Since we have seen some LF results so far, I thought I’ll add some measurement at the upper end of the spectrum, specifically at 90% of the specified bandwidth of 500 MHz.
So, this IMD test is about 450 MHz and 200 kHz frequency offset.
It has already been brought up several times before: for comparable results, we need a bunch of conventions and preconditions. I cannot meet all of them, for instance I cannot guarantee that the dual tone test signal has any lower than -74 dBc IMD.
As has been already discovered, the results also depend on the scope settings. Like a “real” spectrum analyzer, we won’t get impressive results if we feed the scope input with full scale signals (in SA terms: right below the compression level). In general, the lower the input signal(s), the lower the distortion. For testing the third order dynamic range of an SA, we usually have signals in the realm of at least 30 dB below the 1 dB compression point. Consequently, I think it is only fair to use signal levels at least about 10 dB below full scale for the scope as well.
Now let’s calculate the settings for 0 dBm test signals (sine waves assumed throughout these calculations).
0 dBm into 50 ohms means 223,61 mVrms or 632,46 mVpp.
With two 0 dBm signals combined, the peak output voltage doubles to 1.265 Vpp.
The SDS2000X HD ADC input range is roughly 9.6 divisions, so we need at least 132 mV/div to have the dual tone signal within full scale. If we want to keep the signal at or below -10 dBFS, this makes for 4 Vpp full scale.
In the end I chose 500 mV/div, which is equivalent to 4.8 Vpp full scale.
We can get ~74 dBc IMD3 in this scenario, this is equivalent to a TOI (third order input intercept point) of +36.7 dBm.
See attached screenshot:
SDS2504X HD_IMD_500mV_C450MHz_O200kHz_0dBm_L
Please note that this is not a bad result at all. There were (and still are) plenty of low(er) end spectrum analyzers that might not even reach 70 dB third order dynamic range, whereas top instruments can provide up to 110 dB.
For example, the ancient HP8590A has a third order dynamic range of 70 dB at -30 dBm input.