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Two Tone Test with Scope and SA
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mawyatt:
Hello,

I've been looking at various scopes from the usual sources and wanting a scope that could give high Dynamic Range for the standard 2 tone IMD tests using an FFT. The moderately priced scopes mostly seem to have 8 bit ADCs with exception of the R&S which has a 10 bit ADC. Has anyone done any FFT based 2 tone IMD tests at low (~10KHz & 100KHz) and medium (1MHz) and higher frequencies (>10MHz)?

I know this test is more in line with a Spectrum Analyzer, has anyone done these test and frequency range with a modest SA?

I think the new dual channel 16 bit AWGs from Rigol and Siglent might be a good source for the 2 tones, using a resistive combiner at the outputs. 

Anyway, interested if folks have done any of these test and any comments, recommendations are welcome.

Seems this would be an interesting comparison for the various scopes, SA and AWGs now available.

Thanks in advance.

Best,

xmo:
"... standard 2 tone IMD tests..."
--------------------------------------

Standard?  What standard?

IMD tetsts?  Testing what?  Ferrites?  Antennas?  Coax cables? Receivers?  Amplifiers?  Transmitters?
mawyatt:
To help evaluate the instruments, especially the scopes front end circuitry and ADCs. Given quality 2 tone AWG outputs (or other quality signal sources), how do the various scopes compare with each other and with a modest SA.

Are the newer 16 bit AWGs good enough sources for these 2 tone IMD tests for typical scopes?

Best,
tautech:
After some emails chatting about other stuff I got the rundown on what mawyatt considers a Two Tone Test...easy it is too using a single output from both channels of a SDG2042X and its Wave Combine feature.

FYI mawyatt word for word instructions so anyone can join the fun:
The test are very simple, they use two close spaced tones of equal amplitude and show the result of an FFT or direct SA output. Example create a tone of 10KHz in one AWG channel, another tone of 10.1KHz is the other channel. Resistively sum these together and set amplitudes to 1V and look at the FFT result, or SA result. You will see the 2 tones at 1V (or +10dBm(10mw) in 50 ohm system) at 10KHz and 10.1KHz and two smaller tones at 9.9KHz (10KHz -Difference frequency) and 10.2KHz (10.1KHZ + Difference Frequency), these side tones are due to Intermodulation Distortion and tell how good the DUT is, in this case the Scope or SA (assuming tones are really good to start with from AWG). Now do this at 100KHZ and 101KHz, then 1MHZ and 1.0MHz and 1.01MHz and so on. You can use different levels and tone spacing to revel things about the system under test.

The math behind this involves the 3rd Order non-linearity which creates these side tones. Very important test for just about any system, audio, RF or Microwave.

Behind the scenes mawyatt asked me for FFT 2 tone test results using SDS2000X Plus and now I have the 100 MHz model in stock again, here they are and there should be more than enough info in each screenshot to establish the frequencies used that BTW are based on 10 KHz, 100 KHz, 1 MHz and 10 MHz.

Signal source
SDG2042X 50 Ohm output mode 2 active channels combined. 500mV p-p.
Siglent BNC cable
SDS2104X Plus in FFT split screen mode.









And as some sanity check the 10 MHz 2 tone into a SVA1032X.

mawyatt:
Rob,

Thanks for the tests.

These tests show the difference between the two tones and the 3rd Order products is over 50dB indicating the SDS2104 has a respectable front end and ADC performance. Was this with the 2104 in 10 bit or 8 bit mode?

The SA displays shows the AWG output is good. Since the two tones were combined within the AWG in the digital domain, this shows that the AWG DAC and output amplifier have good linearity. BTW, what model SA did you use?

All in all nice performance of the SDS2104 & SDG2042. Hopefully others can do this simple test with other similar priced and featured scopes and we can see how they compare.

Just a note, the individual tones need to set at 1V peak loaded with 50 ohms (+10dBm into 50 ohms, for 0dBm reference 0.316V peak), however this is fine as shown.

Thanks again for running these 2 tone tests!!

Best,
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