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

[Chris Albertson]

On Fri, Feb 20, 2015 at 1:10 PM, Tom Miller <tmiller11147 at verizon.net>
wrote:

>
> ----- 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.
>

I've had poor luck extending "fake" RS232 using cat5 wire.  It works well
if you use differential signaling  Convert the cos level serial to  RS422
and you can go almost a mile using cheap cat-5 wire.  And I've also have
worse luck extending a 1PPS plus using cat-5.  The solution is RS422
signaling for the plus.    But I finally gave up as running a longer
antenna cable has easier.



> tm
>
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-- 

Chris Albertson
Redondo Beach, California