I think that you are confusing the _RATE_
or _SLOPE_ of each individually with
the differential increase per dB of input signal

"Ian Jackson" wrote in message
...
In message t, Old Ed
writes
The original author observed that many practical devices
(e.g., mixers) exhibit distortion levels that rise as the "power"
of the product in question. For example, third-order distortion
rises 3 times as fast (dB scale) as the desired (linear) signal.
Snip
Ed, where the increasing intermodulation distortion is simply a result
of increasing the level of the signals at the input of the mixer (or
amplifier), third order distortion actually rises TWICE as fast as the
desired signal. Third order distortion DOES rise on a 'three dB per dB'
basis, but the wanted signal also rises - at 1dB per dB. The difference
is 2dB. So the relationship is 2dB per dB.

Let's talk loosely, and talk about money.

If I've got twice as much money than Ian has, then
I've got 3dB more.

How much do I have? Don't know.

If Ian has three times as much as Richard, then he
has 4.7 dB more than Richard, and I have 3 + 4.7 =7.7dB
more than Richard.

How much do I have? Don't know.
How much does Ian have? Don't know.
How much does Richard have? Don't know.

OK, assuming that we could deal in 1/10ths of a cent (1 milli-dollar!)
let's assume that Richard has $100 = 50dBm.

Ian therefore has 50 + 4.7 = 54.7 dBm.
And I have 54.7 + 3 = 57.7 dBm.

The answer to your question is that you can start off with an
actual reading in dBm, but everything else relative to that is
in dB only (although it does give a result in dBm).

If the above doesn't answer your question, then, sorry,
but I give up. (Which doesn't mean that my interest is 0dBm
but -173 dBm, ie, indiscernible below the noise)

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

rgds
Jason

Hello All the Kind and Clever People

I think I got what you all explained for me. I will reread them
carefully before asking more in order to save your precious time.
I am thankful to you all.
Thank you so much for people who wrote above with great efforts

Jason

Thank-you for promoting a genuine discussion in this NG.

"Jason" wrote in message
ups.com...
Hello All the Kind and Clever People
I think I got what you all explained for me. I will reread them
carefully before asking more in order to save your precious time.
I am thankful to you all.
Thank you so much for people who wrote above with great efforts

Airy R.Bean wrote:
Thank-you for promoting a genuine discussion in this NG.

We had a lot of gin-ur-wine discussions in college.
--
73, Cecil http://www.qsl.net/w5dxp


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---

Wow -- well written Ian

--
Caveat Lector (Reader Beware)
Help The New Hams
Someone Helped You
Or did You Forget That ?



"Ian Jackson" wrote in message
...
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.
--

In message MF9Nd.29160$xt.24350@fed1read07, Caveat Lector
writes
Wow -- well written Ian

After over 40 years in Cable TV, I think I am beginning to get the hang
of it!
Ian ; ))
--

Hi Ian -

Thanks for trying to clarify, but I think you misread my post
somehow.

I said "...third-order distortion rises 3 times as fast (dB scale)
as the desired (linear) signal."

You said "Third order distortion DOES rise on a 'three dB
per dB' basis, but the wanted signal also rises - at 1dB per dB."

The content of our statements is the same. But you went on
to address the slope DIFFERENCE, which I did not discuss.

I believe Airy is making the same point I am making here
with his (2/5/05 8:25) post.

73, Ed, W6LOL


"Ian Jackson" wrote in message
...
In message t, Old Ed
writes

SNIP

The original author observed that many practical devices
(e.g., mixers) exhibit distortion levels that rise as the "power"
of the product in question. For example, third-order distortion
rises 3 times as fast (dB scale) as the desired (linear) signal.
Snip

Ed, where the increasing intermodulation distortion is simply a result
of increasing the level of the signals at the input of the mixer (or
amplifier), third order distortion actually rises TWICE as fast as the
desired signal. Third order distortion DOES rise on a 'three dB per dB'
basis, but the wanted signal also rises - at 1dB per dB. The difference
is 2dB. So the relationship is 2dB per dB.

If you continued to increase the signal levels, you might expect that
the level of the intermodulation would eventually catch up with - and
overtake - the level of the wanted signal (it doesn't, of course).

The third order intercept point is simply the hypothetical level where
the level of the intermodulation would have risen so much (at 2dB per
dB) that it equals the level of the wanted signal.

Ian.
--

Hi again -

A couple of added thoughts he

1. The term "mixer" has at least two quite different definitions
in the electronics world. To the RF guy, "mixer" means a nonlinear
and/or time-variant device that is used to create sum and
difference frequencies between an input signal and a local
oscillator. But to the audio guy, "mixer" means a highly linear
device used to add or combine audio signals WITHOUT
producing distortion products.

2. As implied above, an RF mixer does not have to be nonlinear;
it can also be implemented as a linear/time-variant device.
(Think of a highly linear switch being chopped at the LO frequency.)
However, the most common practical mixers are those that can be
modelled as non-linear/time-invariant and those that can be modelled
as non-linear/time-variant.

73, Ed, W6LOL

"Airy R.Bean" wrote in message
...
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.

Hi Airy -

I'm not sure what "answer" you're referring to here. But if you're
referring to the question about IP3 applying to amplifiers, the
answer is a resounding "yes"--as noted in posts above.

As a matter of fact, IP3 is often most useful when applied to an
entire chain of cascaded devices such as mixers and amplifiers.

Regarding your comment about an amplifier possibly "clipping...
before non-linearity," that is impossible by definition--because
clipping IS a non-linearity.

But it is possible to find amplifiers that are extremely linear
below clipping, and which clip very abruptly. Such amplifiers
may show relatively poor conformity to the intercept point model,
which was based on more gradual nonlinearities.

73, Ed, W6LOL


"Airy R.Bean" wrote in message
...
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.