2016-05-05 16:31:34 +00:00
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<link href="style.css" rel="stylesheet" type="text/css" />
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<title>osmocom-analog</title>
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</head>
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<body>
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<center><table><tr><td>
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<h2><center>C-Netz</center></h2>
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<center><img src="c-netz.jpg"/></center>
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<ul>
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<li><a href="#history">History</a>
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<li><a href="#howitworks">How it works</a>
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<li><a href="#basestation">Setup of a base station</a>
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</ul>
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<p class="toppic">
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<a name="history"></a>
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History
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</p>
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<p>
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*TBD*
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</p>
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<p class="toppic">
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<a name="howitworks"></a>
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How it works
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</p>
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<p>
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*TBD*
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</p>
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<p class="toppic">
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<a name="basestation"></a>
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Setup of a base station
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</p>
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<p>
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When I first started with testing early C-Netz implementation, I had great trouble getting a response from a mobile phone.
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The main problem was clock speed of the transmitted signal.
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The base stations were all synchronized with a high precision clock.
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All base stations transmit on the same control channel 131 ('Orgakanal'), but at different time slots.
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The phone expects that the time slots repeat precisely every 2.4 seconds.
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The specs say that the phone requires clock speed deviation of +- 1 PPM (Parts Per Million) or less.
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To comply with the specs, the base stations needs to have a clock deviation far below 1 PPM.
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(I guess that there is a tollerance, so maybe a few PPM deviation would make no big problem.)
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We need to calibrate the speed of our transmit signal (and receive signal as well).
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</p>
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<p>
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<font color="red">Importaint: We need calibrate the signal processing clock!</font>
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</p>
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<p>
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First of all, we need a reference clock.
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Since most systems have standard crystals, we cannot use our system clock as reference without any help.
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I decided to use the NTP daemon.
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2016-05-28 17:40:19 +00:00
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After running the system for several days, the clock speed should be accurate enough for our needs.
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2016-05-05 16:31:34 +00:00
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Be sure to choose a place with constant temperature.
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</p>
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2016-05-28 17:40:19 +00:00
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<p>
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In /etc/ntp.conf define one or more servers to geht the time from. I suggest to use the German atom clock of the PTB:
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</p>
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<pre>
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server ntp1.ptb.de
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server ntp2.ptb.de
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server ntp3.ptb.de
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</pre>
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<p>
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This clock provides time base for European radio clocks and most German clocks.
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It is perfect for this project, so please use this clock.
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</p>
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2016-05-05 16:31:34 +00:00
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<pre>
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netbook ~ # ntptime
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ntp_gettime() returns code 0 (OK)
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time dad971f1.9e1b5c44 Sun, May 8 2016 10:06:09.617, (.617605507),
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maximum error 238951 us, estimated error 199 us, TAI offset 0
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ntp_adjtime() returns code 0 (<font color="red">OK</font>)
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modes 0x0 (),
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offset 425.797 us, frequency <font color="red">15.796 ppm</font>, interval 1 s,
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maximum error 238951 us, estimated error 199 us,
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status 0x2001 (PLL,NANO),
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time constant 10, precision 0.001 us, tolerance 500 ppm,
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</pre>
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<p>
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One notices that I have quite an unprecise crystal in my test machine.
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Since I have NTP running, the system clock is now far below 1 PPM.
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</p>
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<p>
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The second problem is the clock of our sound adapter.
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They use own crystals.
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Because they are made for audio signals (and not for high precise base stations), they can't be used without calibration.
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Cheap USB adapters may even have different clocks on TX and RX.
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I found out that the clock speed changes when the USB voltage changes.
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The USB voltage changes, when the current changes, due to different volumes on the output.
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To minimize this effect, don't use extension cables.
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</p>
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<p>
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Since we have precise system clock now, we can measure the speed of our sound adapter.
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Later we will use the measurement result to adjust our signal processor.
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2016-05-28 17:40:19 +00:00
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Be sure to have your machine running for at least 3-4 days (with your sound adapter attached, in case you have USB sound adapter).
