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R&S SMIQ as a replacement for a general signal generator?
G0HZU:
--- Quote ---Since I want to do some experiment to homebrew GHz spectrum analyzer and GPS recievers, which model do you suggest? I can live with poor phase noise performace under 10MHz range .
--- End quote ---
They are both very good signal generators and assuming they are both healthy then I don't think there's a right one or a wrong one. So it may come down to simple human factors as below:
I had the pleasure of using the 2GHz R&S SMH throughout the 1990s as a general purpose sig gen. This generator was too expensive for the company to buy but it was donated to us by the customer on a research task. It is a really nice instrument to use, the whirly wheel has a high quality feel as do the buttons and it even sounds nice when the attenuators click etc. The display is pretty basic but it still looks good.
It hasn't been totally reliable since then (been back more than once for repairs) but we still have it and I think we first got it way back in the early 90s.
The phase noise is pretty good (but not spectacular). I have a library of phase noise plots for lots of our works generators here and here is the SMH at 1500MHz. The noise profile at 1500MHz shows a fairly high pedestal of noise out to 200kHz and you can see this isn't as good as the 8665A.
It's quite good down at lower frequencies although I didn't measure it below 10MHz. But there's a plot of it at 10MHz below.
Hope this is useful.
I would definitely find room for an SMH here, partly through nostalgia and partly because it is such a nice generator to use. However, I didn't ever use any of its modulation modes back then. It was just used as a low phase noise signal source in the tasks I used it for.
For me, the human factors of the 8665A make it a non starter. It is huge and it weighs about 35kg and unless I 'needed' the performance of the YIG + FLL mode up at UHF then I can't see why I would want to have this monster taking up space on my workbench. However, it is an amazing piece of technology and I don't want to appear disrespectful to HP with these views.
However, I just noticed that the 4GHz SMHU is bigger than I thought. The 2GHz SMH we have at work is only about the same size as a Marconi 2019 but the 4GHz SMHU looks to be very big and is going to be very heavy too.
G0HZU:
Here's an old plot of that Marconi 2024 at 1296MHz as a comparison. It still has that odd pip in the noise inside the loop BW and other 2023/4 generators I've tested don't show it.
But you can see that the 2024 is only average up here. I'd expect the ESGD to be about the same here (1296MHz) too although I can't find a plot of one.
However. if I had to choose between the ESG, the 2024, the SMHu and the 8665 for 'just' doing stuff up at UHF then I'd probably pay the extra for the ESGD or even consider an ESGA version if the price was right.
But that would be with a severe warning about its awful phase noise down at LF/HF frequencies :)
G0HZU:
I went through my library of phase noise plots to see which other generators have poor LF/HF/VHF phase noise like the ESGD.
Probably the most relevant is the Aeroflex/IFR341x series as this is a vector sig gen. In this case I measured a 3413 model. See below for a plot of the IFR3413 at 10MHz. I think I plotted this on the same day I measured the Marconi 2024 way back in 2012. You can see that down at 10MHz this generator has close in phase noise that is about 40dB worse than the 2024 making it just as bad as the ESGD on the SW bands.
This range of generators looks very much like a facelifted Marconi 2024 but in reality this is a different animal inside. These generators support IQ modulation so they have to adopt a different frequency plan to include the IQ modulator.
In this case they use a similar scheme as the ESGD with a downmixing plan from UHF to LF to get the low frequency ranges meaning that these generators also have awfully high phase noise down on the SW bands. There is a reason for this and that is that in order to include vector modulation they can't have dividers after the modulator. Also the IQ modulator will probably only work well over a limited range of frequencies and this is usually done up at UHF.
So the (cost/size/power/weight) driven solution here is to throw away phase noise performance and just mix down from approx 1GHz to the low frequencies and avoid the use of dividers.
So in the case of generators like the ESGD (and the IFR) you get 1GHz related phase noise directly mapped down to (say) 10MHz. If you compute this as a 20*logN ratio you get 40dB. So it's no surprise that these compromised generators are about 40dB noisier than they could be down on the shortwave bands because they will have the same phase noise as the mixer drive signal up at around 1GHz.
However, the phase noise performance of the ESGD and the IFR generators will be about the same as the 2024 when used up at high VHF through UHF because the frequency plans will be reasonably equivalent in terms of noise contribution.
Compare below the IFR3413 to the Marconi 2024 plot down at 10MHz to see the 40dB difference :)
davorin:
How much equipment do you own? (o;
G0HZU:
--- Quote ---How much equipment do you own? (o;
--- End quote ---
I do have quite a few sig gens here, but (sadly) the E5052A signal source analyser used to take the plots isn't mine and none of the generators in the E5052A plots are mine either. I do have a Marconi 2024 amongst my stash of sig gens but it isn't the one used for the plots.
The sig gen plots were taken at work using works equipment, usually at lunchtimes on days when I was using the E5052A for design work or verification :)
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