Hello All

I am beginner in RF
I may ask about some silly question and please pardon me
May I know what actually the unit of dbm and db is different from one
another?
If they are different how can we minus the gain in unit of db from a
IP3 in unit of dbm?
Kindly enlighthen
Thank you all


rgds and thanks
Jason

db is a generic term basically meaning: Decibel. A unit for measuring the
relative strength of a signal. Usually expressed as the logarithmic ratio of
the strength of a transmitted signal to the strength of the original signal.
A decibel is one tenth of a "bel".

the key is that it is a ratio... without a reference a ratio is essentially
meaningless. that is why you often hear that station a is 10db louder than
station b... saying that station a was 10db wouldn't mean anything.

dbm has a built in reference. by definition: Decibels referred to 1
milliwatt.
so the reference is 1 mw. do you can say that the power of a source is 0dbm
meaning that it is 1mw, 10dbm is 10mw, 20dbm is 100mw, -10dbm is .1mw, -20db
is .01mw, etc.

i don't know an ip3, but maybe that will give you a start to know what you
are looking for.



"jason" wrote in message
ups.com...
Hello All

I am beginner in RF
I may ask about some silly question and please pardon me
May I know what actually the unit of dbm and db is different from one
another?
If they are different how can we minus the gain in unit of db from a
IP3 in unit of dbm?
Kindly enlighthen
Thank you all


rgds and thanks
Jason

jason wrote:
I may ask about some silly question and please pardon me
May I know what actually the unit of dbm and db is different from one
another?

dBm is referenced to a milliwatt. dB is referenced to
something else and that something else must be specified.
dBd is referenced to a dipole. A web search for "decibel"
uncovered 100 dB of information.
--
73, Cecil http://www.qsl.net/w5dxp


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dB is a ratio for example dB = 10 log10 (P1/P2)
so increasing power from 100 watts to 200 watts is 3 dB (3.01dB actually)
But so is increasing power from 10 watts to 20 watts = 3dB
Since it is a ratio -- it is unitless

dBm is referenced to one milliwatt thus is a discrete power level

In the equation above P2 is always one milliwatt

Thus increasing power from 1 milliwatt to 2 milliwatts is 3dBm
From 1 milliwatt to 10 milliwatts is 10dBm etc

Not sure of your reference to ip3 but suspect you mean input third order
intercept point as used in amplifiers
Google ip3 to get explanations of this and how it is measured.
--
Caveat Lector (Reader Beware)


"jason" wrote in message
ups.com...
Hello All

I am beginner in RF
I may ask about some silly question and please pardon me
May I know what actually the unit of dbm and db is different from one
another?
If they are different how can we minus the gain in unit of db from a
IP3 in unit of dbm?
Kindly enlighthen
Thank you all


rgds and thanks
Jason

From off the top of my head, without any revision.....

IP3, or "Third Order Intercept Point" is an indication
of how good a mixer is, but it is not a physical point!

If you were to plot the wanted output of a mixer stage against
the input signal (ignoring the local oscillator input), you would
get a graph that is a nearly-straight line from the origin
which then starts to flatten off.

At the point of the line where it starts to curve over to
flatness, and therefore starts to be non-linear, other
mixer products, mainly those based upon the third
harmonic of the input signals start to appear in the
output. if you plot these other products on your graph in
addition to the wanted output signal, they grow at a rate (the slope)
which is 3 times greater than was the initial straight line
of the wanted output.

If you take the original straight line of the wanted output, and
extrapolate it so that it meets the other line growing at 3 times
the slope, you get what is known as the "Third Order Intercept
Point". The reason that this is a theoretical point is because the
wanted output has long since flattened off!

The better a mixer is, the higher is IP3 for the outputs of the mixer.

IP3 will be given in terms of the power of the wanted output signal,
say, 50 dBm - other respondents have informed you that this is
50dB (or 10^5) times greater than 1mW, or 100W (Perhaps not
a good figure for an example - a mixer with an output of those
levels could be a PA stage!). In this case dBm gives us the power relative
to the mW.

If we now go back to the flattening off of the curve, at some point,
the curve will be 1dB less than what it would have been had the curve
not been a curve but had carried on as a straight line. This point is known
as the "1dB Compression Point" - In this case we use dB and not dBm because
we are talking relative to some other point on the line.

