[time-nuts] 75Z vs 50Z for GPS receivers

[Didier Juges]

Dr Bruce Griffiths wrote:
> Christopher Hoover wrote:
>   
>>> Most (except for Trimble,..) GPS receivers and antennas
>>> are designed to use 50 ohm cable.
>>> Trimble Bullet GPS antennas have a 50 ohm output impedance.
>>> Trimble literature however is ambiguous in that in the
>>> Resolution T receiver datasheets talk about using RG59
>>> to connect to the antenna.
>>>     
>>>       
>> FWIW, the Thunderbolt manual says this on page 3-5:
>>
>> --------
>> Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and
>> the Bullet antenna are compatible with either 50-ohm or 
>> 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable
>> provides superior transmissibility for the 1.5 GHz GPS
>> signal and a better quality cable for the price. Mismatched
>> impedance is not a problem.
>> --------
>> --------
>> Note - The input impedance of the ThunderBolt RF input &
>> its antenna is 50 ohms.
>> --------
>>
>>
>> I would also add that BNC and N connectors come in both 50Z and 75Z flavors.
>> In fact 75Z BNC connectors are pretty common, being used in professional
>> video applications.  (Check surplus BNC patch cords carefully.)  On the
>> other hand, 75Z N connectors are much less common, but were used in CATV
>> plants, IIRC.
>>
>> -ch
>>
>>
>>
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>>   
>>     
> Impedance mismatch can be more problematic when connecting the PPS 
> output of a GPS receiver to a time interval counter.
> RCA to BNC cables can catch one out if one doesn't notice that these use 
> 75 ohm coax.
>
> 93 ohm RG62 cables with BNC connectors are not unknown, they were used 
> in some nuclear instrumentation.
> I have a few of these lying around.
>
> Bruce
>   
I noticed that when my counter insisted the PPS output of the 
Thunderbolt was 5 Hz, even though it looked fine on the scope at 
200mS/div, until I set the scope's sweep fast enough (20nS/div or so) 
and then I saw the reflections in the ~6 foot cable. In my case, it was 
not an issue of 50 vs 75 ohm cable impedance, simply I had to terminate 
the cable by setting the input impedance of the 5370A to 50 ohm instead 
of the default 1 Mohm. I tried both 50 and 75 ohm terminations 
(externally) and it did not make much difference to the counter, even 
though the signal was definitely cleaner with 50 ohm (that cable was 50 
ohm).


Didier KO4BB