[time-nuts] "Best" GPSDO

[Tom Knox]

"Best" of course is such a vague question. Best For 5MHz today is a Cesium Fountain used periodicly to calibrate 6-12 Symmetricom 5071A option 004 disciplining 5-6 Symmetricom Hydrogen Masers  disciplining 5-6 Oscilloquartz 8607 option 08 BVA's.  I am a part owner in just such a system. Notice the system ends with quartz and I think therein lies the key. I am a big believer that for the best system find the best piece of quartz you budget can afford. On eBay I would currently I would argue that the Datum 1000B's for around 4-500USD is the way to go.  From my experience how you steer it and set up your GPS or other timing source is important but only a major factor if you do something wrong.

Thomas Knox


1-303-554-0307

> From: lists at rtty.us
> Date: Sat, 29 Sep 2012 10:44:07 -0400
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] "Best" GPSDO
> 
> Hi
> 
> For close in noise, you can go from 10 to 120 to 14.5 and the net result will be the same as 10 to 14.5. In the case of 10 to 120, close in might be DC to 50 Hz or DC to 250 Hz. Past that a reasonable crystal oscillator could beat the multiplied 10 MHz. In most microwave chains, the low frequency reference only is responsible for a fairly small range of phase noise offsets. 
> 
> For most radio testing, you use wide spaced tones. What you care about is far removed phase noise rather than close in noise. If you have a tone that's 10 KHz to 100 KHz away from the passband, phase noise at >10 KHz  is what you would worry about. If you are building a radio with 10 Hz selectivity for 40 Hz spaced channels,  you would worry about close in noise.
> 
> For what ever it's worth, I don't lock up the sources I use for most radio testing. I just use free  running oscillators with good noise characteristics past 1 KHz.
> 
> Bob
> 
> On Sep 28, 2012, at 11:31 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 
> > On Fri, Sep 28, 2012 at 5:29 PM, 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.
> > 
> > So in my example case of scaling the 10Mhz t-bolt to 14.5Mhz  Assuming
> > a perfect DDS chip the T-Bolt's phase noise would be scaled up by 20
> > Log(1.45)     I'm assuming this works, that I can go from 10MHz to
> > 120Mhz and then to 14.5MHZ and the total effect is the same as going
> > directly from 10 to 14.5, except for the noise the equipment
> > introduces as added.
> > 
> > You can guess the real question here: "how good does the 10MHz
> > reference need to be to test real-world receivers?
> > 
> > 
> > 
> > 
> > 
> > 
> > -- 
> > 
> > Chris Albertson
> > Redondo Beach, California
> > 
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> 
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