There is a mathematical derivation (which I don't know off-hand) which
shows that the 1dB Compression Point is 10.4dB below IP3.

So, I hope that I have gone some way to explaining (or increasing your
confusion) on the points that you raised!


"jason" wrote in message
ups.com...
May I know what actually the unit of dbm and db is different from one
another?
If they are different how can we minus the gain in unit of db from a
IP3 in unit of dbm?
Kindly enlighthen
Thank you all

Cecil is CORRECT, also, other terms you might run into a

dB I = dB Isotropic (an immaginary point source that radiates
in all directions uniformly) , used for antennas.

dB W = dB, gain or loss , referenced to 1 Watt.

dB is simply the logrithmic ratio of (POWER, Voltage) to
the reference (-1 dB = a loss of 1/10 of your power, -3 dB ,
loss of 1/2 your power, and if positive, the opposite is true
1 dB = 10% power gain, 3 dB = twice the power, ect.
as info, Jim NN7K

Cecil Moore wrote:
jason wrote:

I may ask about some silly question and please pardon me
May I know what actually the unit of dbm and db is different from one
another?


dBm is referenced to a milliwatt. dB is referenced to
something else and that something else must be specified.
dBd is referenced to a dipole. A web search for "decibel"
uncovered 100 dB of information.

"Airy R.Bean" wrote -

The better a mixer is, the higher is IP3 for the outputs of the mixer.

==========================

From a circuit operational point of view, could you please summarise in what
way a high IP3 makes a better mixer?

Am I correct in assuming the device need not be a mixer? Could it be an
amplifier? In which case some of the better or worse parameters would just
become meaningless.
----
Reg.

"Caveat Lector" wrote
dB is a ratio for example dB = 10 log10 (P1/P2)
....snip...
In the equation above P2 is always one milliwatt
___________________

To elaborate, P2 above must be 1 mW only if one wants to calculate dBm.

The equation as it is written above will calculate the relationship in
decibels between any two power values expressed in the same units. For
example, to find the gain of an amplifier in decibels when its input power
is 50 watts and its output power is 1.2 kW:

dB = 10*log(1200/50) = 10*log(24) = 10*1.38 = 13.8

RF

On Fri, 04 Feb 2005 17:19:29 GMT, Jim - NN7K
wrote:
Cecil is CORRECT
A web search for "decibel" uncovered 100 dB of information.

Hi Jim,

I must've missed something here with this dimensionless, referenceless
declaration. I'm not even sure if there isn't an implicit negative
sign to it.

Would this be 100dB above ignorance? 100dB below genius? What is the
zero reference for information? Valid information, and simply
available information is not exchanged at the same rate (100dB above
entropy?). It can't be zero information because that would cause the
computation to be "undefined." Hmmm, maybe the baseline is the
computation of no information. -No- That returns us to the conundrum
of even what is 1dB above "undefined?" SWAG? In that regard:
A web search for "decibel" uncovered 100 dB of information re SWAG

Can there be a ratio of 1 SWAG : 10¹º Facts? When I enter the
universal measure of knowledge into Google, it only returns
"19,400,000 for TITS." This is roughly 500 times below the
declaration made above. Does it follow you must perform 500 SWAGs to
successfully achieve your goal of TITS? This sounds almost about the
same as the computation of probability for a teenage boy on a date
running out of gas on a dark road - and yet historically this occurs
far more often than chance.

73's
Richard Clark, KB7QHC

Thanks for the clarification

I meant to say "For dbm In the equation above P2 is always one milliwatt"

Thanks

--
Caveat Lector (Reader Beware)



"Richard Fry" wrote in message
...
"Caveat Lector" wrote
dB is a ratio for example dB = 10 log10 (P1/P2)
...snip...
In the equation above P2 is always one milliwatt
___________________

To elaborate, P2 above must be 1 mW only if one wants to calculate dBm.

The equation as it is written above will calculate the relationship in
decibels between any two power values expressed in the same units. For
example, to find the gain of an amplifier in decibels when its input power
is 50 watts and its output power is 1.2 kW:

dB = 10*log(1200/50) = 10*log(24) = 10*1.38 = 13.8

RF