SignalScopePage » History » Version 12
max, 04/22/2017 04:04 PM
1 | 1 | max | = Signal Scope = |
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2 | |||
3 | == Installation == |
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4 | |||
5 | 6 | max | Before running the scope app, check that the following standard pre-requisite packages are also installed: |
6 | 1 | max | * GNU Radio |
7 | * The op25 blocks |
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8 | * Frank's gr-fsk4 block |
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9 | * The op25 repeater block |
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10 | |||
11 | 6 | max | The Python Numeric package is now also required. If you're running Debian or Ubuntu: |
12 | {{{ |
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13 | apt-get install python-numeric |
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14 | }}} |
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15 | 1 | max | |
16 | 6 | max | Other than these pre-reqs, no special setup or installation is needed. |
17 | |||
18 | 11 | max | == Targets == |
19 | * Choice of USRP, discriminator-tap, or "audio-IF" hardware modes |
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20 | * Supports either GL or non-GL display mode |
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21 | * Both FSK4 and CQPSK demodulation modes supported |
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22 | * Lower and middle range PC hardware, not just the latest |
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23 | |||
24 | == Running the Program == |
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25 | |||
26 | 5 | max | There are three overall options or modes depending on your hardware; these are |
27 | * USRP |
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28 | * External receiver with discriminator tap connected to sound card |
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29 | 1 | max | * External receiver with IF in the audio sound card range (e.g., 24 KHz), referred to as "audio IF" |
30 | |||
31 | 11 | max | === Running with the disc-tap option === |
32 | 1 | max | |
33 | 3 | max | The signal scope does not require the USRP. If you have a discriminator-tapped receiver, use the "-a" option: |
34 | 9 | max | {{{ |
35 | 3 | max | ./scope.py -a -v 10 -g 50 |
36 | 1 | max | }}} |
37 | 3 | max | |
38 | 11 | max | === Running with the USRP === |
39 | 3 | max | |
40 | It's helpful to find out the current calibration error beforehand (I use kalibrate), for example, +1234 hertz: |
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41 | 9 | max | {{{ |
42 | 1 | max | ./scope.py -f 412.34e6 -RA -g 65 -c 1234 -v 10 |
43 | 3 | max | }}} |
44 | 1 | max | |
45 | 11 | max | === Running in the Audio IF mode === |
46 | 1 | max | |
47 | 5 | max | Receivers equipped with an IF output in the sound card range can be used. This is known as "audio IF" mode. |
48 | A soundcard sampling rate of 96K is used and the IF frequency (typically 24 KHz) is given using the {{{--calibration}}} parameter: |
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49 | |||
50 | {{{ |
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51 | 9 | max | ./scope.py -A -c 24e3 -g 50 -v 10 |
52 | 5 | max | }}} |
53 | |||
54 | 4 | max | == Feature overview == |
55 | |||
56 | * Spectrum plot |
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57 | * Baseband oscilloscope |
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58 | 1 | max | * Eye Pattern Diagram (Datascope) display supporting several standard symbol rates |
59 | 4 | max | * Constellation Diagrams |
60 | * Demodulated Symbol Output |
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61 | 6 | max | * Correlation (including Fast Auto-Correlation) |
62 | 1 | max | * Direct-frequency entry, signal gain and fine-tuning controls |
63 | * User-selectable demodulator (FSK4 or QPSK) |
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64 | 5 | max | |
65 | 12 | max | In the baseband AF (disc. tap) mode, several program functions are disabled (spectrum FFT, constellation diagram, PSK demod, FAC and iDEN correlation) |
66 | because these functions require direct access to the signal with no demodulation. |
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67 | 5 | max | |
68 | In all modes, the {{{--wireshark}}} option is used to write received P25 packet data to Wireshark. |
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69 | 4 | max | |
70 | 10 | max | == Program Options == |
71 | |||
72 | Here is a full list of program options: |
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73 | {{{ |
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74 | Usage: scope.py [options] |
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75 | |||
76 | Options: |
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77 | -h, --help show this help message and exit |
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78 | -a, --audio use direct audio input |
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79 | -A, --audio-if soundcard IF mode (use --calibration to set IF freq) |
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80 | -I AUDIO_INPUT, --audio-input=AUDIO_INPUT |
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81 | pcm input device name. E.g., hw:0,0 or /dev/dsp |
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82 | -i INPUT, --input=INPUT |
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83 | input file name |
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84 | -b Hz, --excess-bw=Hz |
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85 | for RRC filter |
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86 | -c Hz, --calibration=Hz |
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87 | USRP offset or audio IF frequency |
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88 | -C Hz, --costas-alpha=Hz |
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89 | value of alpha for Costas loop |
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90 | -f Hz, --frequency=Hz |
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91 | USRP center frequency |
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92 | -d DECIM, --decim=DECIM |
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93 | source decimation factor |
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94 | -v VERBOSITY, --verbosity=VERBOSITY |
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95 | message debug level |
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96 | -p, --pause block on startup |
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97 | -w, --wireshark output data to Wireshark |
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98 | -W WIRESHARK_HOST, --wireshark-host=WIRESHARK_HOST |
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99 | Wireshark host |
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100 | -R RX_SUBDEV_SPEC, --rx-subdev-spec=RX_SUBDEV_SPEC |
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101 | select USRP Rx side A or B (default=A) |
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102 | -g GAIN, --gain=GAIN set USRP gain in dB (default is midpoint) or set audio |
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103 | gain |
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104 | -G GAIN_MU, --gain-mu=GAIN_MU |
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105 | gardner gain |
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106 | }}} |
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107 | |||
108 | 4 | max | == Spectrum Display == |
109 | |||
110 | [[Image(a.png)]] |
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111 | |||
112 | The controls arranged along the bottom of the page are: |
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113 | * Frequency: to retune, type the new frequency here and press ENTER |
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114 | * Signal Gain: adjusts the baseband (demodulated) signal level |
