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| Two Tone Test with Scope and SA |
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| Performa01:
I’ve never tried to use RF-signal generators for dual tone tests, even though I do have two of them available. A single modern dual channel AWG is just so much handier for this and up to now I’ve not had any desire for such tests at frequencies higher than 500 MHz. I would expect a R&S FSEA30 measurement to provide much better results than what has been shown here. Since I’ve never experienced any problems and always assumed that the analyzer is the weakest spot in tests like this, it becomes obvious that the output stages of a modern AWG must be much less prone to intermodulation distortion from injected signals. Most likely there is no ALC either, even though the accuracy of the output levels can easily compete with the best RF-signal generators, like a specified accuracy of 1 % and +/-0.3 dB flatness over the entire bandwidth. As usual, actual performance is much better than this and there are sweet spots, like 100 kHz - 500 MHz +0.06 / -0.04 dB absolute error at -30 dBm. See this old reply #162 in the thread linked below, showing the response of my (original) SDG6052X and comparing it to an older high performance signal generator. https://www.eevblog.com/forum/testgear/siglent-sdg6000-series-awg_s/msg2621457/#msg2621457 Long story short, the signal quality, stability and accuracy can easily compete – and the insensitive (to intermodulation from outside) output stages are a big plus. The first attached screenshot shows the IMD at 450 MHz with a resistive wideband power combiner (DC – 12.4 GHz) and two additional 10 dB inline attenuators at its inputs, for a total of 26 dB of isolation. Ref_450MHz_O200kHz_-10dBm_Iso20dB We get better than -85 dBc IMD – and mind you, this is not an SA with 110 dB third order dynamic range like the R&S FSEA30. Base line is, that I expect the actual signal to be much better than this. Even without additional isolation, i.e. just the 6 dB of the power combiner alone, I could measure -74 dBc. I can verify that the signal is better than this with another instrument, but only up to 5 MHz: Pico4262_IMD_4MHz_O100kHz_0dBm At 4 MHz I can easily demonstrate the IMD to be better than -97.5 dBc – and once again I hit the limits of the analyzer, not the test signal. The whole point of this posting is to demonstrate, that with the right tools the quality of the test signal needs not be an issue at all. |
| rf-messkopf:
--- Quote from: Performa01 on June 20, 2022, 02:59:27 am ---The whole point of this posting is to demonstrate, that with the right tools the quality of the test signal needs not be an issue at all. --- End quote --- Okay, but you are testing at 200 kHz and 100 kHz spacing. I was testing at 1 kHz. That's a bit more challenging. :) But I'll re-run my measurement at >100 kHz and see what I get. I guess the main problem I'm having with the IMD level is due to the lack of isolation (about 50 dB in my case) between the signal generators. A better splitter (e.g. a 4-way one) should make an improvement. Still not 100% sure but I don't think that the analyzer is the weakest link in my setup. |
| Performa01:
--- Quote from: rf-messkopf on June 20, 2022, 08:23:04 am --- --- Quote from: Performa01 on June 20, 2022, 02:59:27 am ---The whole point of this posting is to demonstrate, that with the right tools the quality of the test signal needs not be an issue at all. --- End quote --- Okay, but you are testing at 200 kHz and 100 kHz spacing. I was testing at 1 kHz. That's a bit more challenging. :) But I'll re-run my measurement at >100 kHz and see what I get. I guess the main problem I'm having with the IMD level is due to the lack of isolation (about 50 dB in my case) between the signal generators. A better splitter (e.g. a 4-way one) should make an improvement. Still not 100% sure but I don't think that the analyzer is the weakest link in my setup. --- End quote --- Well, if you think the frequency spacing is supposed to change the distortion characteristics of a nonlinear device - and that's about the only thing we're interested in – then please find attached the equivalent measurements at 1 kHz spacing. First attachment is the measurement at 10 MHz with the SA, which should be comparable to yours. Ref_10MHz_O1kHz_-30dBm_Iso20dB Thanks to your complaint! Because of this I finally found out (after many years of usage) that my SA performs much better at narrow spans – probably because it uses FFT exclusively instead of swept mode then. Consequently, I now can finally measure an IMD of -92 dBc even at high frequencies! To get a truly comparable result with my earlier measurements, the second attachment shows the 4 MHz test again: Pico4262_IMD_4MHz_O1kHz_0dBm Nothing changed. It is still -97 dBc, hence right at the edge of what this 16-bit DSO is specified for. I know, there are suspicions that it might have something to do with the ALC in the levelled signal generators, but I was talking about an AWG (which apparently doesn’t have nor need that). And then, if I wanted an appropriate test signal to measure distortion products of a DSO, then I would certainly stay away from conditions (like narrow frequency spacing) that might challenge the signal generators and prevent me from getting a clean dual tone test signal. Yes, your FSEA30 certainly is not a weak link, yet it should be easy to fully exploit its phenomenal intermodulation free dynamic range with proper signal sources and distortion-free power combiners. I wish I had one here to demonstrate just that… |
