mirror of
git://projects.qi-hardware.com/ben-wpan.git
synced 2024-12-29 03:46:47 +02:00
fea4ee551a
- usrp/README: changed threshold from 50 to 60, such that the reference "5)" right next to it doesn't look like a mistyped "50"
194 lines
6.9 KiB
Plaintext
194 lines
6.9 KiB
Plaintext
Antenna measurements
|
|
====================
|
|
|
|
The objective of antenna measurements is to determine how much energy the
|
|
antenna transfers at different frequencies. For this, we set up a sender,
|
|
a receiver, connect one to the antennas being tested, and the other to an
|
|
arbitrarily chosen lab antenna.
|
|
|
|
Since none of the items (sender, receiver, lab antenna) are calibrated,
|
|
we can only compare antennas but we cannot determine any absolute
|
|
characteristics.
|
|
|
|
|
|
Preparing a measurement run
|
|
---------------------------
|
|
|
|
Before measuring the characteristics of an antenna, we need to set up the
|
|
test environment and obtain a number of filtering parameters. The filters
|
|
are used to reduce the effect of noise on the measurements and to suppress
|
|
contamination from other sources.
|
|
|
|
1) Install transmitter and receiver. The transmitter is an atusb or atusd
|
|
board, the receiver an USRP2+XCVR2450 with the antenna to test.
|
|
|
|
(The same setup may also work with a USRP1 or UN210, and a RFX2400
|
|
board.)
|
|
|
|
Both should be spaced at least twenty times the wavelength (12.5 cm at
|
|
2405 MHz), or 2.5 m apart. For test runs that can be compared with each
|
|
other, antenna placement and orientation have to be exactly the same.
|
|
|
|
The sender runs tools/atrf-txrx/atrf-txrx, the receiver runs utilities
|
|
from gnuradio.
|
|
|
|
2) Obtain baseline performance values. For example, activate the sender
|
|
with
|
|
|
|
atrf-txrx -f 2455 -p 0.5 -T +0.5
|
|
|
|
Emit a constant wave at 2455+0.5 MHz with a power of 0.5 dBm or 1.1 mW.
|
|
|
|
Monitor the received signal with
|
|
|
|
usrp2_fft.py -f 2455.5M -d 16
|
|
|
|
Record the range in which the frequency peak falls. Variations of a few
|
|
dB are to be expected.
|
|
|
|
3) Generate a series of sample for a specific setting.
|
|
|
|
Example:
|
|
|
|
The following script sets up the transmitter, lets it "warm up" for ten
|
|
seconds, then takes 100 measurements, stored in files tmp00 through
|
|
tmp99 in a directory $PWD/100/.
|
|
|
|
In this setup, the receiver's gnuradio runs on a different host than
|
|
the sender. Therefore we use ssh and pass the directory from $PWD.
|
|
|
|
atrf-txrx -f 2455 -p 2.6 -T +0.5 \
|
|
'sleep 10;
|
|
for a in 0 1 2 3 4 5 6 7 8 9; do
|
|
for b in 0 1 2 3 4 5 6 7 8 9; do
|
|
ssh ws usrp2_rx_cfile.py -d 16 -f 2455.5M -g 46 -N 1124 \
|
|
'$PWD'/100/tmp$a$b
|
|
done
|
|
done'
|
|
|
|
Each measurement obtains 1124 samples, 1024 samples for the FFT and
|
|
100 samples to cut off (see below).
|
|
|
|
4) Determine the shape of the captured waves in the time domain, e.g.,
|
|
with
|
|
|
|
gnuplot
|
|
gnuplot> plot "<./avg 1 <100/tmp00" with lines
|
|
|
|
"avg" outputs the magnitude of the recorded wave, averaging over the
|
|
specified number of sample.
|
|
|
|
Some waves will probably show a peak in the first few samples. We need
|
|
to cut off these peaks in the later processing steps. In this example,
|
|
we will skip the first 100 samples.
|
|
|
|
Besides the initial peak, the waves should be of comparable amplitude.
|
|
|
|
5) Verify the distribution in the frequency domain and determine the noise
|
|
floor.
