[time-nuts] Z3816a and newbie questions

Rex rexa at sonic.net
Sun Jan 9 15:38:43 EST 2005


On Sun, 9 Jan 2005 01:53:11 -0800, "Tom Van Baak" <tvb at leapsecond.com>
wrote:

>Welcome to the list.

Thanks. And, thanks for replies from both you and John.

>
>> Specs I have seen on the Z3801a for 10 MHz output are
>> only 1 x 10^-9 per day. Looking at a link to NIST from
>> www.realhamradio.com it seems that something around
>> 10^-12 might be closer to possible.  I almost always
>> have 8 satellites; never seen less than 6.
>
>That 10^-9 number seems wrong; do you remember
>where you saw it?

I downloaded the "Z3801A GPS Receiver User's Guide" from Symmetricon. It
has Specifications in section 5. It gives "10 MHz Output
Characteristics" as "Frequency Accuracy: < 1 x 10^-9, one day average".

>
>A value 10^-12 per day is more like it -- for either the
>Z3801A or Z3816A, or most GPSDO for that matter.
>
>Note that figures like this are for one day averages.
>Another way to look at it: consider that most modern
>GPS timing receivers can track time to within 100 ns
>(10^-7). One day is close to 10^5 seconds so 100 ns
>per day is on the order of 10^-12.
>
>What you can't say for sure is that your 10 MHz will
>be accurate to 1e-12 any instant you look at it. There
>may be instants or short durations when it is worse,
>say, 5e-12, or 1e-11, or even more. But, on average,
>over a day, it will come out to 1e-12, or better.

That sounds more like what I expected. I feel better now.

>
>> About what accuracy can I reasonably expect from this unit? Is there
>> anything in the GPSCon statistics that gives me a better real grasp on
>> accuracy of my 10 MHz?
>
>It sort of depends on what you mean by accuracy. Time
>accuracy or frequency accuracy? What's your application?
>But a rule of thumb is 100 ns time accuracy, all things
>considered.

Time is not too important to me right now. I want an accurate 10 MHz
reference for test equipment and setting crystal standards. I am into
ham microwave radio and would like to have setability to 1 E-10 or
better.

>
>Below that you enter a can of worms: have you measured
>your antenna delay, your cable delay; splitter or connector
>delays; receiver delay; BNC output delay, and many other
>factors.

I added a guesstimate for cable delay on my unit. Am I correct that
these delays will have no affect on 10 MHz accuracy, and only matters if
accurate time is important?

>
>GPScon can report high-level events that have a bearing
>on accuracy; locked or holdover mode, for example. Or
>less than 2 SV for too long vs. the normal 5 or 6 SV.
>
>But in general, neither the Z38xx nor GPScon can really
>know the accuracy of the output. For this what you do
>is measure the 10 MHz output with a more stable local
>reference (e.g., cesium standard).

Ok, that's what I thought.

-Rex, KK6MK




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