Confusing wording?

[amdx]

On 2/8/2020 3:15 PM, Cursitor Doom wrote:

On Sat, 08 Feb 2020 19:17:55 +0200, Mikko OH2HVJ
mikko.syrjalahti@nospam.fi> wrote:

Nope, it works exactly like that. 1dB is relation (multiply) and 1dBm is
absolute power.

Agreed.

If you increase power from 9dBm by 1dB, that equals 10^(0.1)=1.25
multiplication, i.e. 9dBm + 1dB = 8mW * 1.25 = 10mW = 10dBm.

Agreed. But that's just the "cumbersome method" I used myself earlier
up the thread.

But if you combine 9dBm and 1dBm, you get 8mW+1mW=9mW=9.6dBm.

Which differs from your own answer further up the thread! And I have
no idea what you mean by "combine" either.

After using this calculator to convert dbm to watts
https://www.rapidtables.com/convert/power/dBm_to_Watt.html

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.

> http://www.satsig.net/lnb/db-calculator.htm
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

 

[amdx]

On 2/9/2020 3:07 PM, Michael Terrell wrote:

On Sunday, February 9, 2020 at 1:35:32 PM UTC-5, mpm wrote:
On Sunday, February 9, 2020 at 11:45:47 AM UTC-5, Phil Hobbs wrote:

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


For us RF guys, there's always dBu.

And dBmv for CATV and other TV RF sources. Also the Reference Levels for Cable Modems.

0dBmv = 1mV across a 75 Ohm load.

Here we go!

Mikek

 

[Cursitor Doom]

On Sun, 9 Feb 2020 14:23:39 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-02-09 13:02, Cursitor Doom wrote:
On Sun, 9 Feb 2020 11:21:08 -0600, John S <Sophi.2@invalid.org> wrote:

On 2/9/2020 9:00 AM, mpm wrote:
On Friday, February 7, 2020 at 12:02:37 AM UTC-5, Jasen Betts wrote:

I'm guessing they mean difference between consecutive readings, but
they may mean difference between any two readings (ie between max and
min)

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.


It does not mention +/- 1dB.

"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.


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

Wow! The number of interpretations we've had over this! :-D

 

[Steve Wilson]

On Sunday, February 9, 2020 at 2:10:10 PM UTC-5, Jeroen Belleman wrote:

On 2020-02-09 19:30, Martin James Smith wrote:
On Sun, 9 Feb 2020 03:09:57 -0800 (PST), pcdhobbs@gmail.com wrote:

Unfortunately, however, whenever you see something like "10dBm
plus or minus 1dB" there's no alternative to translating the figures
back to absolute quantities, performing the multiplication (x1.26 in
the case of 1dB) then translating the answer back into dB again.

Why? It's not as though Âą1 dB is a precision instrument. 'Plus or minus 25%' is close enough.

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.

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

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.


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

Agreed. Here's a paragraph from the HP 654A Test Oscillator Manual, Section 1, General Information, Page 7:

The Model 654A is a stable, low distortion sine-wave
signal source with a flat frequency response of +/-0.5% over
the frequency range of 10Hz to 10 MHz. The attenuators
allow the signal to be adjusted in 1 dB and 10 dB steps
from +10 dBm to -89 dBm, and the front panel
AMPLITUDE control allows a continuous adjustment in
level of +/-1 dB from the settings shown on the OUTPUT
LEVEL attenuators. The flat frequency response is
achieved by automatic leveling circuits within the 654A.

https://data2.manualslib.com/pdf7/146/14503/1450284-hp/654a.pdf?14fc1202d71b9cfcd5d9b10508b124c0&take=binary

https://tinyurl.com/rhpqxye

 

[Jasen Betts]

On 2020-02-09, Mikko OH2HVJ <mikko.syrjalahti@nospam.fi> wrote:

Cursitor Doom <curd@notformail.com> writes:

On Sat, 08 Feb 2020 19:17:55 +0200, Mikko OH2HVJ
But if you combine 9dBm and 1dBm, you get 8mW+1mW=9mW=9.6dBm.

Which differs from your own answer further up the thread! And I have
no idea what you mean by "combine" either.

Nope. There I wrote +1dB, not +1dBm.

+1dB corresponds to relative increase of roughly +25%.
+1dBm correspods to absolute increase of 1mW.

no, an increase of 1mw would be +(0dBm)

--
Jasen.

 

[Mikko OH2HVJ]

Jasen Betts <jasen@xnet.co.nz> writes:

On 2020-02-09, Mikko OH2HVJ <mikko.syrjalahti@nospam.fi> wrote:
+1dB corresponds to relative increase of roughly +25%.
+1dBm correspods to absolute increase of 1mW.

no, an increase of 1mw would be +(0dBm)

My bad, of course it's 0dBm.. (I was wondering why somebody thought
9dBm+1dB=9.6dBm, but that's it!)

--
mikko