RadioBanter - View Single Post - Querstions on IP3, and also Re JRC 545 New Radio Rumors ?

Pete KE9OA · November 25th 06, 11:26 PM posted to rec.radio.shortwave

Anytime Bob!

Pete

"Robert11" wrote in message
...
Hi,

Helps a lot.
Very clear explanation.

Appreciate it.

Thanks,
Bob
-------------------
"Pete KE9OA" wrote in message
...
IP3 is a figure of merit for dynamic range of a mixer. As already
mentioned, the higher the number, the better the unit.
Basically, the measurement consists of using three RF generators, with
each RF port generator running through a 6dB attenuator, a low-pass
filter, another 6dB attenuator, and a combiner. The output of the
combiner is connected to the RF port of the mixer under test. The reasons
for the 6dB attenuators are twofold; first of all, they provide 12dB of
isolation for the RF generators that is added to the 25 or so dB
isolation that the combiner already has. This helps to prevent the
generators from "talking" to each other, thus preventing IMD to be
generated in this portion of the test setup. The second reason for these
attenuators is to provide a broadband resistive termination for the
low-pass filters, so that they maintain their design characteristics.
A third RF generator is connected to a 3dB attenuator, a low-pass filter,
and another 3dB attenuator to the LO port of the mixer. The reason for
the 3dB attenuators is to provide a wideband resistive termination for
the low-pass filter so that it retains its design characteristics.
The low-pass filters are very important in this test setup, since when
the signals are squared up in the switching function of the mixer under
test harmonics can cause measurement error. Mini-Circuits has a
requirement of at least -65dBc for all harmonics present in the test
setup. The 6th and 9th harmonics can be especially troublesome when
making IP2 and I.F. port return loss measurements.
As far as injection levels, the LO generator is set to the level required
to illuminate this port. For a Level 7 mixer, this would be +7dBm, or
5mW. With this type of mixer, the level of each tone at the output of the
combiner that is applied to the RF port of the mixer needs to be at least
10dB below the 1dB compression point of the mixer. Since the 1dB
compression point for a typical Level 7 mixer is abour 0dBm, we would be
talking about a maximum level of -10dBm for each tone. -20dBm would be a
little bit better, just to make sure that you are operating within the
linear range of the mixer.
Now that we have the proper test setup, we connect all of this to the
mixer, and we connect the output port, in this case the I.F. port, to a
spectrum analyzer, also making sure that the spectrum analyzer is set up
for maximum dynamic range so that IMD isn't generated in this portion of
the test setup. Your test setup needs to have an IMR at least 10db better
than the device you are measuring, in order to minimize any measurement
error.
Use at least 50kHz separation between your input tones that are applied
to the RF port; the reason for this is so that phase noise sidebands from
the RF generators don't cause measurement error.
Taking a look at the spectrum analyzer, you will see five major tones;
these are, the LO, which should be suppressed by around 30dB or more, an
upper sideband tone, a lower sideband tone, and upper sideband and lower
sideband IMD products. These two IMD products are your third order terms.
Next, measure the difference between your upper sideband tone and your
upper sideband 3rd order term. Do the same for your lower sideband terms.
This difference between the desired sideband and the 3rd order term is
referred to as your IMR.
The reason for measuring both of these terms is because oftentimes, one
of these measurements will yield better results.
Choose the worst result when characterizing the mixer.
IP3 is calculated by this method: IP3 = (IMR/2 + Pin), whereby Pin is
defined as the power level at the output of the combiner of only one of
the input tones that are applied to the RF port of the mixer.
Some of the reviewers make this measurement using a 5kHz offset of the RF
generators that are applied to the RF port of the mixer. This measurement
is only valid if the noise sidebands of the generators are run through a
very selective filter, such as a crystal filter. In this case, you need
to make sure that these levels are well below the level where IMD would
be generated in the crystal filter itself; also, too much power can
shatter a crystal filter. If I remember correctly, it would be in the
ball park of +10dBm.
I remember when I was testing FM communication systems, the test setup
would be similar, with the exception that the spectrum analyzer wasn't
needed and the LO generator wasn't needed since the actual receiver's LO
would be used in this case.
Now, you still need three generators, this time using a 3-port combiner
at the RF port of the input stage of the receiver.
The two interfering RF port generators would be modulated with a 25%
modulation index. First of all, only the desired signal RF generator
would be switched on, using a level that would provide a 12dB SINAD. This
is equivalent to using a 10dB S/N+N ratio in an AM system or a 5% BER in
a digital system. Next, the second and third generators are switched on,
increasing their levels until a 3dB degradation is noted. Now, these two
generators need to be set to frequencies so that their difference
frequency lands on the desired channel where you are making the
measurement. Once again, you need to do this with upper sideband
interferors and lower sideband interferors, since the rejection will not
be symmetrical.
The difference between the levels of your desired signal and your
interfering levels is your IMR. IP3 is calculated in the same manner as
mentioned before.
It is no trivial task when making this kind of measurement. The
measurement itself isn't difficult, but the main thing is to make sure
that test setup created IMD doesn't cause any measurement error. After
you do this a few times, it becomes relatively easy, that is, until you
have to characterize a device such as an RF switch that might have an IP3
of +50dBm. This requires an IMR in your test setup of at least 110dB.
Now, that takes a bit of work!
I hope this long-winded explanation helps.

Pete

"dxAce" wrote in message
...

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 ?

Not sure about a 545 type but there is a rack mount radio coming out:

http://www.universal-radio.com/catal...vr/nrd630.html

dxAce
Michigan
USA