[time-nuts] E4437B phase noise spurs. Any ideas?

[Bill Byrom]

Hi, Wolfgang. Both of your generators appear to be well within their
datasheet specifications based on the spectrums you posted.

The datasheet for that ESG-DP (digital modulation with high spectral
purity) model is at:
http://cp.literature.agilent.com/litweb/pdf/5965-3096E.pdf

The E4437B nonharmonic spur specification is on page 5 (use the ESG-
DP columns):

So according to that table, above 2 GHz the E4437B spurs at >3 kHz
offset **should be below -68 dBc**. It appears to me that the largest
spur at such offsets (at about +3.5 kHz offset from the carrier) is
**actually about -74 dBc**. The largest spur is about -63 dBc at +1 kHz
offset (which is closer than the spur specs).


The R&S SMIQ03B specs are:


The SMIQ03B nearby spurs in your screen capture are at about +/- 9 kHz
offset from the carrier (about -85 dBc), so are too close to be covered
by the spec above. The spurs at >10 kHz offset are about -90 dBc.

I'm guessing that the spurs might be affected by several adjustments
performed during factory alignment. In most modern complex instruments
(including both of these signal generators, I believe) there are far too
many alignments to be made by a human, and a long automated process
using a rack of equipment is used to align the instrument and store
certain constants in nonvolatile memory in the instrument. But there is
no reason to worry based on your results, since they are well within
instrument specifications.

So ... assuming that the spurs aren't coming from your spectrum analyzer
or other sources (and I'm guessing that's a FSU which is very good),
both generators appear to be within specifications (after warmup within
the calibration interval, of course).
 * *The E4437B closest spur covered by the >3 kHz spec is at about +3.5
   kHz, and it's about 6 dB better than the spec.*
 * The SMIQ03B has a 6 dB better spurious spec than the E4437B between 2
   and 3 GHz, but the SMIQ offset for their spur spec is wider than the
   ESG (10 kHz rather than 3 kHz).
 * At other carrier frequencies the spurs may be higher or lower in
   amplitude (and at different offsets). Try tuning the frequency in 1
   kHz steps over a wide range -- I would guess that the spur offsets
   and levels will change in a complex fashion due to the synthesizer.
 * The phase noise specifications normally apply to a smoothed trace and
   do not include narrowband spurs such as the ones you see in your
   screen capture.
--
Bill Byrom N5BB
Tektronix RF Application Engineer
 
 
On Thu, Jul 16, 2015, at 04:15 AM, Wolfgang DL1SKY wrote:
> Hi,
>  
> I just got a used/refurbished E4437B which I wanted to use as a
> all-purpose RF generator primarily for 3-4 GHz.
>  
> Unfortunately, I'm seeing strange spurs for frequencies above 2.4 GHz,
> see the green curve in the attached image.
> The yellow curve is an SMIQ03 for comparison.
>  
> Observations:
>  
> - For frequencies below 2.4 GHz none of the spurs appear.
>  
> - It has an OCXO and I left the device in standby (oven on) for
>   12 hours.
>  
> - If I leave the device ON for 1-2 hours, the spurs go down.
>  
> Anybody else seeing this? Any ideas how to fix this? Does this look
> like a pre-failure sign?
>  
> Regards,
> Wolfgang DL1SKY
> _________________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
> Email had 1 attachment:


>  * E4437B-phase_noise_problem-1.png
>   34k (image/png)