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

[Didier Juges]

Dr Bruce Griffiths wrote:
> 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
>>>>
>>>>
>>>>
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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
>>
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>>
>>   
>>     
> 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
>   
Bruce,

Trimble does not specify the output impedance of the PPS output in the 
Thunderbolt manual, but they say it will provide TTL levels into 50 ohms.
Is 220 ohms impedance a standard value for PPS outputs?
What kind of cable is one supposed to use that has 220 ohms impedance?

Considering the ringing in the Thunderbolt PPS output, the output 
impedance is definitely not 50 ohms. If it were 50 ohms, I should read 
10V into an open circuit, but it only puts out 5V into an open, so the 
output impedance must be very low. I posted 3 pictures of the PPS output 
into an open (1 M ohm scope input impedance), 75 ohms (home-made 
termination for the scope) and 50 ohms. There is no visible difference 
in the ringing between 50 and 75 ohms. The cable was about 4 feet of 50 
ohm good quality cable. The scope is a TDS210 (60 MHz bandwidth). I do 
have faster scopes (150 MHz) but non storage, and sweeping a 1 PPS 
signal at 50nS/div on a non-storage scope does not make good pictures.

The pictures are at http://www.ko4bb.com/Test_Equipment/Thunderbolt

Sorry they have not been cropped.

Then, I switched to one of the "good" 50 feet roll of 75 ohms cable I 
bought on eBay and made more pictures.
The ringing without termination looks very pretty :-)

In my experience when using long cables with fast signals, it is 
important to have at least one end properly terminated to eliminate 
reflections and ringing. As long as one end is properly terminated, 
reflections will not bounce back and forth. Matching both ends optimizes 
power transfer (and frequency response) which, for timing signals, is 
not the most critical.

Didier