[time-nuts] "Best" GPSDO

Said Jackson saidjack at aol.com
Sat Sep 29 02:58:39 UTC 2012


Close in phase noise is of utter importance to radar applications that rely on Doppler frequency shift. Think trying to detect walking intruders at the border.

Frequency synchronization is important if one tries to link up multiple radar units.

Thus good to great Adev and phase noise is needed for those types of applications.

Bye
Said



Sent From iPhone

On Sep 28, 2012, at 17:29, Bob Camp <lists at rtty.us> wrote:

> HI
> 
> Sort of an open ended question, but there is a fairly simple couple answers:
> 
> SInce it's close in phase noise and not far removed, things like PLL's are going to transfer it directly from the reference to the output. It will of course scale by 20 log N where N is the amount you multiplied or divided the reference frequency by. Double the frequency and the phase noise goes up by 6 db. 
> 
> If you look at jitter, measured in the time domain. It will stay constant as you scale frequency. That's provided it's dominated by phase noise in the frequency range that the 20 log N rule applies. If you strip off noise with a filter (or what ever) jitter will go down. 
> 
> If you are building a receiver, phase noise will limit the selectivity of the radio. If you are looking at very tight selectivity, then close in phase noise matters.
> 
> Past that, we get out of the "fairly simple" stuff.
> 
> Bob
> 
> On Sep 28, 2012, at 4:44 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 
>> how important is the close in phase noise of a 10MHz reference?  I
>> means after all, the oscillator you are measuring, say the local
>> oscillator in a receivers or whatever,  is likely not running at 10MHz
>> so you have some step where you convert your t-bolt reference to the
>> desired freq. using either DDS or a synthesizer and I'd bet that step
>> introduces more noise and is the weak link.
>> 
>> I did mean this as a question because I really don't know how to
>> compute the effect.  Say my DDS needs 120MHz clock and I PLL  that
>> 120Mhz clock to my t-bolt and then the DDS as asked to outout 14.5MHz.
>> How do phase noise in the 10MHz t-bolt output effect the DDS' 14.5
>> output.   Or does it even matter compared to all the noise from other
>> sources.  Is there a simple rule of thumb?
>> 
>> OK, maybe we are not really doing any real-measurements, just
>> comparing 10MHz standards to each other? But the above assumes the end
>> goal is some real-world device who performance we want to determine.
>> 
>> 
>> 
>> On Fri, Sep 28, 2012 at 11:50 AM, Murray Greenman <denwood at orcon.net.nz> wrote:
>>> Ulrich,
>>> I think this is a bit like discussing one's favourite wine or favourite
>>> stereo! Especially since many of the participants here will not have the
>>> capability to compare GPSDO performance reliably.
>>> 
>>> I have a few GPSDOs, and it's my impression that of them all the Agilent
>>> Z3815A with MTI260 DOCXO has the best phase noise. Of course these units
>>> were originally equipped with an E1938A, and the later MTI260 version is not
>>> as common.
>>> 
>>> I also have an E1938A, which I operate standalone. It is very impressive,
>>> excellent AD, in fact my best source now that I've given away my HP 5065A.
>>> There is some phase noise well away from the carrier, but I've put that down
>>> to my own construction (switching power supply noise).
>>> 
>>> Another of my favourites with low phase noise is the Trimble/Nortel
>>> NTGS50AA, which has a CMAC/Rakon CFPO-DO OCXO. I've had no opportunity to
>>> measure the AD apart from what Lady Heather reports. This unit is physically
>>> large but is not power hungry and talks to LH very well.
>>> 
>>> I also own a Z3801A, a Samsung GCRU-D and a small homebrew GPSDO with a
>>> CFPO-DO. Sorry, no Timepod for comparisons.
>>> 
>>> 73,
>>> Murray ZL1BPU
>>> 
>>> 
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>> 
>> 
>> 
>> -- 
>> 
>> Chris Albertson
>> Redondo Beach, California
>> 
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> 
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