[time-nuts] Spec An for phase noise measurements

[Mike Feher]

Sometimes, rightly or wrongly, phase noise in the systems I work with are
specified down to as close as 1 Hz from the carrier where the carrier may be
around 30 GHZ. Some of it is a leftover from older satellite systems with
real low data rates where the close in phase noise was considered
significant. Unfortunately, most of the time it is easier to copy an old
requirement, than really figure out what makes sense. Unfortunately, the
contractor is stuck with both meeting and demonstrating compliance.    

The age old question is, and of course depends on the modulation scheme
used, is where does the carrier stop and where does the noise that creates
degradation to BER performance begin. 

Now days we no longer deal with data rates in the few kHz rates but
typically in the 100KHz to 20 or so MHz rates. So, the close in stuff is not
as significant anymore. Some order wire data may still be at low data rates,
but even that has increased significantly.

With higher data rates higher modulation orders are now also in play. I am
ignoring different FEC rates and Turbo Codes. Anyway like 16 PSK modulation
the spacing between significant symbol locations become fairly close, and
even closer at higher rates. The integrated phase noise can indirectly give
one the RMS phase jitter, so, depending on the jitter the anticipated
location of the symbol may be, or then again may not, be close the
anticipated or intended phase. A good rule of thumb is to keep the
integrated phase jitter to be less than 10 % of the Euclidian distance
between symbol locations. With M-ary QAM, both the phase jitter and phase
noise is a concern. 

Just look at DirecTV receivers with free running DROs inside. Not the best
as far as stability or noise, and yet due to the data rates work fine at Ku
band and now Ka.  

Unfortunately the above tirade of mine did not address ham use, where
typically we do deal with real low data rates, and phase noise does become
much more of a concern, and hence our interest in being able to fairly able
to measure it and understand it. There are actually cases where uncorrelated
noise can be of a benefit. 73 - Mike
 
 
Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960
 
 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: Monday, January 21, 2008 5:51 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Spec An for phase noise measurements

David I. Emery wrote:
> 	Spectrum analyzer front ends often either have a blocking
> capacitor (to protect the mixer from DC) or don't.    The kind that
> don't usually start to roll off pretty significantly below 10 KHz, and
> are typically spec'd only to 9 KHz.   I suppose if one wants to live
> REALLY dangerously the cap  might be removable in a few cases (NOT
> RECOMMENDED)....
> 	
> 	But would I be too simple minded to suggest that maybe some form
> of A/D PC/workstation input device with high dynamic range and decent
> sample rate (certainly available in high end audio stuff to 192 KHz)
> would be the logical vehicle for close in measurement in a quadrature
> locked PLL type phase noise setup ?   Otherwise why would you care about
> performance below 9 KHz ?
>
> 	Why exactly does one need a wideband SA that goes down to 100 Hz
> (common spec) or 30 Hz (nicer and newer...) for this ?   Maybe I am
missing
> something here.... (probably am, I often do...)
>
>   
If one is measuring the phase noise of a good OCXO with a frequency of
less than 100MHz, there isnt usually much of interest above 10-100KHz.
However when measuring the phase noise of a VCO there may be significant
phase noise out to several MHz (limit depends on the VCO frequency phase
noise measurements at offsets above half the source frequency are
problematic.). The range of interest is even wider with microwave
sources, so the answer depends on what sources you are trying to measure.
For phase noise measurement of most OCXOs a soundcard system will
suffice and with accurate calibration a modern sound card will have
better performance over the [20Hz, 20kHz] range than most surplus
spectrum analysers. It is particularly important in the flicker phase
noise region that the noise bandwidth of the filter be significantly
less than the offset frequency. It is also important that a filter with
steep skirts be used for accurate measurement in this offset frequency
region where the phase noise spectrum is far from flat. These
requirements are easier to meet with an FFT filter than to implement
such a filter in hardware.

> 	And should one want an actual SA for these measurements instead
> of a soundcardish thing and FFT software, I know that LF/HF boxes with
> better specs than any of the affordable general coverage SA families on
> Ebay show up regularly for not too much money.
>
>   
Thus for characterising the phase noise of OCXOs like an HP10811,
FTS1200 etc there is a good case for using a sound card based system.
Since sound cards have stereo inputs implementing a correlation phase
noise measurement system is also possible.

Bruce

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