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Two Tone Test with Scope and SA
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David Hess:

--- 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 sampling process before conversion *is* mixing, and the same types of circuits are used.  (1) The difference is that an impulse replaces the local oscillator signal so while a mixer has fine resolution in frequency, a sampler has fine resolution in time.

Aliasing in a sampling analog-to-digital converter is identical to downconversion, and subsampling receivers take advantage of this.  The impulse being used in place of the continuous local oscillator signal contains harmonics which mix with higher frequencies to allow this.

(1) Microwave mixers are a good source for microwave samplers when the transformers are removed from the RF and IF ports.
SilverSolder:
Just for fun, here is what the tone test looked like in 1975!

I fired up an ancient HP 3571A Spectrum Analyzer -  a 100% analog spectrum analyzer full of gold, crystal filters, and engineering love. 

The two tones are generated by HP3325A x2 summed into a 50 ohm load.

The analog output from the 3571A was tracked on the scope in roll mode for display.

It seems to me the performance is not embarrassing at all, considering this thing is 45 years old.  The noise floor is a bit high, but the 3325A signal generators are not exactly low noise...   The 3571A is capable of -130dB, perhaps a little better, performance on a clean signal.  The ancients knew how to build stuff!  (It probably didn't cost $450)


mawyatt:

--- Quote from: SilverSolder on August 03, 2020, 12:07:00 am ---Just for fun, here is what the tone test looked like in 1975!

I fired up an ancient HP 3571A Spectrum Analyzer -  a 100% analog spectrum analyzer full of gold, crystal filters, and engineering love. 

The two tones are generated by HP3325A x2 summed into a 50 ohm load.

The analog output from the 3571A was tracked on the scope in roll mode for display.

It seems to me the performance is not embarrassing at all, considering this thing is 45 years old.  The noise floor is a bit high, but the 3325A signal generators are not exactly low noise...   The 3571A is capable of -130dB, perhaps a little better, performance on a clean signal.  The ancients knew how to build stuff!  (It probably didn't cost $450)


(Attachment Link)

--- End quote ---

Those old instruments can still hold their own :-+

Brings back fond memories, along with the superb Tek analog scopes of that era :)

Best,
rf-loop:

--- 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 ---



They are not named as Mixer but other way, how many "mixer" you find in one oscilloscope... think about it. Do you know what all circuits can produce mix. Sorry... I feel it was necessary..  :) but not so seriously.

Is it good to know even basic fundamentals...

Basic: When two frequency signals,  f1 and  f2,  are input to any nonlinear device.... etc...  do you know any other nonlinear devices than these what are commercially named as Mixer. Ok lets start from bottom, do you know diode... how about transistor etc etc.
tggzzz:

--- Quote from: rf-loop on August 03, 2020, 02:21:40 am ---Is it good to know even basic fundamentals...

Basic: When two frequency signals,  f1 and  f2,  are input to any nonlinear device.... etc...  do you know any other nonlinear devices than these what are commercially named as Mixer. Ok lets start from bottom, do you know diode... how about transistor etc etc.

--- End quote ---

Precisely.

Don't forget to add "capacitor" to that list.
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