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Coax connector 50ohm v. 75ohm C band

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Ex Member

May 30th, 2010 at 5:24am  
Please help..

 what would be the effect on RF measurements when using a cable assembly with 50ohm connector and a 75ohm connector on the other end using a 75ohm coax cable.
 Further to this, can I just use a 50ohm cable assembly for all RF measurements?
note: RF is C-band..

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« Last Edit: May 30th, 2010 at 2:04pm by Admin1 »  
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Eric Johnston
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Reply #1 - May 30th, 2010 at 11:27am  
Ideally you need the same source and load impedences as well as connectors and cable type.

If you have a mixture, the effect is partially reflected signals which results in amplitude ripple due to partial signal cancellations/additions across the frequency band.  This will add errors to measurement accuracy - but it depends what you are trying to measure or observe - if you are simply looking to see a particular small carrier on a spectrum analyser then maybe the ripple errors and absolute levels don't matter.  The partial reflections also cause mulitpath delay effects, like shadow images on analogue TV and inter-symbol interference in digital systems.  Similarly don't crush coax cables at regular intervals (e.g. using wall clips, clamps or cable ties).

If you are connecting a C band BUC with a coax output via coax to the feed OMT you should must use 50 ohm Andrew Heliax cable (or similar) and Andrew 50 ohm N type connectors at both ends.

Be aware that 50 ohm N type and 75 ohm N type plugs exist.  Never put a 75 ohm N type plug to a 50 ohm N type socket - the centre pin won't connect properly as the 75 ohm centre pin is very slightly smaller diameter.

I agree with the desirability of using 50 ohm for all C band RF measurements.

Best regards, Eric.
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« Last Edit: May 31st, 2010 at 6:39pm by Admin1 »  
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Oasis Networks
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Reply #2 - May 31st, 2010 at 2:04pm  
Just another small note -

Impedence mismatch might lead (it depends on the signal frequency and cable length) to a specific phenomenom of standing waves, that might lead after several years to cable break. This is as a result of hitting at certain points along the cable that after some time will lead to cable break. I think I still have this article somewhere if more detailed explanation is required.
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Ex Member

Reply #3 - Jun 2nd, 2010 at 5:12am  
Thanks Eric..

Hi Oasis,
  will appreciate if you could send me the link of the article you mentioned.

Thnkx and regards
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Oasis Networks
Ex Member

Reply #4 - Jun 2nd, 2010 at 8:48am  

I have it printed and I can scan and send by email.

Please contact me privatly and provide me with you email address and I will mail it to you.

Kind Regards,
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