Marconi 2041/2022 sig gens and phase noise as viewed on a SA

[mark03]

The delivery man brought a new toy yesterday, a Marconi 2041 "low noise" sig gen:


I stumbled across this on *bay last week for what seemed like a decent price, then made a low-ball offer, which to my surprise was accepted.  $600 shipped (plus tax) for a high-quality 2.7-GHz sig gen in working condition.  Despite some signs of abuse, it indeed appears to work as advertised.   This thing is built like a tank, and heavy!  I intend it to replace its little brother, a Marconi 2022.

I have no test equipment to accurately measure phase noise, only an Agilent 4396B spectrum analyzer, but I was curious what I could learn.  First, here is the specified phase-noise performance of the 2041, then the SA:



At 1 GHz, if everything is working as it should (and maybe even if it isn't...), the SA phase noise should dominate at offsets greater than ~ 100 Hz, and indeed that seems to be the case:



There are some close-in spurs at harmonics of 60 Hz.  If I look at the SA's 20-MHz calibration output and compare with the 2041 set to 20 MHz, the spurs are visible on both, so I guess this is happening within the SA.  First, the 2041 at 20 MHz:

and then the SA cal output:


Does this indicate a problem with my SA, or is it expected?

Finally, comparing the 2022, also at 20 MHz to match the previous two plots, the noise sidebands are comparatively huge:



I looked for a phase-noise plot for the 2022 but couldn't find one.  In several places I've read it has "good" phase-noise performance at HF, and this seems fairly horrible, so I'm wondering if my unit has a problem?

[hendorog]

You got a great deal there!

Member GH0ZU has mentioned his lab has many 2022's, and no doubt he will have intimate knowledge of its phase noise characteristics.

Any chance the spurs are coming from your External Reference which is flagged on the SA screenshots? Or have you coupled the references together, which is probably not what you want to do measuring phase noise?

Image from the Marconi 2022 datasheet showing the phase noise spec is attached.
Link to document:
https://www.avionteq.com/document/Marconi-2022D-specification-sheet.pdf

[mark03]

The OCXO option on the 4396B is implemented as a separate output BNC which you need to jumper to the reference-input BNC with a short cable, and when you do, the ExtRef indication comes on, even though the external reference is actually internal   But it would be a good idea to switch to the internal reference (i.e. not the OCXO) for comparison.

I'm unsure how to interpret the 2022 phase-noise spec.  Presumably those numbers (-120 and -130 dBc/Hz) are the "floor" at some distance out from the carrier, but how far?

[hendorog]

Thanks, that makes sense

Yes I don't see anything to explain their spec either. Strange they have written it like that. 

[hendorog]

I found this, there is an image in this thread with the 2022 phase noise - from GH0ZU

https://www.vintage-radio.net/forum/showthread.php?t=108457&page=2

[mark03]

Good find, thanks!  I took some matching data at 144 MHz and it agrees almost perfectly, so I guess my 2022 is healthy.  It just looks bad next to the 2041.

I'm still learning the best way to set up this SA for phase-noise measurements.  I think my earlier plots were slightly off because there is actually a "noise" mode which displays directly in dBm/Hz.  This is slightly different than the normal display even when RBW is 1 Hz; apparently the noise bandwidth isn't exactly 1 Hz.  Anyway, the net effect was to make my original measurements somewhat pessimistic.

I tried switching to the 4396B's internal-internal frequency reference (i.e. not the OCXO), but the 60-Hz harmonic spurs are still there.

[ha3flt]

I'm a bit late, but I have a Marconi 2022D and I can confirm those noise figures. It's a reliable generator with a nice OCXO and output, but noisewise is not really the best lab. instrument. At this point mine seems to be even worse by several decibels than shown above... I even tried using a decent GPSDO/OCXO external reference for it but there wasn't a significant change - except the 5-6 Hz difference. :-)

[Leo Bodnar]

Here is my Marconi 2041 at 10MHz - if this helps.
Cheers
Leo