[time-nuts] Looking for advice to get a submillisecond setup

[Jim Lux]

On 2/20/15 6:30 AM, Chris Albertson wrote:
> I think the easiest cable to make really long, if one must be long is the
> antenna cable.   Use 100 meters of the kind of cable they use for cable
> TV.  It comes double shield and has those compression type F connectors.
> The cable can cary both the GPS signal and power for the amplifier that is
> built into the antenna.  If the cable is very long, You would need another
> in-line amplifier, again powered by the cable itself.

that's fine if you have a separate antenna(w/preamp) and receiver..

But something like the Garmin GPS-18x and other similar inexpensive 
receivers have integrated antennas.




>
> And yes, a gps antenna needs a good view of the sky, but the receiver
> itself can be 100+ meters away from the antenna


I think you're getting into receivers that are well into the hundreds of 
dollars range, if bought new.

For an inexpensive "NTP for few hundred dollars to get better than a 
millisecond" end of things, I think the integrated GPS antenna/receiver 
with a suitable computer right next to it is the way to go.  Then you're 
just running a network cable and power.


4 pair Cat5 sometimes works ok with RS232, sometimes not. At the 4800 
bps of the garmins, it would probably work ok.
At least you're sending power from the same place as you're 
generating/consuming the signals, so you don't have the common mode 
voltage difference problem.

I like that idea in general.. a pair for power, a pair for TxD, a pair 
for RxD and a pair for 1pps. I'm not sure you'd want to connect the 
"ground" at both ends of the signal pairs, though.  What does the supply 
current to the GPS-18x look like?  Maybe it really doesn't make any 
difference.  Hopefully your computer's RS232 "input" isn't drawing 10s 
of mA.