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| Two Tone Test with Scope and SA |
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| mawyatt:
--- Quote from: G0HZU on June 14, 2022, 05:40:17 pm ---What you are overlooking is that the IMD terms are coherent. If you combine two coherent terms in phase the level on the spectrum analyser goes up 6dB with respect to the power of each tone and not 3dB. I'm not sure I need equations to prove this as this should be common knowledge. You have made the common mistake of just doubling the power. The power goes up by 6dB in the in-phase/coherent case which is a multiplication of four not two. Put the numbers in to my earlier equation for the case where the IMD terms are at the same level. This is when d = 0. You should get 6.02dB as the answer (not 3.01dB). --- End quote --- Nothing overlooked, the two signal generators are NOT coherent, as shown in just about every app note anywhere these source are completely independent sources. The IMD products are computed a 2F1-F2 and 2F2-F1, where F1 and F2 are completely independent sources, so the common mistake is yours :-+ Best, |
| G0HZU:
Wow, just wow... this is taking much longer than it should... Have a look at page 13 of this white paper from Rohde and Schwarz https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ma219/1MA219_2e_IM_Distortion.pdf If you think they are as clueless as me you could also look at page 21 of this article from Keysight. https://www.testunlimited.com/pdf/an/5980-3079EN.pdf They use the same equation as me. The same equation I've been using for decades. --- Quote ---Nothing overlooked, the two signal generators are NOT coherent, as shown in just about every app note anywhere these source are completely independent sources --- End quote --- You simply aren't listening to what I'm saying. The generated distortion terms are coherent in both stages. That's the point I'm trying to make. |
| mawyatt:
--- Quote from: G0HZU on June 14, 2022, 05:45:44 pm ---A good practical example would be to take two identical 12dB gain amplifiers and put them in series with an attenuator in between them that has 12dB attenuation. So you end up with the same 12dB gain of a single amplifier. Then compare the narrowband IMD level of this arrangement to the IMD seen on a single 12dB gain amplifier with no attenuator. Note that this analysis assumes a perfect measuring tool. Or you could consider the second amplifier stage to be the limiting factor of the measurement tool if you like. For a narrowband system the IMD of the first and and the second amplifier usually sums in phase so the IMD levels for the two amplifiers in series will usually be about 6dB worse (higher in level) compared to the case where you just measure a single amplifier on its own with no attenuation. This is because the two IMD terms sum together in phase so you get twice the voltage (four times the power) hence a 6dB increase in IMD level seen with the dual amplifier setup. --- End quote --- Well this won't work as you state either!! The two identical 12dB gain amplifiers the second "sees" the IMD of the 1st attenuated by the 12dB attenuator, however the source signal level will be the same for the 2nd, so the net result will NOT be twice IMD level as you claim, but slightly more IMD than a single amp. Please note that typical amplifiers and most circuits the IMD grows faster than the input signal level grows, so in your example the 1st and 2nd amplifier would have the same IMD if the second one had the 12dB attenuator input, but the 1st would have the IMD also attenuated by 12dB, so the 2nd would see an input with the same signal level as the 1st and the 1st IMD attenuated by 12dB, thus likely not much additional overall IMD. Anyway, this is why noise is usually determined by the first stages and linearity by the later stages in well designed systems, and from the posts shown here the new SDS2000X HD is a very well designed data acquisition system disguised as a scope ;) Best, |
| G0HZU:
--- Quote from: mawyatt on June 14, 2022, 06:16:50 pm --- --- Quote from: G0HZU on June 14, 2022, 05:45:44 pm ---A good practical example would be to take two identical 12dB gain amplifiers and put them in series with an attenuator in between them that has 12dB attenuation. So you end up with the same 12dB gain of a single amplifier. Then compare the narrowband IMD level of this arrangement to the IMD seen on a single 12dB gain amplifier with no attenuator. Note that this analysis assumes a perfect measuring tool. Or you could consider the second amplifier stage to be the limiting factor of the measurement tool if you like. For a narrowband system the IMD of the first and and the second amplifier usually sums in phase so the IMD levels for the two amplifiers in series will usually be about 6dB worse (higher in level) compared to the case where you just measure a single amplifier on its own with no attenuation. This is because the two IMD terms sum together in phase so you get twice the voltage (four times the power) hence a 6dB increase in IMD level seen with the dual amplifier setup. --- End quote --- Well this won't work as you state either!! The two identical 12dB gain amplifiers the second "sees" the IMD of the 1st attenuated by the 12dB attenuator, however the source signal level will be the same for the 2nd, so the net result will NOT be twice IMD level as you claim, but slightly more IMD than a single amp. Please note that typical amplifiers and most circuits the IMD grows faster than the input signal level grows, so in your example the 1st and 2nd amplifier would have the same IMD if the second one had the 12dB attenuator input, but the 1st would have the IMD also attenuated by 12dB, so the 2nd would see an input with the same signal level as the 1st and the 1st IMD attenuated by 12dB, thus likely not much additional overall IMD. Anyway, this is why noise is usually determined by the first stages and linearity by the later stages in well designed systems, and from the posts shown here the new SDS2000X HD is a very well designed data acquisition system disguised as a scope ;) Best, --- End quote --- If you won't take my word for it then read the app notes I linked to in post #109 written by R&S and Keysight. There is a graph there if it helps. The graph shows that the IMD level rises 6dB when the IMD level in stage 1 is the same as the IMD in stage 2. The two identical amplifiers in series (with an attenuator between them) is a neat way to demonstrate this. It can be demonstrated on a decent RF simulator or you can do the tests on a pair of real amplifiers and use a decent spectrum analyser. I've demonstrated both methods in the past. The result is usually very close to a 6dB increase. This assumes the IMD terms generated in each stage are in phase so they sum and this gives a 6dB boost. See the blue curve below. It shows a 6dB boost just as I told you it would. |
| SilverSolder:
Are you saying that if we have two signal generators running, and we are able to change the phase between them, we should see a difference in the result? |
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