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

Bob Camp lists at rtty.us
Fri Jan 25 18:44:48 EST 2013


Hi

The other nice thing about TDR's is that the units come out in units of time. If you are working on a timing system, there's much less opportunity to goof things up by a conversion error. 

Bob

On Jan 25, 2013, at 6:37 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> 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
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