Products > Test Equipment
R&S SMIQ as a replacement for a general signal generator?
G0HZU:
Just to show Davorin how poor the ESGD is when it comes to its phase noise performance in the shortwave bands I set up a very simple experiment/comparison.
I compared my E4433B against an old Marconi 2019 that dates back maybe 35 years. The Marconi 2019 was a regular sight in test and service departments throughout the 1980s and 1990s. We even had a couple of them in our design labs in the 1990s as the phase noise performance was pretty good across 2MHz-30MHz.
The performance of the 2019 isn't so impressive once you get up into the VHF/UHF ranges and they aren't very reliable either but even this old generator can be seen to totally outclass the ESGD for close in phase noise on the SW bands.The 2019 uses dividers to synthesise outputs down to about 2MHz so it achieves good (but not spectacular) noise performance in the SW bands.
I set each generator to a few kHz outside the passband of a 10.7MHz crystal filter and set the level to +4dBm to account for filter and cable loss. So the sideband phase noise that leaks through the filter in its passband can be read as -dBc/Hz. The leaked carrier can be seen on the right of the analyser plot and it's level is about 60dB lower than normal because it is being attenuated by the filter stopband.
If you look at the plot below there are two traces stored on this old HP8568B analyser. Can you guess which is which? Yes, the one that is over 30dB higher in noise level is the ESGD. When measured on a signal source analyser they typically produce about -100dBc/Hz at a few kHz offset down on the SW bands and maybe -108dBc/Hz at 10kHz.
The creaky old 2019 that I've owned for about 20 years manages about -133dBc/Hz at about 5kHz offset at 10MHz CF. The ESGD shows about -101dBc/Hz at the same offset in the plot below.
A Marconi 2024 should manage about -138dBc/Hz here and I'd expect the HP8640 to better this by several dB. I'm guessing maybe -145dBc/Hz but I've not had my hands on an 8640 for many years. But a decent 8640 should deliver about -115dBc/Hz at 5kHz offset up at its cavity oscillator at 320MHz and the dividers should clean this up by 30dB when dividing by 32 to get 10MHz. So it could (reasonably) be expected to achieve -145dBc/Hz at 5kHz ofset at 10MHz CF. Thats over 40dB better than the ESGD.
So anyone choosing to replace a regular sig gen for use on shortwave (AM bands) should be aware how poor the ESGD is for phase noise down here. Because it is at least 30dB off the pace here it can't be taken seriously as a lab sig gen for typical performance checks of a decent SW receiver for example. Even the 2019 will struggle here when testing a decent Ham radio for blocking or adjacent channel performance despite being >30dB better
than the ESGD.
G0HZU:
I dug out an old E5052A library plot of a 2024 set to 10MHz and +13dBm.
This is one of the 2024 generators at work and it showed a curious blip in the phase noise at about 1.5kHz offset (green arrow)
But you can see that this generator manages about -140dBc/Hz at 5kHz offset at 10MHz. Apart from the blip, this generator was slightly better than a few other 2023 and 2024 generators I measured in comparison.
The red trace is hand drawn by me and represents the typical phase noise of an ESG4433 at 10MHz. It is much, much worse as you can see.
The blue trace is also hand drawn and it represents the typical phase noise of a simple homebrew LC oscillator at 10MHz. i.e. something you could make with a transistor, and a few resistors, caps and an inductor.
analogNewbie:
I'm also looking for a signal generator these days.
Anyone has the experience with Agilent 8665A? This heavy guy takes about 6 minutes or more to do the cool boot calibration. Does anyone know what is this guy calibrating? I think the phase noise performance of 8665A is not bad, any comments on this unit?
Thanks
G0HZU:
The 8665A is an early PSG model. I've never used one because when these were being sold 20-25 years ago the company couldn't afford the PSG range. By the time the company grew enough to buy a £££ sig gen we bought the later PSG models. So I have zero experience of them.
However, I think that the early range of PSG models use a precision delay line based discriminator as part of a PLL+FLL. This allowed a useful reduction in phase noise compared to a basic single loop PLL if the user selected this FLL mode. However, this complex/expensive FLL mode may be a factory option. I'm not sure.
This mode will be slower to tune and lock but gives better phase noise.
But (I think) the delay line in the FLL and the VCO tuning has to be calibrated and in some PSG models these delay lines are even heated by an oven to make them more stable so 'maybe' this is what takes all the time to stabilise after a cold boot.
But all of this is just a guess. Maybe someone on here has one and has serviced it etc.
In terms of phase noise, I think this model still resorts to using a mixer to get its lowest frequencies (eg LF through to VHF) so I would guess that its phase noise could be as bad as the ESGD down here unless its FLL 'noise cleanup' mode is enabled. In this mode it will be much better than the ESGD but I doubt it will be as good as something like a basic Marconi 2024 down in the SW bands. Note: the Marconi 2024 also uses a mixer to get its very lowest frequencies but this applies below about 10MHz and the phase noise is still much better than the ESGD below 10MHz despite this.
Up at UHF>>xGHz the 8665A phase noise performance will be extremely good. Much better than a 2024 because it has the FLL mode to exploit in order to achieve low phase noise across maybe 2kHz to 100kHz offsets and it uses a YIG oscillator.
Basically, the 8665A isn't a generator that would be on my wish list unless I really 'needed' its low phase noise up at UHF. It's very big, very heavy and offers limited modulation capability.
Maybe someone on here has one and can comment?
analogNewbie:
Thanks for the detailed info about 8665A. The close in phase noise is not as good as 8662/8663A, but the 1k+ phase noise is pretty good.
I want to ge one with 4GHz capability and phase noise shoud be good. Now I found one 8665a and one R&S SMHU. The second hand ESG series are all over the place and the prices are much higher than these old heavy tanks.
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 .
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