[time-nuts] Lucent RFTGm-II-XO / Lucent RFTGm-II-Rb / NT BW50AA

Wed Feb 13 02:31:49 UTC 2019

Hi

The normal approach as you head from HF up through stages to microwaves is to stop
at various points along the way. Just where depends on the actual noise you measure on 
the sources you have. (Yes you have to measure them yourself). 

Once you have your noise plots, you can pick likely crossover points for a PLL. You then 
design the PLL to hit those targets. After that you measure the result to see how close you
got to your target. Then it’s off to teak this or that to improve things. 

The step by step process continues through VHF to UHF to microwaves as you move through
various steps. Indeed you may change your plan once you see what your measurements tell 
you about each design iteration. 

A lot also depends on just what your target actually is. Running PSK 32 at 22 GHz is a bit 
different than running WBFM HDTV at 5 GHz. What works for one may not be ideal (or even 
close) for the other. 

So yes, it’s a great way to spend a couple of years fiddling around …..

Bob

> On Feb 12, 2019, at 7:41 PM, Jeff Blaine <KeepWalking188 at ac0c.com> wrote:
> 
> OK.  I would still be interested to see if anyone has plots on the close in noise profile of the Lucents.  The XO part has an Efrom OCXO and I can assume it's similar to a generic OCXO.  But the RB is an unknown - I have no idea what to expect from that.
> 
> On a separate task, one of these days I would like to build up a set of 125 & 500 Mhz sources for lab and gear use disciplined by a GPSDO 10Mhz feed.  The current source is a 125 Mhz homebrew OCXO and a pair of doubler/amp/filter sets.  I assume they are pretty clean close in but the frequency moves around a bit more than I would like as the OCXO part of the equation is nothing special.  They drive some SDR and DDS sig gens here in the shack so the PN is a factor as well as the frequency accuracy.
> 
> 73/jeff/ac0c
> alpha-charlie-zero-charlie
> www.ac0c.com
> 
> On 12-Feb-19 5:15 PM, Bob kb8tq wrote:
>> Hi
>> 
>> None of the GPSDO’s out there are great for multiplying direct to microwaves. It’s not so much
>> the phase noise as all the spurs (though phase noise does vary a bit). For an instrument frequency
>> source, they *assume* the input will be dirty and clean it up inside the device. So again, pretty much
>> any of the gizmos likely will do ok.
>> 
>> As you go to longer gate times than one second, the accuracy changes a bit on most GPSDO’s.
>> Just how much is a “that depends” sort of thing. If you get out around 200 seconds it’s a good
>> bet that your 0.1 ppb is more like 0.01 ppb.
>> 
>> Bob
>> 
>>> On Feb 12, 2019, at 4:30 PM, Jeff Blaine <KeepWalking188 at ac0c.com> wrote:
>>> 
>>> Hi Bob,
>>> 
>>> Thanks for your comments.  The highest precision thing in my lab is a 5338a and that's only if I set the gate time to something really long so that it renders a full 12 digits of info.  Other than a "I wanna be a time nut so let's see what it will do" - I never need anything near that in reality.  So let's say what I want to be able to see is 0.1 Hz resolution on a 50 Mhz test frequency.  So your 0.1 ppb is a good observation.
>>> 
>>> Do you have any thoughts on PN of the 3?  Ive not found plots yet for the Lucent boxes but know the NT I have is middling on a good day.
>>> 
>>> 73/jeff/ac0c
>>> alpha-charlie-zero-charlie
>>> www.ac0c.com
>>> 
>>> On 12-Feb-19 9:17 AM, Bob kb8tq wrote:
>>>> Hi
>>>> 
>>>> Pretty much all of these big old telecom beasts are “good enough” for “typical use”. The gotcha
>>>> is that both of those terms are very much lacking in precision. Since this is a precision sort of
>>>> thing, that may not be ideal :)
>>>> 
>>>> If 0.1 ppb is good enough for what you are doing, then they all should be fine. If you need 0.001 ppb,
>>>> then none of them are likely to do what you need to do. Between those two limits, things like antennas,
>>>> survey locations, lab temperature, and the accuracy definition you use all get into play. Yes, you *can*
>>>> have some unusual cases (your lab goes from -20 to +90 (F of course …) on a daily basis. For anything
>>>> reasonable, the limits above are pretty good.
>>>> 
>>>> If time rather than frequency *is* the goal, then one of these GPSDO's is not really what you want.
>>>> The time pulse output from a good GPS module is likely closer to UTC (after some math) than the
>>>> output of the GPSDO.
>>>> 
>>>> Bob
>>>> 
>>>>> On Feb 11, 2019, at 9:57 PM, Jeff Blaine <KeepWalking188 at ac0c.com> wrote:
>>>>> 
>>>>> I have these 3 systems operational here and wonder what is the "best" of the sources?
>>>>> 
>>>>> The local use for the reference is as a time base for test equipment.  Nothing hyper critical in the way of end-use requirements - but after running the NT GPSDO for a few years and watching the Lucent box collect dust, I wonder if I have got that backward?
>>>>> 
>>>>> The lucent has a 10 Mhz output on the front panel of the RB but the XO would need a tiny bit of modification to have 10 Mhz instead of the current 15 Mhz.
>>>>> 
>>>>> There is a lot of data on http://leapsecond.com/u/sims/gpsdo17/ but I don't see the RFTG XO on there - which I assume is due to it's native 15 Mhz Fo vs. 10 Mhz for the rest of the field.
>>>>> 
>>>>> Appreciate any advice you may have for a (very) novice 'nut.
>>>>> 
>>>>> 73/jeff/ac0c
>>>>> alpha-charlie-zero-charlie
>>>>> www.ac0c.com
>>>>> 
>>>>> 
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