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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] 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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.