In article ,
"Pete KE9OA" wrote:

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.

When a receiver is being characterized for IP3, the 1st mixer is what
is being characterized in every case.

What? All the rest of the radio circuitry has nothing to do with IP3?

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.

This is something that is generally done in the industry today. It is
more or less the testing standard.

Then explain why this is a industry standard way of making these
measurements.

I don't understand why the amplifiers are need in the first place.
Maybe their generators have weak outputs.

Most RF generators today have an output of at least +15 to +20dBm, so
generator output is not an issue. The main reason for selecting these
amplifiers is twofold. First of all, they have very high reverse
isolation, and second, they have very low harmonic distortion and
very good multitone response.

Makes no sense to me. Why would a separate amplifier not have the same
problem as the output amplifier in the generator? The attenuators on
the generator outputs before the power combiner and the power combiner
itself (resistive type) are are supposed to provide the isolation of
the generator outputs.

Why is some additional amount of reverse isolation the amplifier
provides needed?

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.

From the above statement, I have to deduce that you haven't really
made too many mixer characterizations. If you had, you would have
discovered that a "complex" setup is necessary when characterizing
low distortion mixers. If you are measuring the IP3 of the newer RF
switches and digital step attenuators, this "complex" setup just
barely makes the grade.

Yeah, I have not made any measurements on a mixer.

You mean solid state switches and step attenuators? Why are additional
amplifiers, attenuators, and filters needed for those?

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.

No, I don't see your point.

Pretty simple point, it is my observation of RF test setups measuring
this, that, or the other people have created when having problems
making a measurement over many years time. It's not an inditement it
just raises my suspicions.

There has to be a reason that you use an amplifier to boost a signal
followed by attenuators to cut it back down. There has to be a reason
you think you need some additional amount of reverse isolation in the
fixture.

My guess is that the DUT input and output impedance is not a flat 50
ohms over the measurement range and the reflections play havoc with the
measurements. If that is the case then fix the DUT design not add junk
to the fixturing that needs to be calibrated out of the DUT
measurements.

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.

Perhaps this setup is adequate for unity gain amplifiers, but it
doesn't even come close if you have to characterize high performance
mixers. You might thing the setup is working well, but you would
discover very quickly that one of the newer mixers isn't meeting its
specifications. understand their problem with that. above.

The unity gain amplifier was just an example that I used because their
is no gain and it might be a reason amplification is needed somewhere
in the fixturing with the attenuation needed for isolation of the
generator outputs.

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.

I am not sure about that.......if they are not following these
guidelines and if different reviewers are using different topologies
with their test setups, there is no standard to compare by.

Bingo.

A similar problem occurs with microscope specifications. As an
example, with planachromatic objectives one manufacturer might
specify a plan objective as having a flat field with good color
correction over at least 90% of the viewing field while another
manufacturer would use 85% as a specification.

Gotcha.

Here is another problem when making IP3 measurements, or any
distortion measurement...........when you are looking at distortion
components that are at -100dBc or lower, even connectors can
contribute to the measured result. When selecting attenuators, the
ones with the transverse heatsink fins do have lower IMD properties,
so these are the ones to use.

My work is mostly small signal so attenuators I use don't have fins but
ya got me why transverse cooling fins would make an IMD difference.

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.

That is why I provided the PDF link from
Mini-Circuits..............they are pretty succint in the appnote.

I pretty much did not like what I read there. Good company, good
products but that pdf left me desiring more explanation of some of
their conclusions.

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.

I have no doubt about your ability.......it appears that you have
quite a bit of experience in this field.

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.

I have noted the same thing, as long as I was working within the
linear range of the DUT. As far as noise testing, I think you are
talking about phase noise. If that is the case, Agilent has a new
phase noise/VCO test system that can measure down to below -160dBc.
We were using it with some of the new VCOs (in-house design). Still,
when measuring down at that level, external factors can affect the
measurement. When were were testing the VCOs at -40C, there were some
spikes in the phase noise spectrum. The firing of the SCRs in the
environmental chamber were causing perturbation of the tuning voltage
from radiated emissions. These were very low frequency harmonics of
60hZ...............the 3rd and 4th harmonic were causing the
problems.

Yeah, chambers of any type and vibration tables always add another
dimension of "fun" when the DUT is not making specifications and don't
you just love it when the problem turns out to be test equipment
instead of the DUT.

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...

I remember when I was working at Rockwell-Collins. They had buffered
10MHz signals running through all of the labs that we used to connect
all of the generators and spectrum analyzers to. This was also
important when characterizing the frequency hop synthesizers. Same
thing at Motorola.......we used to sync all of the equipment when we
were measuring BER of some of the newer radios. Those R&S generators
were pretty good. When we were looking at new generators years back,
we had samples of the Agilent E series and of the R&S SMY-01s. Those
SMY-01s were pretty nice, but not as clean as the 8642s. We used them
as the reference oscillator with new synthesizer designs. At wide
frequency offsets, the phase noise of the synthesizers were about the
same with both generators, but at a 7kHz offset there was a 25dB
disparity between the two, with the 8642 coming out better. Another
thing about the SMY-01s...............many of them had failures in
the phase lock loop section. I haven't seen this problem with the
newer R&S units..........they have been very reliable. Wouldn't be
bad for a home unit! I remember the 8640s used frequency locking, and
if you connected the generator to a high resolution frequency counter
you would see the output frequency warp up and down by a few Hz ov er
a period of time. That is the reason that I still have my old Boonton
103D. This generator has phase noise in the -143dBc range and it has
a dedicated OCXO for the phase locking function.

Oh well...........time to get ready for work again.............nice
talking (bantering?) with you. I don't mind intelligent debates!

I don't care for debates, I'm interested in learning. You are right
that I've never characterized a mixer so maybe you can explain the
reason for the filters, attenuators and amplifiers in the test setup. I
explained why I thought them unnecessary and I do not understand why
they are necessary as a standard way of characterizing a mixer. Sure
you can connect everything together without the DUT and characterize
the fixturing. These measurements can either be a floor you can't
measure below or ceiling you can't measure above on the DUT or used as
a fixture compensation on the DUT measurement depending on the
measurement type but generally fixture compensation is to be avoided
unless necessary. It is a much better situation that the fixturing be
transparent to the measurements.

--
Telamon
Ventura, California