Confusing wording?

us 25%' is close enough.

Cheers

Phil Hobbs (percentages don't have to be translated to absolute units either)

Don't give up your day job

I'm surprised that this generates so much discussion. It's really that simple. A figure in dBm is a power. A figure in dB is a gain. So x dBm + y dB = (x+y) dBm.

Adding dBm to dBm is a different animal altogether. There you have to convert back to watts. So for example 10dBm + 10dBm =

13dBm because 10mW + 10mW = 20mW, but that isn't what the original question was about.

Jeroen Belleman

I don't think you need to add m to your statement. The m is simply the reference. It could be anything such a volt, watt, amp, or any other reference. Disregard it.

Think in terms of recording data as suggested from 100kHz to 3GHz every

25MHz into a spreadsheet. That's about 120 data points. Now find the highest and lowest. What is the difference between the two? The spec says not more than 1dB. It doesn't matter whether it is dBV, dBW, dBI, or any other reference.

My read of this is +/- 1.0 dB across the entire range and not from any two adjacent measurements in 25 MHz steps.

I get here by realizing there are around 120 such measurement steps in the specified range. If 1.0 dB variance were allowed with each step, that spec quickly becomes meaningless.

But either way, poorly worded spec.

I'm not all that educated about all the dbxx things, but, as I see it you can have a relative difference in db, but if you are measuring the level (which he is) then the measurement is with reference to one of those references listed. Mikek

For us old-timey analogue voice/datacom types, there's also dBA, dBC, dBm0, and dBrnC0 ("dibrinco"). ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

It does not mention +/- 1dB.

How do you know what the meter's reference is? It is measuring *some* level. The meter may be calibrated such that it reads volts across a known resistance and reads dBm printed on the face. Should it be reading volts and refer to a resistor reference? Forget the dBm until the end of the final analysis is done.

Er. You could go back to basic definitions... P = 10^(PdB / 10), (i.e antilog10(PdB/10) )

10dBM + 1dB = 11dBm. so 11dBm = 1mW * antilog10(1.1) = 1mw * 1.258 9dBm is left as an exercise for the student - or just use an online calculator!

"The difference between the readings should not be greater than 1.0dB."

Which I take to mean the same thing. But the wording's not entirely unambiguous. It would have been better to say, "the difference in readings should be within 1.0dB [either way]" which more clearly implies +/- 1dB.

Yes, but the point I was trying to make is that in the case of the original question, you can't just add 1dB to 9dBm as some here were trying to suggest. You have to convert these figures into what they really mean then convert the answer back into dBm again. There's no shortcut - except for the case of 1, 3, 4, 6dB and whatnot,, the commonly encountered values that we just know off the top of our heads after years and years of dealing with this stuff. If the question had been ' what's xdBm +/- 17dB, even some big shot like you would have to convert back to absolute values and work it out the long way.

And dBM for movies. For example, I was watching some biopic on Whitney Houston last night and IMO it was 3dBM longer than it needed to be. :)

--



- Winston Spencer Churchill

For us RF guys, there's always dBu. :)

But you can! That's precisely why deciBells are so useful!

Not at all. xdBm +/- 17dB = (x +/- 17) dBm. Work it out the long way, if you have to, but you'll find it's spot-on.

Jeroen Belleman

I'd be inclined to read it as 1 dB p-p, i.e. +-0.5 dB.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

He said so in a follow up post.

It is measuring *some*

In his original post the OP said he set the original value to 9db. Then came back and corrected that to 9dbm, which is one scale on 4 of my meters. When I saw his correction, I also corrected my db to dbm, in my followup post. So yes he set the original value to 9dm as in, reference to a milliwatt, as far as I know setting a level to 9db, without reference to something, is meaningless and useless. But I would need a 3rd or 4th opinion, as we have each side of the argument and neither of us will be satisfied with input from the other. Phil chimed in with some old timey references, but didn't add that I was right, but he didn't say I was wrong either. I'm waiting for others to confirm or explain. Mikek

After using this calculator to convert dbm to watts

Convert 8 dbm and 9 dbm to watts, then do a division with those to answers, take that answer and put it into the next calculator.

Lower left side, dB calculator Conversion Ratio to dB

The end result I get,is 0.998 db.(truncated input numbers) This holds whether it is the ratio of 1dbm to 2 dbm or 50 to 51 dbm.

Try it and then you can convince yourself that it is as easy to make the conversion as it seems.

Mikek

I did use the calculators in my 2:48 post, and list them for any one else to use to prove what Tom Gardner and a few others say is correct.

Repeated,

After using this calculator to convert dbm to watts >

Convert 8 dbm and 9 dbm to watts, then do a division with those to answers, take that answer and put it into the next calculator.

The end result I get,is 0.998 db.(truncated input numbers) This holds whether it is the ratio of 1dbm to 2 dbm or 50 to 51 dbm.

Try it and then you can convince yourself that it is as easy to make the conversion as it seems.

Mikek