Intermodulation Products - RF Cafe Forums
Post subject: Intermodulation products Posted: Sat Nov 29,
2008 12:40 pm
Joined: Tue Aug 26, 2008
As an RF amplifier
designer I have seen 5th order lower than 3rd order products.
My question is if we see 5th order higher than 3rd order products
then what is the cause? Is it normal to see that?
Post subject: Posted: Tue Dec 02, 2008
Joined: Mon Jun 27, 2005
This is all depends
on the technology of the power transistor and the non-linear characteritic
derived from it. This is very unusual behavior. At which back-off
do you see this behavior?
I personally have not encountered
in that, but I have seen in data sheets that IMD5 do not always
increase with input power but can also decrease and then increase
Post subject: Posted:
Wed Dec 03, 2008 3:43 am
Feb 17, 2006 12:07 pm
Location: London UK
years ago I was trying to relate the input>output transfer function
to the harmonic levels for a TWT. I did some routine in MATHCAD
that worked, using the Fourier Transform function they have. It
seemed to work fine, although I never got to measure the ultimate
levels during testing. I think if you can obtain a very accurate
transfer curve, it would be possible to model it with a Fourier
However, there is a complication I foresee:
The load match to harmonics can be so complex that it could grossly
distort the measurements in a real circuit. The Fourier Transform
would assume a perfect 50 ohm match, but in reality, the circuit
you have could fortuitously show a much better S11 at the 5th harmonic
than at the 3rd.
Post subject: Posted:
Wed Dec 03, 2008 8:39 am
Aug 26, 2008 5:18 am
FP1189 and I am checking IP3 with two tones at 10dBm below p1dB.
Post subject: Posted: Mon
Dec 15, 2008 10:01 am
Joined: Thu Oct
19, 2006 6:02 pm
It is important to make the distinction
between apparent IM3, IM5 and real IM3, IM5. In a 2 tone test the
spurs at plus and minus the difference frequency are considered
to be IM3 and the spurs at plus and minus twice the difference frequency
are considered to be IM5.
What we think of as IM3 is a sum
of IM3 and IM5. It usually doesn’t matter because IM5 is usually
smaller than IM3. An example.
Usual IM3, Usual IM5, ( A + A
+ B) or 2A - B
Usual IM5, (A + A + A - B - B) or 3A - 2B
IM5 that mimics IM3, (A + A + A - A + B - B - B) or 2A - B
I know this looks like slight of hand but it is real. Try a
2 tone simulation with a pure first order and a pure fifth order
characteristic. You will get 2A - B. This condition won't happen
in the real world but it tells what is hiding beneath that apparent
A common example is some medium signal Class-A amplifiers
(where IP3 is >> PIdb + 9.5db) as they approach P1db have
IM3 growing faster than 3db/db. This is usually due to the IM5 at
that same spur frequency getting a bigger vote. Depending on the
phasing, IM5 could subtract from IM3. This is unlikely in a broad
band Class-A but is more probable in a narrow band and/or Class-B
Post subject: Re:
Intermodulation productsPosted: Thu Apr 09, 2009 10:40 am
Joined: Wed Apr 08, 2009 7:53 am
Any way it is an uncommon behavior for a mid power Class
A.. I´ve seen this effect on high power LDMOS Class AB amplifiers
with high bias points, but not on med power Calss A..
by doubting on the test bench.. If the SA is not set correctly,
your measurement could be affected by the SA own intermodulation,
and perhaps the DUT IM3 is being canceled by the SA IM3.. like a
predistortion effect, and that is why you see the IM3 lower than
it should on the screen... Check the SA setup.. Or change the cable
length from the DUT to the SA to see if you have a different response.
Or perhaps you have big harmonics on the signal generators.. try
to filter them..
Or perhaps you have low isolation on the tones
combination at the input.. and so you are being affected by the
reverse IP3 of the generators (an other predistortion effect).
With this answer i´m not trying to underestimate no one, this is
just something that happend to me in the past.