Querstions on IP3, and also Re JRC 545 New Radio Rumors ?
In article ,
"Pete KE9OA" wrote:
Very true on all of your points, but the initial question was about
the IP3 of receivers, so the mixer specification is what is being
talked about here, not amplifiers. When characterizing amplifiers,
you have either input IP3 or output IP3 to contend with, so it gets a
little bit more complicated.
The formula I quoted IP(n)= Pin + (delta P/n-1) is a classical
derivation of a 2 tone result in the passband of a broadband
circuit such as an amplifier. Delta P is the difference in the
output tone level and the intermodulation product level. You can
use it for the input IP2, IP3, et etc. The formula can be used to
calculate any intermodulation product as long as the following
conditions are met:
1. The tones and intermodulation products you want to make a
measurement on all have to be in the circuits passband.
2. You have to be in the circuits linear range.
3. You have to be within the dynamic range of the measurement
equipment.
The Mini-circuits pdf is about a making these measurements on a
mixer and so it requires a third generator as the Lo.
IMR is intermodulation ratio. The definition appears to be the
delta of the input tone level power and the measured spurious
response, which is the intermodulation product I speak of or in
other words is a difference dBc (dB below carrier, this being the
input tone).
No, we are talking about the delta between the 3rd order product and
its associated tone at the I.F. (output) port of the mixer under
test.
Maybe that is what you were talking about but I'm trying to stay more
general. The formula and explanation I provided can be applied to RF
circuits in general not just mixers. Mixers comprise just one to three
stages in a radio generally. The rest of the circuits are filters and
amplifiers and detectors. With the right generalized approach you can
analyze any linear circuit or the whole radio.
This being the case then IP3 = Pin + (IMR/2) has the same meaning
if there is no gain. If there is gain then you would get a
different answer. I think you are better off using the formula I
referenced as both input power and gain or loss are accounted for.
True.
The test setup has several amplifiers so I don't know how they
actually expect to make a measurement on the DUT.
Actually, this is very easy........those amplifiers have a very high
IP3, so they introduce very low measurement error. This setup will
allow you to have an IMR of at least 110dB. I didn't have those
amplifiers on hand, so I used circulators for the required isolation
when I characterized that MCL digital step attenuator.
Well, its easy if you know all the specifications of the amplifiers or
have made the measurements on the test setup by itself. This is
something I can't do and would be making an assumption about.
I don't understand why the amplifiers are need in the first place. Maybe
their generators have weak outputs. The test setup is overly complex and
has unneeded equipment. When I see something like this it generally
means that someone is having problems making the measurement and not
understanding the problems they are having. It could be they are having
reflections from the DUT screwing up the measurements and the amplifiers
and attenuators is their way of dealing with it. I could keep making
pointless speculations about this but you must see the point by now.
The need of attenuators and power divider on the generator outputs is
well understood. The generator outputs must have some degree of
isolation from each other so the test setup itself does not generate the
intermodulation products to be measured. I've made these measurements on
unity gain amplifiers with just two generators, two attenuators, a power
combiner and a spectrum analyzer. That's all you need.
Also troubling to me that they state the IP3 measurement can only
be made at some input power level and that it you will get a
different result at a different input power level. Well, you will
get the same result at different power levels as long as you
account for it and conditions #2, and #3 above so I don't
understand their problem with that.
As long as you are within the linear range of the DUT, this is true.
Yes, that is one of the conditions I stated and is a direct logical
requirement of this type of measurement.
They are also using filters. Using filters is OK as long as you
don't violate condition #1 above.
If you are using low-pass or bandpass filters at the output of each
generator and make sure that the tones are at the required level,
this is a non-issue.
Well OK for you to say that but I'm not so sure other people that make
measurements on SW radios and publish the results have paid proper
attention to this.
Sometimes, you might only have a filter that has a corner frequency
very close to your highest frequency of interest. Part of calibrating
the test setup is making sure that you have the correct power level
at every frequency that you are making the test at. What I would do
is measure the power level of the RF generators and LO generator at
every frequency of interest, and either use a correction factor for
setting the generator output manually, or I would enter the
correction factor into the Labview program when applicable. Since we
are making sure that the power levels are correct at all frequencies,
condition #1 is being met.
Very conscientious of you to follow proper procedure in calibrating out
the affects of support circuitry. It is the right approach but again for
the sake of discussion I can't know what it is you or Mini-circuits may
or may not be doing unless it is explicitly stated.
On a final note, I haven't done any multitone testing of
amplifiers..........my tests were limited to harmonic distortion,
noise figure, S-Parameters, and 1dB compression point. As I have
mentioned in the past, it sounds like you have been in the industry,
and I appreciate your input. One thing I didn't mention was a piece
of test equipment that makes these tests a little bit easier. Instead
of using a swept spectrum analyzer, a Vector Signal Analyzer (VSA) is
used. This instrument has a very wide dynamic range, with a noise
floor of -140dBm, even in a very wide passband. Since this is really
an FFT analyzer vs a swept analyzer, you aren't limited by very long
sweep times of the swept analyzer. Another new tool that has become
available from Agilent is the PSA. This is a spectrum analyzer with
added functions, but the best thing about this analyzer is the very
low sideband noise from its internal LO. This makes it possible to
look at the phase noise sidebands from an 8657 for instance, even at
1MHz away from the carrier. There were several different generators
from Agilent, Rohde and Schwarz, and Fluke, but the quietest units
they had around were still the 8642B. When I characterized one of
those unit at a 100kHz offset, I measured the noise down at -154dBc.
The new R&S stuff isn't bad, but the 8642 generators are still "king
of the hill".
I have some experience making these measurements. One thing about being
in electronics is change. I'm sure you know the drill. I have
experience in many areas out of necessity. Digital, analog, RF, mixed
signal, electromechanical, military, computers and so on.
Agilent make some good equipment. Always always have since they were
HP. The company where I work now has an Agilent PSA. The thing is still
slow on the noise measurements and I can't see how that type of
measurement could be speeded up while maintaining accuracy. I checked
the formula I posted against the IP2 and IP3 measurements it calculated
on some wide band amplifiers and they matched so I have some confidence
in them AND I ran the generators at different output levels and got the
same IP2 and IP3 numbers so it can be done with consistency at
different levels so there you have it Mini-circuits personnel.
The Agilent PSA is an OK piece of equipment that automates some
measurements.
Anritsu bought Wiltron a US manufacture of RF signal generators with
very good phase noise. The R&S claim to fame I saw as important at the
the time I was buying them was phase locking ability. Most RF generators
have frequency lock but the R&S generators have phase lock. Most people
(very smart people by the way) didn't understand the difference and
would fail BER measurements due to the multiple generators in the test
setup being frequency locked that occasionally slipped a cycle on a
telecommunications link. What is a few bits lost every few minutes
between friends anyway? Oh well, details, details...
--
Telamon
Ventura, California
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