Root/usrp/README

Source at commit 49e7c83796bc04941e9dbcec69bc0751563ff4d4 created 2 years 4 months ago.
By Werner Almesberger, atusb/: use ""VDD" symbol from kicad-libs
1Antenna measurements
2====================
3
4The objective of antenna measurements is to determine how much energy the
5antenna transfers at different frequencies. For this, we set up a sender,
6a receiver, connect one to the antennas being tested, and the other to an
7arbitrarily chosen lab antenna.
8
9Since none of the items (sender, receiver, lab antenna) are calibrated,
10we can only compare antennas but we cannot determine any absolute
11characteristics.
12
13
14Preparing a measurement run
15---------------------------
16
17Before measuring the characteristics of an antenna, we need to set up the
18test environment and obtain a number of filtering parameters. The filters
19are used to reduce the effect of noise on the measurements and to suppress
20contamination from other sources.
21
221) Install transmitter and receiver. The transmitter is an atusb or atusd
23   board, the receiver an USRP2+XCVR2450 with the antenna to test.
24
25   (The same setup may also work with a USRP1 or UN210, and a RFX2400
26   board.)
27
28   Both should be spaced at least twenty times the wavelength (12.5 cm at
29   2405 MHz), or 2.5 m apart. For test runs that can be compared with each
30   other, antenna placement and orientation have to be exactly the same.
31
32   The sender runs tools/atrf-txrx/atrf-txrx, the receiver runs utilities
33   from gnuradio.
34
352) Obtain baseline performance values. For example, activate the sender
36   with
37
38   atrf-txrx -f 2455 -p 0.5 -T +0.5
39
40   Emit a constant wave at 2455+0.5 MHz with a power of 0.5 dBm or 1.1 mW.
41
42   Monitor the received signal with
43
44   usrp2_fft.py -f 2455.5M -d 16
45
46   Record the range in which the frequency peak falls. Variations of a few
47   dB are to be expected.
48
493) Generate a series of sample for a specific setting.
50
51   Example:
52
53   The following script sets up the transmitter, lets it "warm up" for ten
54   seconds, then takes 100 measurements, stored in files tmp00 through
55   tmp99 in a directory $PWD/100/.
56
57   In this setup, the receiver's gnuradio runs on a different host than
58   the sender. Therefore we use ssh and pass the directory from $PWD.
59
60    atrf-txrx -f 2455 -p 2.6 -T +0.5 \
61      'sleep 10;
62       for a in 0 1 2 3 4 5 6 7 8 9; do
63         for b in 0 1 2 3 4 5 6 7 8 9; do
64           ssh ws usrp2_rx_cfile.py -d 16 -f 2455.5M -g 46 -N 1124 \
65         '$PWD'/100/tmp$a$b
66         done
67       done'
68
69   Each measurement obtains 1124 samples, 1024 samples for the FFT and
70   100 samples to cut off (see below).
71
724) Determine the shape of the captured waves in the time domain, e.g.,
73   with
74
75   gnuplot
76   gnuplot> plot "<./avg 1 <100/tmp00" with lines
77
78   "avg" outputs the magnitude of the recorded wave, averaging over the
79   specified number of sample.
80
81   Some waves will probably show a peak in the first few samples. We need
82   to cut off these peaks in the later processing steps. In this example,
83   we will skip the first 100 samples.
84
85   Besides the initial peak, the waves should be of comparable amplitude.
86
875) Verify the distribution in the frequency domain and determine the noise
88   floor.
89
90   gnuplot> plot "<./fft -s 100 -d <100/tmp00" with lines
91                             ^
92                          skip initial peak
93
94   The spectrum should be U-shaped, with narrow peaks tens of dB above
95   the noise floor near the beginning and the end. Note that the noise
96   floor is curved and not perfectly flat.
97
98   From this, we pick level of the noise floor. The value should be at or
99   slightly below the highest peaks of the noise between the large peaks
100   at the end of the spectrum.
101
102   This noise floor value is used to filter uninteresting samples later
103   on, removing a constant bias from the results.
104
105   In this example, we'll use a noise floor value of -60 dB.
106
1076) Determine the "interesting" frequency range. For this, we consider all
108   the spectra of the measurements:
109
110   gnuplot> plot "<for n in 100/tmp*; do ./fft -s 100 -d <$n;echo;done" \
111     with lines
112
113   There should be a thick noise band in the middle, with pronounced
114   narrow peaks at the edges. If there are one or two signals on top of
115   the noise band, some measurements have been compromised and need to be
116   removed or redone. We will do this in the next step.
117
118   When zooming into the left peak, the "bins" which contribute to the
119   peaks can be identified. The range should be chosen with some
120   tolerance, since the frequency may shift a bit during the measurement
121   process.
122
123   By not considering bins far from the peak, less noise is included in
124   the final result, complementing the filtering by noise threshold from
125   step 4). Restricting the bins also eliminates the second peak at the
126   end of the spectrum.
127
128   In this example, we'll use a range from 0 to 20.
129
1307) Obtain the peaks from all measurements
131
132   gnuplot> plot "<for n in 100/tmp*; do ./fft -s 100 0 20 60 <$n;done" \
133     with lines
134                                                   ^ ^ ^ ^
135                                                   | | | |
136                                   skip, from step 4 | | threshold, 5)
137                                             lowest bin highest bin
138
139   This should yield a jagged more or less horizontal line with values
140   differing by not more than 1-2 dB. If there are any large outliers,
141   they have been contaminated and should be dropped.
142
1438) The final result for one measurement run can be obtained as follows:
144
145   for n in 100/tmp*; do ./fft -s 100 0 20 60 <$n;done | ./range -v 2
146
147   In this example, "range" eliminates all outliers more than 2 dB from
148   the average and reports this.
149
150   The output are three numbers: the average (after eliminating
151   outliers), the minimum, and the maximum.
152
153
154Performing a measurement run
155----------------------------
156
157The script "fscan" performs 100 scans of the 26 channels used by IEEE
158802.15.4. The frequency scan is the inner loop, so that slow changes
159in environmental parameters (background noise, temperature, etc.) will
160affect the spread of the results over the entire frequency range instead
161of causing seemingly frequency-dependent distortions.
162
163The script is written for a setup that uses a pair of hosts, both
164sharing the same file system.
165
166Usage: fscan out-dir [tx-power]
167
168The output directory must not exist yet. The transmit power is in dBm and
169defaults to 2.6 dBm.
170
171Example: ./fscan testant
172
173The full run takes approximately half an hour.
174
175The results are filtered and averaged by the script "evscan". This script
176contains the filtering parameters obtained in the preparation, described
177above.
178
179Example: ./evscan testant >testant.out
180
181The output is a graph with frequency, average signal strength, minimum
182and maximum. The format is compatible with gnuplot's "with errorbars"
183(or "with errorlines") plot style.
184
185Finally, the results can be plotted with the script "plscan", which uses
186gnuplot to output either in a window or to a PNG file.
187
188Usage: plscan [-o pngfile] file ...
189
190More than one graph can be plotted in the same run. The file name is used
191as the title for each graph. Titles are truncated at the last dot.
192
193Example: ./plscan testant.out
194

Archive Download this file



interactive