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..

tnx

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.

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.

Thanks Eric..

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

Thnkx and regards

Hello,

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,
Nimrod