[time-nuts] Failure mode in GPS receivers

bg at lysator.liu.se bg at lysator.liu.se
Sun Dec 17 16:29:57 EST 2006


On Sun, December 17, 2006 22:09, David Forbes said:
> At 12:04 PM -0800 12/17/06, Bruce Lane wrote:
>>On 17-Dec-06 at 19:54 Poul-Henning Kamp wrote:
>>
>>>In message <200612171136230661.088C8673 at 192.168.42.129>, "Bruce Lane"
>>>writes:
>>>>Fellow clock-tickers,
>>>>
>>>>        In the group's experience, what's the most common failure
>>>>  mode for GPS receivers, especially older ones, that could stop them
>>>>  from hearing the sats?
>>>
>>>Antenna damage.
>>
>>	Are you sure? Especially considering that the other two
>>devices connected to the same antenna are working just fine? ;-)
>>
>>	Just thought I'd remind folks of that -- My main antenna
>>feeds a four-way splitter, and two of the three devices hooked to it
>>are functioning quite well.
>
> Bruce,
>
> The point is that some receivers require more RF signal from the
> antenna than others to achieve the same satellite tracking
> performance.
>
> You might try improving the S/N ratio of the failing receiver by
> bypassing the splitter and connecting it directly to the antenna,
> netting a 6 dB signal increase, to see if it gets happier.

GPS antenna splitters are often amplified (up to 20dB and more to
compensate for long cable attenuation), or at least doing 0dB, like the HP
ones.

The S/N ratio often displayed by the receivers are not directly related to
the signal level. What is important is to have a signal level that the AGC
on the RF-front end can handle. These are different between receivers.

That said, its no harm in trying without the splitter.

> Failing that, is there any diagnostic information presented by any of
> your receivers that would tell if the signal strength to them had
> changed recently?

Good idea. And check what oem receiver is in the box. It might be a fairly
easy one to source a replacement.

Checking the receiver antenna connector for a DC-offset on the centerpin
might tell you at least that little part of the receiver is not dead.

--
   Björn




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