| rf-messkopf:
--- Quote from: Performa01 on June 20, 2022, 09:27:40 am ---Well, if you think the frequency spacing is supposed to change the distortion characteristics of a nonlinear device - and that's about the only thing we're interested in – then please find attached the equivalent measurements at 1 kHz spacing. --- End quote --- The frequency spacing would certainly not change the characteristics of the DUT, but it gets harder to measure levels of 3rd order product reliably at narrow spacing. Anyway, thanks for testing at 1 kHz. --- Quote from: Performa01 on June 20, 2022, 09:27:40 am ---I know, there are suspicions that it might have something to do with the ALC in the levelled signal generators, but I was talking about an AWG (which apparently doesn’t have nor need that). And then, if I wanted an appropriate test signal to measure distortion products of a DSO, then I would certainly stay away from conditions (like narrow frequency spacing) that might challenge the signal generators and prevent me from getting a clean dual tone test signal. --- End quote --- Not all signal generators have an ALC. Some of the old Marconi ones always run open loop, I think the 2024 is an example. I'll repeat the test with two function generators and see if that makes any difference. And it does not necessarily have something to do with the ALC (which was disabled anyway), but the IMD seen can also be due to nonlinearities in the generator output stage. Also, the two signal generators (R&S SML and SMU200A) both have electronic attenuators. Maybe that has an influence as well. --- Quote from: Performa01 on June 20, 2022, 09:27:40 am ---Yes, your FSEA30 certainly is not a weak link, yet it should be easy to fully exploit its phenomenal intermodulation free dynamic range with proper signal sources and distortion-free power combiners. I wish I had one here to demonstrate just that… --- End quote --- Not so easy with the stuff I have in the drawer it seems. :) |
| Performa01:
--- Quote from: rf-messkopf on June 20, 2022, 10:07:56 am ---Not all signal generators have an ALC. Some of the old Marconi ones always run open loop, I think the 2024 is an example. I'll repeat the test with two function generators and see if that makes any difference. --- End quote --- I only had (and still have them) 3 RF signal generators in my whole life. So my practical experience with these is limited. Since none of them appears to have any settings for the ALC, I cannot know whether they have it or not. But at least my first generator, a Wavetek 3000, settles its output amplitude in a way that it's quite obvious that there must be some ill-designed control loop for it ;) Without the ALC, it's hard to see where there is a difference. If we ignore modulation, there should not be any substantial differences between the output stages of a signal generator and an AWG. We have an output amplifier, which is of course prone to intermodulation when fed with external signals and a step attenuator (which usually covers a much wider range in the signal generator, but nothing that cannot be rectified by using an external device for additional attenuation). My Siglent AWGs are somewhat old school because we hear several relays click when dialing the output amplitude within its range of -56 dBm and +25.5 dBm (at frequencies up to 40 MHz). Yet I suspect that there will be some additional electronic attenuator to manage the fine steps in between. Since this is just a switch, i.e. a negligible serial resistance within the signal path, I do hope that it won’t cause any significant distortion. BTW, for all these recent measurements, I've not used the SDG6052X where I demonstrated the amplitude accuracy, but the new SDG7102A, which covers the frequency range up to 1 GHz and can still maintain +13 dBm at that frequency. Normally irrelevant, I want to make clear that I do not compare apples to bananas, by using a 20 MHz AWG, rather than something with a proper high power HF output stage. Of course, in this context it is totally irrelevant what’s been used, as long as it results in a distortion-free dual tone signal that's fit for the purpose. |
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