Author Topic: 50/75 Ohm Measurement Offset  (Read 2081 times)

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Offline metrologistTopic starter

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50/75 Ohm Measurement Offset
« on: May 22, 2024, 11:26:10 pm »
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?
 

Offline Wallace Gasiewicz

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Re: 50/75 Ohm Measurement Offset
« Reply #1 on: May 23, 2024, 01:59:15 pm »
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
 

Offline metrologistTopic starter

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Re: 50/75 Ohm Measurement Offset
« Reply #2 on: May 23, 2024, 04:25:15 pm »
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.
« Last Edit: May 23, 2024, 04:27:08 pm by metrologist »
 

Offline ConKbot

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Re: 50/75 Ohm Measurement Offset
« Reply #3 on: May 29, 2024, 06:32:19 pm »
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.
« Last Edit: May 29, 2024, 06:42:56 pm by ConKbot »
 

Offline metrologistTopic starter

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Re: 50/75 Ohm Measurement Offset
« Reply #4 on: May 30, 2024, 01:44:25 am »
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.
« Last Edit: May 30, 2024, 01:47:28 am by metrologist »
 

Offline cncjerry

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Re: 50/75 Ohm Measurement Offset
« Reply #5 on: June 03, 2024, 05:39:45 am »
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
 

Offline Joel_Dunsmore

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Re: 50/75 Ohm Measurement Offset
« Reply #6 on: June 24, 2024, 07:33:11 pm »
As stated above, it is probably 5.7 dB offset, because most people use a special attenuator pad, called a mismatch loss pad, that presents 75 ohms to the input when loaded with 50 ohms on the output, and vice-versa.  BUT, we also have done the thing where we put a series 25 ohm resistor in line with the 50 ohm instrument to get 75 ohms looking in.  It does mean the instrument looks back into 100 ohms but that is accounted for in the 1.6 ish dB offset. So it depends upon what kind of offset loss pad you use. Or, you can use a proper transformer and not have any offset loss (well except for slight resistive loss in the wire).
   And the power meters just use a connector with no loss in front of them and just compensate with the power loss from mismatch calculated into the power table.  This is the little adapter used to connect a power sensor to the reference cal port of the older power meters which is always 50 ohms.
 
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Offline metrologistTopic starter

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Re: 50/75 Ohm Measurement Offset
« Reply #7 on: June 25, 2024, 02:42:23 am »
Thanks, Joel  :-+

I'm seeing it as a minimum loss pad as shown here: http://ham-radio.com/k6sti/match.htm
 


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