# [time-nuts] CSAC Project(was CSAC purchase)

Bob kb8tq kb8tq at n1k.org
Mon Jan 29 14:22:47 EST 2018

```Hi

Questions are good, it’s how you figure things out. We’re talking about a
“practical” timing device design. It’s not as crazy a topic as it might seem.

Some basic math:

You get to a million seconds at about 11.6 days. A millisecond error over that
period is one ppb. If you are off 1 ppb at T=0 and stay there, you will be off
by 1 ms. If you drift so you are off by 1 ppb at the end, you will be off by less
than 1 ms.

At some point, you do need “rough numbers” to work out what you are going to do.

Holding a few microseconds (not milliseconds, we just jumped a factor of a thousand)
on a GPS based wall clock / watch is quite practical, even with poor GPS access ( = crummy
antenna).  With a reasonable antenna 100’s of ns are very practical, even with a
cheap GPS module.

The practical question would be: What am I getting from the CSAC? One basic answer
might be “holdover” at less than 1 us / day. Another basic answer might be autonomous
operation in a location where GPS simply isn’t available.

Bob

> On Jan 29, 2018, at 12:44 PM, Ronald Held <ronaldheld at gmail.com> wrote:
>
> Bob;
>  I see what you are saying.  I will wait until I get the chips d more
>         Ronald
>
> Hi
>
> As mentioned multiple times in the archives. As you get into the single digit
> milliseconds, the human eye simply can’t keep up. A watch that is 1 ns off
> and one that is 1 ms off are both “good enough” if you are looking at it with a
> normal eyeball.
>
> From a design standpoint 1 ms / day / week is *way* different that 1 ns over the
> same sort of period. Design constraints *do* make a big difference.
> It’s important
> in any project to get them sorted early.
>
> If you are spending \$5K on a CSAC, tossing in another \$100 on a GPS isn’t
> going to even get into the roundoff error. You *will* need the GPS gizmo to
> keep the CSAC calibrated. It is only a question of how often the beast gets
> used.
>
> Bob
>
> On 1/29/18, Ronald Held <ronaldheld at gmail.com> wrote:
>>
>>
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```