[time-nuts] Cable length calibration

[Charles Steinmetz]

Scott wrote:

> When using a TDR its best if cable is unterminated as the discontinuity at the end is helpful as a marker.

Someone else previously asked how you're expected to do TDR on a GPS 
antenna cable since everything is matched (therefore, presumably, no 
reflections).

There are ALWAYS reflections.  As one message in this thread noted, TDR 
easily shows sharp bends in coax.  Plus, a GPS antenna will be "matched" 
only at a relatively narrow band of frequencies centered on the 
frequency(ies) it receives (L1, L1+L2, etc.).  Also note that GPS 
antennas specified as having 50 ohm outputs are often (if not usually) 
used with 75 ohm cable.  (Trimble actually advises this.)

You can do the TDR situationally (when the receiver is powered up, and 
whenever an antenna fault is cleared), or repetitively during operation 
(e.g., once a minute).  I see no reason why either strategy would not 
work well on a GPS antenna cable.

There is no need to use signals that would damage a GPS antenna preamp.

Best regards,

Charles