|
|
|
|
gnuplot> plot "<./fft -s 100 -d <100/tmp00" with lines
|
|
^
|
|
skip initial peak
|
|
|
|
The spectrum should be U-shaped, with narrow peaks tens of dB above
|
|
the noise floor near the beginning and the end. Note that the noise
|
|
floor is curved and not perfectly flat.
|
|
|
|
From this, we pick level of the noise floor. The value should be at or
|
|
slightly below the highest peaks of the noise between the large peaks
|
|
at the end of the spectrum.
|
|
|
|
This noise floor value is used to filter uninteresting samples later
|
|
on, removing a constant bias from the results.
|
|
|
|
In this example, we'll use a noise floor value of -60 dB.
|
|
|
|
6) Determine the "interesting" frequency range. For this, we consider all
|
|
the spectra of the measurements:
|
|
|
|
gnuplot> plot "<for n in 100/tmp*; do ./fft -s 100 -d <$n;echo;done" \
|
|
with lines
|
|
|
|
There should be a thick noise band in the middle, with pronounced
|
|
narrow peaks at the edges. If there are one or two signals on top of
|
|
the noise band, some measurements have been compromised and need to be
|
|
removed or redone. We will do this in the next step.
|
|
|
|
When zooming into the left peak, the "bins" which contribute to the
|
|
peaks can be identified. The range should be chosen with some
|
|
tolerance, since the frequency may shift a bit during the measurement
|
|
process.
|
|
|
|
By not considering bins far from the peak, less noise is included in
|
|
the final result, complementing the filtering by noise threshold from
|
|
step 4). Restricting the bins also eliminates the second peak at the
|
|
end of the spectrum.
|
|
|
|
In this example, we'll use a range from 0 to 20.
|
|
|
|
7) Obtain the peaks from all measurements
|
|
|
|
gnuplot> plot "<for n in 100/tmp*; do ./fft -s 100 0 20 60 <$n;done" \
|
|
with lines
|
|
^ ^ ^ ^
|
|
| | | |
|
|
skip, from step 4 | | threshold, 5)
|
|
lowest bin highest bin
|
|
|
|
This should yield a jagged more or less horizontal line with values
|
|
differing by not more than 1-2 dB. If there are any large outliers,
|
|
they have been contaminated and should be dropped.
|
|
|
|
8) The final result for one measurement run can be obtained as follows:
|
|
|
|
for n in 100/tmp*; do ./fft -s 100 0 20 60 <$n;done | ./range -v 2
|
|
|
|
In this example, "range" eliminates all outliers more than 2 dB from
|
|
the average and reports this.
|
|
|
|
The output are three numbers: the average (after eliminating
|
|
outliers), the minimum, and the maximum.
|
|
|
|
|
|
Performing a measurement run
|
|
----------------------------
|
|
|
|
The script "fscan" performs 100 scans of the 26 channels used by IEEE
|
|
802.15.4. The frequency scan is the inner loop, so that slow changes
|
|
in environmental parameters (background noise, temperature, etc.) will
|
|
affect the spread of the results over the entire frequency range instead
|
|
of causing seemingly frequency-dependent distortions.
|
|
|
|
The script is written for a setup that uses a pair of hosts, both
|
|
sharing the same file system.
|
|
|
|
Usage: fscan out-dir [tx-power]
|
|
|
|
The output directory must not exist yet. The transmit power is in dBm and
|
|
defaults to 2.6 dBm.
|
|
|
|
Example: ./fscan testant
|
|
|
|
The full run takes approximately half an hour.
|
|
|
|
The results are filtered and averaged by the script "evscan". This script
|
|
contains the filtering parameters obtained in the preparation, described
|
|
above.
|
|
|
|
Example: ./evscan testant >testant.out
|
|
|
|
The output is a graph with frequency, average signal strength, minimum
|
|
and maximum. The format is compatible with gnuplot's "with errorbars"
|
|
(or "with errorlines") plot style.
|
|
|
|
Finally, the results can be plotted with the script "plscan", which uses
|
|
gnuplot to output either in a window or to a PNG file.
|
|
|
|
Usage: plscan [-o pngfile] file ...
|
|
|
|
More than one graph can be plotted in the same run. The file name is used
|
|
as the title for each graph. Titles are truncated at the last dot.
|
|
|
|
Example: ./plscan testant.out
|