[time-nuts] "Better" gps antennas than a Symmetricom 58532A

[Magnus Danielson]

On 01/24/2013 03:52 PM, John Lofgren wrote:
>> Should I make it a habbit of TDRing my GPS antennas, receivers and
>> splitters?
>>
>> Cheers,
>> Magnus
>
> I think that question ties into some of the other responses to the original post.  The value of doing the TDR measurement would probably depend on your cable lengths and how likely you think it is that a connector mismatch would cause cable reflections that might smear the GPS signals.
>
> Since you're fortunate enough to have a TDR, it might be interesting to do it just to see how much mismatch there really is.  If the resolution of the measurement is good enough you should be able to see all of the connectors.  If nothing else, it might tell you if you have a bad cable end or a loose connection.

Electrical TDRs excel on short distances, as they have great time 
resolution, but the dynamics isn't great. VNAs has great dynamics, but 
tend to lack the time resolution. The high frequency losses of cabling 
does however make some of the details go lost anyway, and it also kicks 
in for reflections. Sufficiently lossy cables makes impedance mismatches 
less critical as the reflection at the sink side would need to traverse 
the cable twice, as well as being reflected at the source side. So 
low-loss cables also calls for lower reflections (rather than impedance 
matching really) in order to achieve the higher system performance.

TDRing may not be the ultimate tool, but it is highly educative at least. :)

> Maybe the Italian guys should have run an optical TDR on their timing setup before doing the neutrino measurements :)

Actually, that would have meant that they where suspecting things from 
start. Also, validating connections means unhooking them, allowing for 
human errors on each location.

Cheers,
Magnus