its ENOB by using massive oversampling at these low frequencies.
nctnico:

--- Quote from: mawyatt on August 02, 2020, 12:54:18 pm ---In my case I was interested in the linearity performance of the scope front end and ADC. Also looking at the AWG output with a SA provides a view into how good the AWG signal is. The 2 tone tests was a simple means to show such and why I asked. If you are interested in the limits of the scopes FFT capability, then other tests like you've suggested might be considered.

Honestly I'm impressed by all the scopes behavior, even the low cost "entry' level devices seem to be performing better than I expected.

--- End quote ---
AFAIK the two-tone test is primarily a way to see how good the mixer is in a spectrum analyser. A DSO doesn't have a mixer so it is not a surprise this effect doesn't occur.
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