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

[Tom Miller]

----- Original Message ----- 
From: "Jim Lux" <jimlux at earthlink.net>
To: <time-nuts at febo.com>
Sent: Friday, February 20, 2015 10:25 AM
Subject: Re: [time-nuts] Looking for advice to get a submillisecond setup


> 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.
> _______________________________________________

One needs to be careful with extending the 18X "RS232" signal. It really is 
not true RS232 but more like a 5 volt CMOS like signal.

If you are using it in a wood frame house, it might work fine indoors. But 
in a steel/concrete multi-story commercial or industrial building, I would 
be more surprised if it worked than not.

tm