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| Two Tone Test with Scope and SA |
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| rf-loop:
--- Quote from: tautech on June 15, 2022, 02:12:58 am --- --- Quote ---Anyway, speaks highly of the SDS2000X HD. Do you know if the front end section of the SDS2000X HD & ADC are the same as the SDS6000 series 12bit ADC version sold only in China, or the one for LeCroy? --- End quote --- Have no idea Mike, maybe 2N3055 or Performa01 can help you with this one....that's if their NDA lets them. :-X --- End quote --- due to NDA and so on... But why even need for answer this just anything more than read from public data sheets and rest can know if think even bit. Answer to this question is obvious. SDS6204 H12 Pro and SDS6204 H12 Pro what is up to 2GHz/1GHz front end vs SDS2000X HD 500MHz front end... no, they are not equal aka same. And even more, then in question there was "& ADC". How it can even think if they are same. Example 6k H12 Pro models have 5GSa/s 12bit ADC's 2GHz models have one for every channel. 1GHz models and below have shared ADC's, one for two cannel so that these have 5GSa / 2,5GSa/s Then very different animal... 2kXHD have two 2GSa/s 12bit ADC's shared one for two channels so 2GSa / 1GSa/s. No need any deep knowledge. It can read just from data sheet. And these SDS2kXHD ADC's are "perhaps" normal industrial models what who ever can buy from several different vendors. No need Sherlock Holmes to quess what they are. Naturally front ends are more or less close relatives. No one is so stupid that start every time from empty board. Development and design of something new is a continuous and cumulative, but also self-correcting learning process |
| G0HZU:
If it helps I have an early/old Tek spectrum analyser here that has a digital IF. When used down at below 40MHz it is effectively a 40MHz baseband scope sampling at 102.4MHz but the display is trapped in the frequency domain. This is not a swept analyser, it samples the input and generates an FFT just like a scope. If I inject two clean tones I can generate IMD in this analyser at about -83dBc if I drive it towards FSD of the ADC. If I then deliberately sum in a 'pre-distortion' tone at the source input at -83dBc at one of the IMD3 frequencies and then rotate the phase of this the analyser shows the expected behaviour where the displayed IMD3 term rises 6dB to -77dBc when the IMD3 terms are in phase and the IMD3 term vanishes on the FFT display when the phases cancel. This shows that the equal IMD of the source can cause summing at up to 6dB on the display and it can also cause cancelling although significant cancelling is highly unlikely to happen as I'd expect the source IMD terms to normally be in phase. |
| Performa01:
Everything said by our RF-guru G0HZU should be taken seriously. Up to now, he mainly talked about the (close to) +6 dB boost for the distortion products in case of two clearly (and inevitably) corelated distortion sources, but of course this assumes zero phase shift, hence zero runtime – or at least test signals whose wavelengths are long compared to the physical dimensions of the test setup. Once the distance between distortion sources happens to be half the wavelength, we get phase inversion and instead of a boost, the distortion components will cancel out. EDIT: Doh! Nobody notified me of my mistake. Of course the runtime is almost irrelevant, since the distortion products in question are almost the same frequency as the test signals! In my test, the frequency was 450 MHz and half its wavelength is about 33 cm. While it is highly unlikely that such massive distances will occur between potential distortion sources within the DSO frontend, it would be perfectly possible that an odd multiple of this distance exists (as electrical length) between the external generator and any non-linear stage within the DSO. As a consequence, there is indeed a chance of cancelling out distortion products in this scenario. My claim that I cannot guarantee the IMD performance of the test tones to be better than -74 dBc came from the fact, that I’ve measured this with my SA. It’s pure coincidence that the measurement result with the DSO was about the same. Since I don’t happen to own a R&S FSEA30 boat anchor with 110 dB third order dynamic range, I could not know whether I hit the limit of the generator or the analyzer. With two 10 dB pads for the external power combiner, hence a total isolation of 26 dB, any IMD products should be killed reliably. Now I could indeed measure an IMD of at least -85 dBc and can be confident that my source will be vastly better than this, when it already measured as -74 dBc IMD with only 6 dB isolation. With the 10 dB pads in place, I cannot maintain a 0 dBm signal anymore. Even though I could have used about -3 dBm, I voted for repeating the test with a signal level of -10 dBm. This totally changes the scope settings and its internal nonlinear transfer curves. Originally, I had 0 dBm signals at 500 mV/div. This means that there is an internal 20 dB attenuator active, and the PGA (Programmable Gain Amplifier) might have to deliver about 6 dB of gain. For an equivalent setting at -10 dBm, I would have to set the channel gain to 160 mV/div. With this, the attenuator is still in place, but the PGA now is at 16 dB gain, which might cause more distortion. In general, I think that the PGA might be the main source of nonlinearity in a wide bandwidth scope frontend, whereas the unity gain input buffer as well as the ADC should be rather benign in this regard. Look at the attached screenshot for -10 dBm at 160 mV/div. The IMD isn’t quite as good, but it still happens to be another sweet spot: SDS2504X HD_IMD_160mV_C450MHz_O200kHz_-10dBm We can also try to replicate our former sweet spot without the internal 20 dB attenuator. For this, we need a -20 dBm test signal at 50 mV/div, see third screenshot: SDS2504X HD_IMD_50mV_C450MHz_O200kHz_-20dBm With this setup, we can be confident that the generator signal is way better than that, so we need not speculate where the distortion comes from. |
| tautech:
--- Quote from: mawyatt on June 15, 2022, 02:26:50 am --- --- Quote from: tautech on June 15, 2022, 02:12:58 am ---I suppose I should do a 2 tone with my SDS6204A however with what I have here 200 MHz is the current limit of toys available. --- End quote --- Would be interesting to see how good this SDS6204A is, I'm sure others are interested as well. --- End quote --- Okay so I had a little play, SDS6204A running 500 Mpts fixed Mem depth with 189 & 190 MHz 0dB tones from SDG6022X. All except the last were at 4 Mpts and last was at 8 Mpts FFT with some averaging. Ref levels +5dB and -40dB.....most of which is visible in menus or OSD. |
| mawyatt:
--- Quote from: G0HZU on June 15, 2022, 08:30:16 am ---If it helps I have an early/old Tek spectrum analyser here that has a digital IF. When used down at below 40MHz it is effectively a 40MHz baseband scope sampling at 102.4MHz but the display is trapped in the frequency domain. This is not a swept analyser, it samples the input and generates an FFT just like a scope. If I inject two clean tones I can generate IMD in this analyser at about -83dBc if I drive it towards FSD of the ADC. If I then deliberately sum in a 'pre-distortion' tone at the source input at -83dBc at one of the IMD3 frequencies and then rotate the phase of this the analyser shows the expected behaviour where the displayed IMD3 term rises 6dB to -77dBc when the IMD3 terms are in phase and the IMD3 term vanishes on the FFT display when the phases cancel. This shows that the equal IMD of the source can cause summing at up to 6dB on the display and it can also cause cancelling although significant cancelling is highly unlikely to happen as I'd expect the source IMD terms to normally be in phase. --- End quote --- Think your test show we can't use the RSS method to evaluate/estimate/separate DSO sources of IMD as it can potentially introduce a 3dB optimistic result because of correlation. Thanks for taking the effort for this testing. Best, |
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