Can anyone provide any insight into why i’m getting the results I am?
The aim of this test was to measure the OIP3 during transmit, then
determine the IIP3.
Conditions
I conducted two-tone tests using f1 = 4 MHz and f3 = 5 MHz.
The LO was set to 2400, 2450, 2500, 4900, 5400 and 6000 MHz (I’m using
the
XCVR2450 with the USRP N210)
The Amplitude variable has been set to 0.25, and the gain has been
stepped
from 0 to 35dB.
Measurements
I’ve measured the power amplitudes, also the gain has been calculated at
each step (since the gain request may not actually get the specific
gain).
Measurements have been taken with an agilent spec amp at 300Hz.
I’ve then calculated the OIP3 and subtracted the actual gain to get the
IIP3.
Results
However, my data is a bit odd… An example has been provided in the
table
below. It seems that my intermodulation products are not increasing at
3dB
for each 1dB…
What does this imply?
Examples of my transmit IIP3 results are attached, these seem somewhat
in
the jagged ball park of the receiver results below for the WBX. http://code.ettus.com/redmine/ettus/documents/16
Not an apple to apple comparison I know… but I don’t have much to go
on…
am I missing something basic (I am still… in my opinion… new)
(Note: units below are MHz for frequencies, and dB for gain, dBm for
the
output power.)
Let me preface this by saying I mostly lurk on this list, and haven’t
touched a USRP of any variety, so I’m unfamiliar with their gain lineup.
However, looking at this data, a couple of things come to mind.
1-You’re sweeping the gain by 35 dB, and the amplitude is moving by 28
dB,
which seems to indicate to me that something strange is going on with
the
levels.
2-I graphed the levels of the tones, and they aren’t moving in amplitude
until the gain setting is 15 or so, which seems unusal to me. What does
the
spectrum analyzer display look like when you are doing this? Do you see
visible tones at deltaF? What about at other frequencies? Are you using
a
real SA or the USRP?
3-The other thing to look at and worry about are IP5/IP7 tones, which
can
really be a problem when you’re approaching P1dB on the output power.
I’m
suspicious that at higher gain settings, you’re starting to see these
tones
show up.
Here’s the google spreadsheet I imported your data into.
Thanks for the responses! Again, let me point out i am quite new to alot
of
the IIP3 concepts and still have my areas of confusion regarding the
board’s operation, so if I say anything weird or counter-intuitive
please
correct me!
To start with some info from Dave’s queries:
1- Thats right… these are the sorts of things i’m looking to
characterise.
I’m not sure if this would be considered some fault with my
USRP/daughterboard or within the expected variance in behaviour for the
daughterboards
An example shot is attached. I’m using a real Spec Amp. Thats right
regarding the tones… I noticed that too. However one thing that might
be
confusing about this test is that i’m not directly increasing the power
by
1dB at a time (in which case I’d expect the 2F1-F2 and 2F2-F1 tones to
increase by 3dB…) im changing the output power by varying the gain…
(rather then power directly). So that seemingly would impact the IIP3
values…(e.g. im gathering this would explain why my 2F1-F2 and 2F2-F1
tones aren’t increasing by a 3:1 ratio…)
Will that affect my measurements for the 2F1-F2 and 2F2-F1 tones
though
for this calculation?
Ben
(Re: OIP3) thanks thats a good point… o_0 I should adjust that.
(Re: Useful) But knowing the OIP3 will surely still give me an
indication of when my transmitter will start showing non-linearities…
won’t it? I’m not planning on using a USRP as a receiver…
Re: Clipping. I expect your right, i’ve used an amplitude of A = 0.25
to
avoid the DAC from saturating / causing clipping… so ideally… (if i’m
right) any non-linear behaviour will be due to the OIP3 rather then the
DAC…?
On Sun, Sep 16, 2012 at 8:51 AM, Michael H. [email protected] wrote:
characterise. I’m not sure if this would be considered some fault with my
USRP/daughterboard or within the expected variance in behaviour for the
daughterboards
so a starting point would be to do a single tone measurement, sweeping
the
gain. That would let you make sure that the gain is relativly linear and
figure out the relationship between the gain steps and output power. You
can also see how bad the DAC spurs are with the transmitter, something
that’s important. to understand.
In general, the most accurate IP3 measurements are when the IP3 tones are
only 3 to 6 dB above the noise floor. Doing this lets you be assured
that
you’re not measuring anything else, (IP5 etc). averaging is also a good
idea, as that reduces noise in the measurement. You’re measuring with a
100
Hz RBW/VBW, you might be better served by increasing that to 1 or 3 kHz
and
trace averaging. I’d also reduce the output power until the IP3 tones
are 6
dB above the noise floor, and use that single measurement to calculate
your
OIP3. As your output power increases, Higher order intermodulation
products
will start to come up at the same frequency and give you a bad
measurement,
something that’s no longer OIP3.
Will that affect my measurements for the 2F1-F2 and 2F2-F1 tones though
for this calculation?
Yes, IP5/IP7 tones can start showing up at higher output levels, close
to
compression. If you widen up the bandwidth of the measurement to go out
to
catch the IP5/7 tones, I bet you’ll see them with your current
measurement
setup.
Ben
(Re: OIP3) thanks thats a good point… o_0 I should adjust that.
(Re: Useful) But knowing the OIP3 will surely still give me an
indication of when my transmitter will start showing non-linearities…
won’t it? I’m not planning on using a USRP as a receiver…
OIP3 measurement is a reasonable transmitter quality metric. There are
others that are system specific you could use, but it’s all measuring
the
same thing.
Re: Clipping. I expect your right, i’ve used an amplitude of A = 0.25
to avoid the DAC from saturating / causing clipping… so ideally… (if i’m
right) any non-linear behaviour will be due to the OIP3 rather then the DAC.
it’s worth making sure of that by measuring the DAC output directly, if
possible.
Hm, I don’t understand what you are attempting to measure. Measuring the
input IP3 (IIP3) for a transmitter doesn’t mean anything. All that
matters
is output IP3 (OIP3). The e-mail you linked to is Marcus explaining how
to
measure the IIP3 through the receive-only port of a daughterboard, using
the same daughterboard as a signal source. You are attempting to measure
the IIP3 of a transmitter only - whatever data you collect won’t tell
you
anything meaningful.
David -
Gain levels are approximate, and it is also possible that at the upper
amplitudes the ADCs are clipping anyway.
Cheers,
Ben
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