50/75 Ohm Measurement Offset

[metrologist]

Hi All,

If my measuring instrument is designed for a 50 ohm system and I measure a 75 ohm device, what is the correct offset to apply? For example, return loss?

[Wallace Gasiewicz]

VSWR into a 75 Ohm load from a 50 Ohm device will yield a 1.5 VSWR   
This is used as a quick check by some folks to see if everything is working OK.   

1.5 VSWR  is 13.979 dB Return Loss which means 96% of power goes into the load

[metrologist]

OK, thanks. I was wondering why my instrument manual says a 7.5 dB offset is applied when selecting 75 Ohm impedance.

And an online converter says 1.5 VSWR is 0.18 mismatch loss.

[ConKbot]

Was it a 7.5 db offset or a 5.7 db offset? If the latter, then it's expecting Min loss pads to be installed. https://www.keysight.com/us/en/product/11852B/adapter-50-ohm-type-n-f-75-ohm-type-n-m.html
 
With a good pad, it presents a good match to both ports (see the 27db return loss on that page)
For stuff like an amp, as noted by 1.5:1 mismatch you'll probably be fine without, as if your amp oscillates with 1.5:1 on the input or output, heaven help you in real life. But if evaluating filters, having the right load on the filter can make all the difference.

[metrologist]

Thank you for the continued interest.

I learned that the 7.5 dB offset is applied to spectrum analyzer applications for amplitude measurements when the mfg's impedance transformer is used. I think that means that the reference level is offset 7.5 dB due to the mfg specific loss specifications for their impedance transformer device.

There is an "Other" or a simple Reference Level Offset option to achieve the same result.

It just seemed odd to me that the Impedance menu would use some specific offset value without stating what it was.

[cncjerry]

On a 3586 selective level meter when you switch between 50 and 75 ohm, there is an offset of 1.76db applied.  They mention it in the 3586 manual.  I think the 3586 can add or subtract it automatically.   Power is 10 log 75 divided by 50.  S0 1.76dB would be .176 and 10^.176 is ~ 1.5.  I might have the units upside down.

Jerry