IP3
 

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

Richard Clark wrote:

Jim - NN7K wrote:
Cecil is CORRECT
A web search for "decibel" uncovered 100 dB of information.

Would this be 100dB above ignorance? 100dB below genius?

100 dB compared to the answers one gets on a newsgroup. :-)
--
73, Cecil http://www.qsl.net/w5dxp


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On Fri, 4 Feb 2005 16:31:00 -0000, "Airy R.Bean"
wrote:

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!

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 generally agree with your description of IP3; but I would add
a few points.

The IP3 model was first published in a now-classic article
back in the 60s. (I could probably dig up the specific
reference, if someone really wants to know.)

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.

If the subject distortion is plotted against input/output levels,
and approximated by a best-fit straight line, that line will
intersect a similar linear extrapolation of the desired signal
at a point dubbed the "Intercept Point."

The utility of all this is that you can use a single specification--
intercept point--to make quite good predictions of distortion
levels over a wide range of input conditions.

But it is important to remember that IP is only a MODEL,
and an empirical one at that. Real devices will never follow
the model exactly and completely--as you note in your
discussion of the saturation region.

73, Ed, W6LOL

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

Hi Reg -

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. I am going to start by assuming that you are in the former
category.)

More below...


"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?

A higher IP3 simply means that the hypothetical mixer can handle
bigger signals before it produces a given level of 3rd order IM
distortion products.

Whether or not that makes it a "better" mixer would depend on
many other specifications, such as noise figure, loss/gain, bandwidth,
and even size and power consumption.

Am I correct in assuming the device need not be a mixer?
Could it be an amplifier?

If you're asking whether other components such as amplifiers
can have IP3 specifications, the answer is definitely yes.

In which case some of the better or worse parameters would just
become meaningless.

I can't figure out what you're trying to say in this last sentence.
But correct and relevant specifications are never "meaningless."
(At least I can't think of any such pathological examples.)

----
Reg.

Hello to all that Helps
You are all very kind and genius.
It is first time I asked question in newsgroup and I received so many
kind answers.
I am so happy. Thanks a lot

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??
Please verify

Thank you

rgds and thanks
Jason

On 4 Feb 2005 20:02:05 -0800, "jason" wrote:

Hello to all that Helps
You are all very kind and genius.
It is first time I asked question in newsgroup and I received so many
kind answers.
I am so happy. Thanks a lot

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??
Please verify

Dear Jason,

I am sorry to say that I cannot agree with your conclusion.

In my opinion, in the general case, you can NOT add dB and dBm. They
have different applications and were not intended to be added
together.

A quantity expressed in dBm is intended to convey, or imply, a
specific power level in a specific load impedance. On the other hand
the dB is NOT so defined. Therefore, I cannot think of a situation in
which you could simply add them together and have a result that anyone
would be able to iterpret at face value.


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

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?
Thank you

rgds and thanks
Jason

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?
Thank you

rgds and thanks
Jason

For "meaningless" just read "not applicable".

Thanks.

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.
--

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.

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


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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.