Accurancy calculation and dB vs dBm  

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AlbertoM
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10/05/2012 10:49 am  

I need to chose a type of spectran HF in relation with the accurancy of measuring.
You declare +- 2 dB for 6060 or +- 1dB for 60100.

I'm thinking to measure UMTS cell signal in download.
My experience tell me that is rare to find 0 or 1 dB into a common civil house. ( 2 V/m is strong value )

The difference of 1-2 dB is not important near -30 -40 dB but is to large near 0 dB.

What is the conditions in use measure with 6060 ( for example) and declare +- 2 dB ?


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Moderator
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10/05/2012 11:38 am  

Sorry, we have no idea what you are writing about. Can you please explain more in detail?

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AlbertoM
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10/05/2012 12:42 pm  

The Accuracy is the degree of closeness of measurements of a quantity to that quantity's actual (true) value ( i think you know very well) (attached).

You write +2 dB , -2 dB for HF 6060.
Without clarifications I can think :
Case 1:
Generation of signal reference with 2000 MHz (UMTS) with -30 dBm. The Spectran HF-6060 display a value from -28 to -32 dBm.
Case 2:
Generation of reference signal with 2000 MHz (UMTS) with -0 dBm. The Spectran HF-6060 display a value from -2 to +2 dBm.

I need to use V/m and not logarithmic values.
In the case 2 the error in V/m is bigger then case 1.

My question is :What is the dBm of reference signal to define accurancy +- 2dBm ?


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10/05/2012 1:25 pm  

There is no "level of dBm". dBm is dBm so the accuracy in dB is valid at any dBm power level.

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AlbertoM
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10/05/2012 2:04 pm  

Good.

Using HF-6060 (+-2 dBm) and Hyperlog antenna to measure UMTS the exposure limit in V/m, you tell me that happen this:
I reed into the display 0 dBm and I must declare to my customer the value of V/m from 2.7 V/m to 11,5 V/m approximately . ( this value are in relation with pag 60 of manual column "UMTS" ).


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10/05/2012 3:13 pm  

No, your graph is totaly wrong. There isn't such a step between 1dBm and 0dBm existing.

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AlbertoM
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10/05/2012 6:21 pm  

Sorry . I'm wrong.
Check this second calculation, please .
I suppose a gain 4.9 dBi , and 2100 MHz. We can see around 1 V/m that the difference +- 1 dB have not a big importance.

Is correct ?


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Moderator
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10/05/2012 6:47 pm  

Now you got it :D

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AlbertoM
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11/05/2012 10:38 am  

OK.
I increased the details of my excel ( if you want I will attach it).
I find, with your help, what I asked. I'm not sure can you confirm ?
I calculate near the common frequencies of downlink GSM UMTS the % of difference in V/m consider +- 1dBm ( 60100 ) . I find 12%.
We know that into the house the exposure value can be near 1 V/m in particular condition ( BTS very close ). Suppose the real value 1 V/m my accurancy is from 1.12 to 0.88.

Confirm ? :?:


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11/05/2012 11:41 am  

Looks OK for me but the accuracy is not +/-1dBm it is +/- 1dB, thats a big difference!

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AlbertoM
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11/05/2012 12:40 pm  

I can understand when I'm speaking about accuracy in relation of physical values ( dBm ... logarithmic ratio of mW ) .
I don't understand why you declare accuracy of instrument in dB : Logarithmic ratio of what ?

From Wiki :

"...The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level ... "

"...The decibel symbol is often qualified with a suffix, that indicates which reference quantity or frequency weighting function has been used. For example, dBm indicates a reference level of one milliwatt ... "


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11/05/2012 1:16 pm  

Now you just wrote the answer :D

But don't worry, even a lot of pro-users and even "big" companys are mixing dB and dBm from time to time and thy even write it in manuals or product informations (shame on them).

But if you still don't understand it, please just google a little bit more e.g. at http://www.radartutorial.eu/18.explanat ... 15.en.html

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