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| Two Tone Test with Scope and SA |
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| G0HZU:
See the attached video showing a dual stage amplifier vs a single stage amp. Each amp has 13.1dB gain and there is a 13.1dB attenuator between them. The IMD3 terms are -57dBc for the dual amplifier. When the second stage and the attenuator is removed then the IMD levels drop to about -63dBc. This is only a simulation so not exactly conclusive but I have done similar tests on a pair of 'real' and identical ERA-1SM test amplifiers. These produce about 12.2dB gain so I did the test with a 12.2dB attenuator in between them for the dual amplifier scenario. The IMD change was very close to 6dB when I changed to a single amplifier with no attenuator. It was within about 0.3dB of 6dB. |
| 2N3055:
--- Quote from: G0HZU on June 14, 2022, 09:39:32 pm ---In the case of a DSO I guess a lot depends on the signal handling performance of every stage of the analogue front end before it gets into the digital domain. All these stages can generate IMD ahead of the ADC. The same applies to a spectrum analyser. Often it's the front end that defines the IMD performance. --- Quote ---A) With the attenuator the total IMD should be just slightly more than the IMD of the second amp since this distortion products of the 1st are attenuated by 12 dB. --- End quote --- I'm confused why you come to this conclusion. This is for a classic two tone IMD test. The 12dB attenuation is offset by the 12dB gain of the first stage. So the second stage is being driven by a signal that now has (say) -60dBc IMD. The second stage will amplify these IMD terms by about 12dB but it will also generate coherent -60dBc IMD terms of its own at a similar level at the output. These two mechanisms are usually in phase so the IMD terms will appear 6dB worse for the dual amplifier compared to the single amplifier case. The single amplifier is being fed the same amplitude drive level but there are no IMD terms in the drive signal. So it just generates IMD3 terms close to that predicted by the datasheet for that drive level. The dual amplifier will have IMD terms 6dB higher. --- End quote --- You keep repeating same things and mentioning first stage and second stage and spectrum analysers. What does it have to do with two tone performance of oscilloscope in question? How do you map your statements to that? So plainly, if you feed two-tone into scope input like Mike did (noncoherent two tones) and get some IM distortion (whatever the number) isn't that IM distortion of the scope. He is not trying to measure any external DUT. He is simply trying to characterize the scope internal IM distortion. What am I missing here? |
| G0HZU:
You can think of the two tone source itself as the first stage. I got the impression the IMD performance of this was unknown. Normally, a high performance two tone source will use two sources separated by a high isolation (linear) combiner and there will be lowpass filters in place as well. |
| G0HZU:
You can also experiment with the type of resistive combiner. It's often risky to use a classic delta or star resistive combiner as these have limited port to port isolation. I've built and used alternative resistive combiners that offer higher port isolation at the expense of some loss in the combiner. The alternative is to add attenuators before the combiner. |
| tautech:
--- Quote from: G0HZU on June 14, 2022, 10:50:03 pm ---You can think of the two tone source itself as the first stage. I got the impression the IMD performance of this was unknown. Normally, a high performance two tone source will use two sources separated by a high isolation (linear) combiner and there will be lowpass filters in place as well. --- End quote --- Mostly the 2 tone sources in this thread have been created in SDG models with their 2ch wave combine feature although I don't remember now if the very early one in this thread I did for Mike had locked phasing however I don't think that was needed or even a requirement from Mike. Anyways, riveting discussion that we can all learn something from. :) |
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