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2016-05-05 16:31:34 +00:00
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The machine must have network connection to the NTP server.
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I suggest to use 'ntp1.ptb.de' and 'ntp2.ptb.de' as reference.
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Use command line option '-M' or '--measure' to measure my sound adapter.
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Let it run for one hour.
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</p>
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<pre>
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# src/cnetz/cnetz -k 131 -M
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...
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dsp.c:218 notice : Clock: RX=4.75 TX=36.84; Signal: RX=135.38 TX=-378.58 ppm
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dsp.c:218 notice : Clock: RX=10.61 TX=31.09; Signal: RX=135.38 TX=-378.58 ppm
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dsp.c:218 notice : Clock: RX=3.01 TX=26.55; Signal: RX=135.38 TX=-378.58 ppm
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...
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dsp.c:218 notice : Clock: RX=19.46 TX=19.45; Signal: RX=19.39 TX=19.42 ppm
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dsp.c:218 notice : Clock: RX=19.45 TX=19.50; Signal: RX=19.39 TX=19.42 ppm
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dsp.c:218 notice : Clock: RX=19.48 TX=19.47; Signal: RX=19.39 TX=19.42 ppm
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dsp.c:218 notice : Clock: RX=19.44 TX=19.44; Signal: RX=19.35 TX=19.43 ppm
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dsp.c:218 notice : Clock: RX=19.44 TX=19.49; Signal: RX=19.35 TX=19.43 ppm
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</pre>
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<p>
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One can see that the measured clock from the sound card (sample rate) differs about +19.45 PPM.
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Because I did not do any correction to the signal processing, the same error applies to the signal.
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Use command line option '-S 19.45,19.45' or '--clock-speed 19.45,19.45' to compensate the clock error by adjusting the signal processor.
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2016-05-28 17:40:19 +00:00
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Let it run again for one hour and you will see that clock deviation is below 1 PPM.
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2016-05-05 16:31:34 +00:00
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</p>
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<p>
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<font color="red">Importaint: We need to connect to PLL and discriminator!</font>
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</p>
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<center><img src="c-netz-burst_full.png"/></center>
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<p>
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*TBD*
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</p>
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<p>
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*TBD*
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</p>
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<p>
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2016-05-28 17:40:19 +00:00
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<font color="red">Importaint: We need to transmit correct polarity, use auto polarity detection always!</font>
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2016-05-05 16:31:34 +00:00
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</p>
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<center><img src="c-netz-burst_part.png"/></center>
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<p>
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2016-05-28 17:40:19 +00:00
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C-Netz uses FSK (Frequency Shift Keying) by directly modulating the carrier, rather than modulating a tone, which is then transmitted via FM.
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2016-05-05 16:31:34 +00:00
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The advantage is much higher data rate.
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2016-05-10 17:23:11 +00:00
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The base station's carrier frequency is raised by 2.5 KHz to transmit a logical "1" and lowered by 2.5 KHz to transmit a logial "0".
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(The mobile station transmits reversed polarity.)
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The mobile station will only detect the correct polarity.
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2016-05-05 16:31:34 +00:00
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The three sync words of "11100010010", is shown in the picture above.
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In the picture above, the polarity is wrong.
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2016-05-10 17:23:11 +00:00
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(The logical "1" points down and the logical "0" points up.)
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2016-05-05 16:31:34 +00:00
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You must check, with what polarity your transmitter is doing a positive frequency deviation.
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Also you must check your sound card's polarity on positive value beeing sent.
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2016-05-28 17:40:19 +00:00
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To change polarty, use command line option '-F yes' or '--flip-polarity yes'.
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2016-05-05 16:31:34 +00:00
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(Note: don't confuse with '-P'!)
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</p>
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<p>
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2016-05-10 17:23:11 +00:00
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This software does not care about what polarity it receives from the mobile station.
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It just checks for sync words with both polarities and chooses right polarity automatically.
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2016-05-05 16:31:34 +00:00
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</p>
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[<a href="index.html">Back to main page</a>]
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</td></tr></table></center>
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