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Two Tone Test with Scope and SA
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_Wim_:

--- Quote from: nctnico on August 02, 2020, 07:13:36 pm ---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.

--- End quote ---

The scope's ADC also has non neglectable IMD distortion, but probably only significant at higher frequencies than the 10kHz typically tested here.

For example the datasheet of TI ADC08D500 (8-bit 1GSPS) states an IMD of -50db at 121Mhz. 
jjoonathan:

--- Quote ---I think the RTO1024 is an 8-bit scope
--- End quote ---
Yep, it's 8 bit! The "16 bit HD Mode" marketing wank is one of the more egregious examples out there. It was good sport to watch Lecroy and Keysight take shots at R&S for it. Of course, in the scheme of things, it's not *that* big of an issue. Obviously it didn't stop me from buying one (for unrelated reasons).

Still, there actually is a kernel of truth at the center of the marketing turd: R&S did a better than usual job at averaging. Many implementations are stuck at a pre-Signal-Processing-101 level. They use boxcar filters before downsampling and don't calibrate their PGA settings to ensure good dithering across the instrument bandwidth. R&S brought things up to Signal-Processing-101 spec. That's nice. It plays especially well with the fully digital trigger, which still works great when you've got de-embedding and filtering on. The third timebase knob (record length) also shines in these situations.

I was hoping to provoke a response from someone with an Infiniium S. There was a big split in ADC philosophy this generation, with Keysight taking the "farm out to a bunch of slow CMOS cores, individually calibrated" strategy and R&S (Lecroy too, I think) taking the "one core and step on the gas" strategy. I wanted to see what that looked like in the low frequency "average to the moon" regime. There's still time, I suppose :)
egonotto:
Hello,

jjoonathan wrote: "I was hoping to provoke a response from someone with an Infiniium S"

perhaps you can ask Shahriar. He can make test with Infiniium S and Infiniium MXR and Tek MSO 5 and MSO 6.

Best regards
egonotto
mawyatt:

--- Quote from: nctnico on August 02, 2020, 07:13:36 pm ---
--- 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.

--- End quote ---

Don't know about others, but we often used the 2 Tone Test as a verification of the custom chips we developed (for example to compare against simulations) and the linear systems involved (LNA, active filters, attenuators, VGAs and so on) well before any intended non-linear functions (mixers) were encountered, we used the 2 Tone Test on these (mixers) as well. Also used the 2 Tone Test to characterize the high resolution DACs and ADCs we were involved with. In fact this 2 Tone Test was one of the main tests involved in a new type ADC developed (non-uniform sampling ADC) that utilized simultaneous time and amplitude quantization. This ADC has the unique property on not requiring a pre anti-aliasing filter as the input signal itself imposes the time & amplitude quantization that allows post conversion anti-aliasing filtering. Someday these may end up in our test equipment  :)

In the area I recently retired from (chip design), the 2 tone test was utilized for just about any "analogish" circuit or system, linear or non-linear, to help characterize performance. This test is also a very valuable tool for calculating the classic IIP3 and OIP3 points without having to sweep the input power levels assuming a well behaved system.

Anyway, don't think the 2 Tone Test should only be relegated to systems involving mixers (like SA), but can be highly beneficial in all sorts of "linear" and "non-linear" systems, including the data acquisition systems we call Digital OscilloScopes :)

Best,
jjoonathan:
> perhaps you can ask Shahriar. He can make test with Infiniium S and Infiniium MXR and Tek MSO 5 and MSO 6.

He's already gearing up for a shootout. That's what the preselector was for. Can't wait  ;D
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