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

[Dr Bruce Griffiths]

Didier Juges wrote:
> 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
>>>
>>>
>>>
>>> _______________________________________________
>>> time-nuts mailing list
>>> time-nuts at febo.com
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>
>>>   
>>>     
>>>       
>> 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
>
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>
>   
Didier

I found, as expected when using a 75 ohm cable to connect a 220 ohm GPS 
PPS output to an HP5370A terminated in 50 ohms, the threshold setting is 
a bit more critical than when using a 50 ohm cable.
Its advisable to trigger on the incident wave and not on a subsequent 
reflection. This is less of a problem if the PPS source has an output 
impedance closer to the cable impedance.

Bruce