Copyright: 1996 - 2024
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
while tying up your telephone line, and a nice lady's voice announced "You've Got
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Intermodulation products - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
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August of 2003 and was quite well-attended for many years. By 2010, Facebook and
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Forums activity dropped off precipitously. If the folks at
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but until then it is probably not worth the effort. Regardless, there are still
lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
|-- Amateur Radio
Gripes & Humor
-- CAE, CAD, &
Test & Measurement
Post subject: Intermodulation products Posted: Sat Nov 29, 2008
Joined: Tue Aug 26, 2008 5:18 am
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 10:28 am
Joined: Mon Jun 27, 2005 2:02 pm
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?
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
Post subject: Posted:
Wed Dec 03, 2008 3:43 am
Joined: Fri Feb
17, 2006 12:07 pm
Location: London UK
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 Transformer.
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
Joined: Tue Aug 26, 2008 5:18 am
Device is FP1189 and I am checking IP3 with two tones at 10dBm below
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
Usual IM3, Usual IM5, ( A + A + B) or 2A - B
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
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 or Class-AB.
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
I´ll start 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