[time-nuts] Re: Stability question

[Bob kb8tq]

Hi

Over the years, there have been many generations of GPS gear. 
Even today there is a range of stuff out there. Any time you dive into
this or that box there is a long list of things to wonder about.

1) Does it expect a 50 db, 30 db, 20 db or 12 db gain antenna? Some
front ends are designed with very little gain at the device end. Others
have quite a bit of gain in the device. To much or to little antenna gain
and they overload or drop out. 

2) Is it driving 12V, 5V, 3.3V or something else? What does the antenna
expect? How much current will it source? How much current does the
antenna need? A miss match here could be a non issue or it could be
a really big deal. A 150 ma antenna on a source that is limited to 70 ma
might not be easy to spot. Stacking inline amps is a good way to run
up the current ….

3) How much loss *does* your cable have? Real / full spec LMR 400 has
a loss of ~ 5 db / 100 feet at GPS. ?MR 400 from who knows who might 
be very different. 200’ of cable would be around 10 db. RG-58 is something
over 20 db / 100 feet. The typical setup is designed to work with 20 to 50’
of RG-58. If the cable is ok, is the same true of the connectors ….

4) How good is your splitter? You would *think* this would never / ever 
be an issue. Unfortunately stuff does happen and even splitters can be
damaged. Working out that the splitter is 30 db rather than 3 db is not
a lot of fun. 

5) Is the antenna ok? Again, stuff happens. Antennas do die. A bird nest
on the antenna is *not* helpful. If the 30 db antenna is now an 18 db, some
devices may be happy and others may not work at all. 

6) Does your device need something odd? This might be a down converter
at the antenna end. Even today, some very high end gear runs this way. 
(it lets them use real long runs of cheap cable. That’s nice when the dog 
decides the cable is a chew toy … I have data on this … :) ). 

7) Are you after something beyond simple L1 GPS? Things like Glonass run
on other frequencies. Inline amps and antennas may or may not be happy
with these frequencies. 

So lots and lots of this and that. The list could go on for a bit. I suspect 
that’s enough bandwidth for now. 

Bob

> On Mar 2, 2022, at 6:49 PM, Jack Davis <jack.davis at engineer.com> wrote:
> 
> 
> I have a Datum Exactime ET-6000TCXO that I use to lock all my test equipment to 10 MHz.  My GPS antenna is approximately 200 feet from the receiver and I have intermittent GPS signal levels and associated unlocking of my ET-6000TCXO.  I have tried using bigger less loss coax cable but its just not enough to make it reliable.
> 
> I see there is a Symmetricom Kit 142.6150 consisting of a model 140-615 GPS antenna / downconverter and a model 150-615 upconverter available to allow longer runs of coax cable.  From what I can tell there is a local oscillator in the antenna that down converts the 1,575.42 MHz GPS signal to 4.092 MHz for lower cable loss.  The up converter on the receiver side apparently another local oscillator that up converts the 4.092 MHz IF to 1,575.42 MHz.
> 
> My question is: will the two local oscillators and the up and down conversion cause phase noise or frequency instability of the GPS signals into my ET-6000TCXO?  It does have some sort of flywheel correction internally but is it enough to keep the 10 MHz timebase stable?
> 
> Thanks,
> Jack  K6YC
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