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115 | * Fine Tune: adjusts tuning frequency over +/- 3000 Hz range |
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116 | * Demod: Selects demodulator (currently used in Demodulated Symbols only) |
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117 | Except for the signal gain control, these controls are only available in USRP RX mode. |
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118 | |||
119 | == Eye Pattern Diagrams == |
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120 | |||
121 | The scope input source can be connected either before or after the symbol filter using the Viewpoint toggle. |
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122 | |||
123 | Also the proper speed must be selected from the available options. |
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124 | |||
125 | [[Image(b.png)]] |
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126 | |||
127 | == Constellation Diagram == |
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128 | |||
129 | 7 | max | [[Image(f.png)]] |
130 | |||
131 | The signal scope also features an angular population graph (shown above) in addition to the traditional constellation display. |
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132 | In this mode the symbol magnitude (distance from center) is discarded. Instead the circle is sliced into segments and a count |
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133 | of symbols found in each segment is plotted. This is similar to a histogram except that a straight line is drawn between each |
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134 | result, and that the results are arranged in polar form instead of rectangular form. |
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135 | |||
136 | 8 | max | With this display, the zone at the exact center of the plot can be used precisely to measure the degree of separation or margin |
137 | between the symbol decision points. The plots below illustrate the difference, with poorer convergence showing on the left image: |
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138 | 1 | max | |
139 | 8 | max | [[Image(mhp7a.png)]][[Image(c.png)]] |
140 | |||
141 | The two-color mode is used in these images, providing natural relief to highlight the distinctive feature of π/4 DQPSK in which |
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142 | successive symbols are chosen from two distinct constellations (each containing four possible symbol values) separated by 45° |
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143 | |||
144 | 4 | max | == Demodulated Symbols == |
145 | |||
146 | [[Image(d.png)]] |
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147 | |||
148 | 1 | max | == Correlation == |
149 | |||
150 | [[Image(e.png)]] |
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151 | |||
152 | 9 | max | Cross correlation allows rapid identification of signals with known characteristics. Frame Sync (FS) signatures of several commonly |
153 | used radio systems are included. |
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154 | |||
155 | By convention correlation results are usually displayed using positive correlation peaks only. In this system |
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156 | however it is possible (and legal) for negative correlation products to be produced. This can occur for two wholly separate reasons: |
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157 | * If the hardware polarity is inverted |
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158 | * When the FS symbols are purposely inverted as an integral part of protocol processing (commonly used in certain protocols but not used in P25) |
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159 | |||
160 | The first case commonly happens when using the disc-tap method of hardware connection, because the actual polarity of the signal seems to vary |
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161 | randomly among different sound cards and receivers. In one actual case, two PC's of the same PC brand bought from the same store had opposite polarity. |
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162 | |||
163 | The P25 software framer automatically detects the proper polarity and issues a message if negative polarity data is received: |
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164 | {{{ |
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165 | Reversed FS polarity detected - autocorrecting |
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166 | }}} |
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167 | |||
168 | The automatic correction applies only to software framing and doesn't help with correlation. For correct results for both software framing and |
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169 | correlation, you should correct the polarity reversal problem at its source; this is done using negative values for the {{{--gain}}} parameter at |
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170 | program start time: |
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171 | |||
172 | {{{ |
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173 | ./scope.py -a -v 10 -g -50 |
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174 | }}} |
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175 | |||
176 | The second cause of negative correlation peaks is that some protocols (although not P25) make use of both normal- and inverted-polarity FS |
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177 | sequences as a standard part of their processing. Instead of clogging the GUI menu with several choices that are merely inverses of others, |
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178 | 1 | max | just for the sake of always having positive-peaked correlations, we allow the correlation graph to reflect the natural polarity of the data. |
179 | 9 | max | Thus both + and - peaks are shown, allowing quick diagnosis of incorrect hardware polarity (see above), and allowing identification of the |
180 | 12 | max | particular family and sub-protocol in use. |
181 | 9 | max | |
182 | == Auto Correlation == |
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183 | |||
184 | 1 | max | Also included is Frank's Fast Auto Correlation (fac): |
185 | |||
186 | 8 | max | [[Image(g.png)]] |
187 | 9 | max | |
188 | For further details about Fast Auto Correlation refer to Frank's page at http://sites.google.com/site/radiorausch/ |
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189 | 8 | max | |
190 | == BUGS == |
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191 | |||
192 | Possibly bugs exist, here are a few of the known ones as of this writing |
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193 | 1 | max | * Symbol filters totally brain damaged (need separate filters for each speed) |
194 | * When switching modes using the notebook tabs, leftover data from before may appear momentarily |
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195 | * Highest and lowest speeds are not well tuned resulting either in sluggish updates or CPU exhaustion |
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196 | 11 | max | |
197 | == CREDITS == |
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198 | |||
199 | I ripped off the "tabbed notebook" theme (and code) from Stevie; it already had the spectrum, baseband, and decoded symbol displays. I added the data scope, constellation scope, and cross correlation features. |
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200 | |||
201 | The Fast Auto Correlation (fac) code came from Radiorausch. |
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202 | |||
203 | Special thanks to Mossmann for leaving one commented-out "print" statement in c4fm-decode.py. Of course it was tempting to wonder "what would happen" if that statement were un-commented. The final result? The cross-correlation feature. |
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204 | |||
205 | Special thanks also to GPL(v3), for encouraging these mashups. |