In article
,
Telamon wrote:

In article ,
"Robert11" wrote:

Hello,

Saw the term "IP3" used in discussing sw radios.

Guess I'll never learn if I don't take the risk of showing my
ignorance, so: what does the abbreviation IP3 stand for, please ?

Also, any info. or rumors re a new JRC 545 type (555 ?) radio coming
out next year ?


IP3 - third order intercept point. That does not mean much to you does
it.

It is a measurement of intermodulation products of two signals. That
probably does not mean much to you either.

Generally it is a measurement of an amplifiers ability to amplify
signals without generating other mixing products. If an amplifier
produces these other mixing products it steals the power from the
signals you are putting at its input limiting the amplification it could
produce on those input signals and so it turns out that IP3 directly
impacts the -1 dB compression point of an amplifier.

The -1 dB compression point is a point where the output of an amplifier
fails to track the input by 1 dB or in other words the gain rolls off 1
dB at some point from what the gain of the amplifier is otherwise.

An intermodulation product is the result of two signals (a mixing
product) that you might be familiar with such as the sum and difference
of two signals. If you took the direct sum or difference then you would
be talking about IP2. This measurement is basically a measure of the
difference of the sum or difference signal (whichever is larger)
compared to the original two signals. A perfect radio circuit would not
produce any other signal mixing products (other than a mixer because the
object of a mixer is to produce the sum and difference signals) so when
it come to IP2 a larger number is better since it is a measure of the
original signal levels (usually the same level for both generators)
compared to the sum and difference signal generated by the amplifier or
whatever circuit the two signals are passing through.

IP3 is the same measurement as IP2 except it is the second harmonic one
one input generator mixed with the sum or difference of the second
generator frequency. Those mixing harmonics levels produced are once
again compared to the original signal levels of the two generators.

To make the measure simple you can set both generators to 0 dB and then
make a measurement of the appropriate mixing products for either IP2 or
IP3. Lets say the IP2 was -66 on the difference and -68 on the plus. The
IP2 would then be 66 dB, which is the worst of the two. Lets say 2 times
generator 1 frequency plus the generator 2 frequency product had the
highest level of -75 dB of the IP3 possibilities. The the IP3 would be
75 dB.

Generally you don't care about IP2 and IP3. This specification only
matters when the radio has to deal with very strong signal levels. Best
example of this in the USA would be local AMBCB stations reducing the
input sensitivity of a radio on short wave or other AMBCB stations.

The IP3 75 dB I stated in the example was not correct. Here is the
general formula: IP(n)= Pin + (delta P/n-1) where if the input power of
tone 1 is used then delta power is the difference of the tone 2 output
power and the inter-mod product power in dBm. If you keep both input
tones at the same level things are easier to calculate so let's say the
amplifier has unity gain and we set up and measure in dBm:

Input tone power (both) -4.0
Output tone power (both) -3.8
(2*F1) - F2 power was -59.7
then the IP3 would be +24

--
Telamon
Ventura, California