But why we can add or minus gain and IP3 which are in different unit(db
and dbm)?
Anyone knows?
Thank you

rgds
Jason

"Jason" wrote
But why we can add or minus gain and IP3
which are in different unit (db and dbm)?
_______________

The algebraic summation of decibel values is a mathematically legitimate,
and convenient way to determine system performance. Decibels are based on
logarithms. Adding/subtracting logs or (decibels) is easier than
manipulating the real values they represent. The final dB value in an
analysis can be converted back to whatever units are desired.

For example, below is an analysis of a UHF radio link system over a
free-space path. The 5 watt power of the transmitter is first converted to
dBm so it can be used with other dB values present to analyze the system.
The same result is reached when multiplying tx power in watts by system
gains and losses expressed as decimal values, but that process is more
awkward -- at least when using a pencil & paper or a pocket calculator
(computers don't care).

TX PWR OUTPUT 36.99 dBm
TX ANT 19.20 dBi
RX ANT 19.20 dBi
TOTAL GAINS 75.39 dB

DISTANCE 18.00 Miles
FREQ 950.00 MHz
PATH LOSS 121.26 dB
LINE LOSS TX 1.80 dB
LINE LOSS RX 3.00 dB
CONN LOSS 1.00 dB
OTHER 0.00 dB
TOTAL LOSSES 127.06 dB

RX SIGNAL -51.67 dBm (584 uV)
RX SIGNAL REQ'D -90.00 dBm
RAW FADE MARGIN 38.33 dB

RF

Visit http://rfry.org for FM transmission system papers.

In message .com,
Jason writes
But why we can add or minus gain and IP3 which are in different unit(db
and dbm)?
Anyone knows?
Thank you

rgds
Jason


Think of it this way:
dBm indicates an absolute value. db indicates a relative value.
For example:
0dBm = 1mW
0dBm + 3dB = 1mW x 2 = 2mW = 3dBm
0dBm + 10dB = 1mW x 10 = 10mW = 10dBm
3dBm + 10dB = 2mW x 10 = 20mW = 13dBm
20dBm - 30dB = 100mW/1000 = 0.1mW = -10dBm

What you can't do is to add dBm values directly.
If you have power combiner, and add 10dBm and 13dBm, you can't add 10dBm
and 13dBm and get 23dBm. 23dBm would be 200mW (because 20dB is x 100,
3dB is x 2, so 100 x 2 =200), and this is incorrect.

What you have to do is to convert the dBm values into mW, then add the
mW.
10dBm = 10mW
13dBm = 20mW
Total power = 30mW (and not 200mW)
30mW can then be converted back into dBm (= appx 14.5dBm)

Do you see the pattern?
Ian.
--

On 4 Feb 2005 21:04:19 -0800, "Jason" wrote:

Dear Bob,

I understand what you mean. But in the RF lecture, gain is given in db
unit, while IP3 is in dbm unit, then in order to find overall IP3 for a
cascaded system, the gain and IP3 which in different db unit are add or
deduct from one another without effort to change the unit. WHy is it
so?
Anyone can help?
By the way, how can I get notification from this newsgroup whenever
there is new contribution? No email notify me on this. How to do the
correct setting?


Assume that your system has IP3 = X dBm measured at point Y in the
system.

Assume further that there are amplifier stages A and B following point
Y, and that those amplifiers contribute a gain of A dB and B dB.

At the output of B the power level, in dBm, will be X + A + B.

In this way you avoid the confusion by clearly stating that the power
level at the output is X + A + B. In that context, the values can be
added, because you have made it clear that it is a power level in dBm
- not a gain in dB.

Regarding the auto-notification. I do not know of any such system.
Perhaps that is an opportunity for someone to develop a valuable
product.


Bob, W9DMK, Dahlgren, VA
Replace "nobody" with my callsign for e-mail
http://www.qsl.net/w9dmk
http://zaffora/f2o.org/W9DMK/W9dmk.html

The higher the IP3, the higher is the implied 1dB Compression
Point and therefore the higher up the straight line before
curvature starts and therefore the bigger signal handling
capability before in wanted products come along/

"Reg Edwards" wrote in message
...
"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?

I don't know the answer to that, and you are as capable of
looking it up as I am.

If an amplifier, then perhaps you'd run into clipping problems
or rail problems before non-linearity?

"Reg Edwards" wrote in message
...
"Airy R.Bean" wrote -
The better a mixer is, the higher is IP3 for the outputs of the 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.

If the poster is "Airy" or "Reg" then it is likely to be
a genuine question and answer session, or an attempt
to promote discussion.

If the poster is "Brian Reay", "Spike" or "Frank Turner-Smith", then
it will be a rather silly and childish attempt at baiting.

"Old Ed" wrote in message
k.net...
It looks like I saw your post before Airy, so I'll respond.
(Note: I sometimes have difficulty telling when some of the posters here
are really looking for answers, and when they are just trying to bait each
other

You could start off with a signal measured in dBm, perhaps
the output of a transmitter.

Thereafter you could add and subtract anything in dB (but NOT dBm).

Here's an example (my figures are made-up and not intended to
be realistic). let us calculate the E-M-E signal strength of our
transmission bounced off the moon.

TX output = 1000W = 60dBm.

Antenna gain (Assume a big dish) = 50dB

Effected Radiated Power (ERP) = 110dBm

Path loss to Moon = 80dB

Path loss due to bouncing off Green Cheese = 30dB

Path loss back from Moon = 80 dB again

Total path loss = 190dB

Antenna Gain = 50dB

Received Signal Strength = 110 -190 + 50 = -30dBm = 1 uWatt.

So, we started off with dBm, then added or subtracted dB (which
gave us dBm again, but we only added or subtracted dB)

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

So from what you have all explained, can I bold enough to say that I
can add any value in db and any value in dbm together without
converting to one db or dbm unit because they are in the ratio form and
having virtually the same 10log (P1/P2) formula and nothing else more??

It's just a continuation of humorous electrical loose-talk.
For example. no change to anything, such as moving
all the gear around in the shack, is worthwhile
unless it produces a "3dB Improvement"

"Richard Clark" wrote in message
...
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.

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.

Why, Thank-you!

In the case of amplifiers, presumably we are talking
about the effects of unwanted strong signals driving the
amplifier into its non-linear region (and therefore acting
as a mixer)?

"Wes Stewart" wrote in message
...
On Fri, 4 Feb 2005 16:31:00 -0000, "Airy R.Bean"
wrote:
From off the top of my head, without any revision.....


Pretty good explanation for a mixer, however, IP3 relates equally well
to amplifiers.

In receivers, IP3 is used as a figure of merit and describes how a
receiver will handle weak signals in the presence of other stronger
signals. It is as explained earlier, a theoretical value.