203 lines
6.3 KiB
HTML
203 lines
6.3 KiB
HTML
<html>
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<head>
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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>SDR setup</center></h2>
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<center><img src="sdr.jpg"/></center>
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<ul>
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<li><a href="#intro">Introduction</a>
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<li><a href="#limesdr">LimeSDR</a>
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<li><a href="#uhd">UHD</a>
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<li><a href="#mchan">Multiple channels</a>
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</ul>
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<p class="toppic">
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<a name="intro"></a>
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Introduction
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</p>
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<p>
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A far more convenient way is to use an SDR.
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Why would you (not) want to do that?
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<ul>
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<li>Not a bunch of cables and radios, just one device.
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<li>Simulate multiple channels with a single SDR channel.
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<br>Keep control channel while using voice channel(s).
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<li>No audio level (deviation) calibration is required.
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<li>The polarity of the frequency deviation (FFSK) is always correct.
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<li>No radio mod for discriminator output required.
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<li>No radio mod for modulator input required.
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<li>Don't care about correct emphasis.
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<li>Carrier FSK never returns to zero, no AC coupling.
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<li>Switching to channel 19 (B-Netz) works without transmitter hack.
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<li>More experience is required to run an SDR.
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<li>The device must be full-duplex.
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<li>In 2017, they are still more expensive than analog radios.
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</ul>
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</p>
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<p>
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It may sound weird, but you are able to simulate multiple channels with only one SDR.
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The I/Q data of an SDR allows to shift the frequency.
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By adding multiple frequencies, shifted by different offset, allows to transmit multiple down-link channels at a time.
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By using a filter, the mix of received up-link channels can be separated.
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Multiple channels require multiple CPU usage or even more.
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</p>
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<p>
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How to use an SDR and install the driver software and drivers is beyond the scope of this document.
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I assume that you successfully used your SDR with Gnu Radio,
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know how to select antenna connectors,
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know about DC calibration,
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know about clock settings,
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know about transmit and receive gains,
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and so on...
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</p>
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<p class="toppic">
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<a name="limesdr"></a>
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LimeSDR
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</p>
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<p>
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If you have this device, you need to install the SoapySDR, then the LimeSuit and finally run configure with Osmocom Analog, compile and install.
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Run Osmocom Analog with --help again, and you should see a bunch of option for SDR.
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In case of B-Netz, I use the following parameters:
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</p>
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<pre>
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# bnetz --sdr-soapy \
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--sdr-tx-gain 50 \
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--sdr-rx-gain 30 \
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--sdr-samplerate 5000000 \
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-s 100000 \
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-k 17
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</pre>
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<p>
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In order to change from analog sound card to SDR, you need <b>--sdr-soapy</b> option.
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In my setup I use antennas directly connected to the SDR.
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Being about 1-10 meters away, I use the <b>gain</b> as defined above.
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The <b>sample rate</b> must be 5 MHz minimum.
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The default <b>bandwidth</b> follows the sample rate, if not specified using <b>--sdr-bandwidth</b>.
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Higher sample rate causes more CPU, RAM and USB load.
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The local oscillator frequency causes the transmitted signal to be noisy, so I shift it 1 MHz away, using the default <b>--sdr-lo-offset</b>.
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The audio processing rate of 100 KHz (<b>-s 100000</b>) is used to generate two channels: <b>17</b> and 19.
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Note that channel 19 is not given here, but will be used automatically.
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With B-Netz, the transmitter switches from any voice channel to the paging channel (19) whenever the phone gets paged.
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</p>
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<center><img src="display-iq.png"/></center>
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<p>
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The RX gain is quite important.
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You can monitor it by pressing 'q' key.
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You will get an ASCII art plot of received IQ data.
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Press 'q' again and you will get a logarithmic view of IQ vectors.
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Press 'q' again to turn off this view.
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Please monitor this view when the phone tries to call or register.
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If the dots are yellow, or red, the received signal may overdrive the ADC.
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Especially when the linear view shows a deformed circle (or even rectangle), reduce RX level.
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</p>
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<center><img src="display-spectrum.png"/></center>
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<p>
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Press 's' to get an ASCII art graph of received frequency spectrum.
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The spectrum's bandwidth is defined by the sample rate (-s) and not by the SDR sample rate!
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If you don't get nice peaks, but maybe several peaks, you might over-driven the input.
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Note that the peak will spread by the frequency deviation, so the peak might look noisy on the top.
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If the peak is quite low, check the input gain.
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</p>
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<p class="toppic">
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<a name="uhd"></a>
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UHD
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</p>
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<p>
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Most things apply to the things stated at the LimeSDR section.
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I have good experience, even without using IF offset or special resampling (--sdr-samplerate) or special bandwidth settings.
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</p>
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<p>
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You can run any UHD device using SoapySDR or UHD API.
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Use "--sdr-uhd" to use the UHD API.
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</p>
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<p>
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</p>
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<p class="toppic">
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<a name="mchan"></a>
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Multiple Channels
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</p>
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<p>
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Multiple channels can be useful, to keep the control channel, while establishing one or even multiple voice channels.
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One nice feature of SDR is that a spectrum is transmitted and not just one modulated carrier.
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Because the carrier is actually generated in software, we can generate multiple carriers that fit in our spectrum.
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I suggest to have at least one channel space between channels you want to generate.
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I suggest to use channel 1 and 3 or even channel 1 and 4 for NMT system.
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The software will notice you, if the channels wont fit into the spectrum.
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In this case increase the sample rate (-s) to generate a wider spectrum. (--sdr-samplerate must not need to be changed)
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</p>
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<p>
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Note that the CPU usage increases for the main thread.
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I suggest to monitor the usage using "top -H".
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You will see each thread. Be sure not to get too close to 100 percent.
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I suggest to stay below 60% for each threads.
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</p>
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<p>
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<b>Example: C-Netz</b><br>
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Two channes are created, one the OgK (control channel) and one SpK (speech channel).
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The distance between channels is 20 KHz (two channels).
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Because C-Netz uses only odd channel numbers for 10 KHz spacing, we use channel 131 (OgK) and 135 (SpK).
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</p>
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<pre>
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# cnetz --sdr-soapy \
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--sdr-rx-gain 50 \
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--sdr-tx-gain 30 \
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--sdr-samplerate 5000000 \
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-s 100000 \
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-k 131 -k 135 \
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-C 0,0
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</pre>
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<p>
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<b>Example: Test TV signal</b><br>
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Transmit PAL 'FUBK' test image on TV channel 21.
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</p>
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<pre>
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# osmotv --sdr-soapy \
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--sdr-tx-gain 60 \
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--sdr-bandwidth 60000000 \
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-r 13750000 \
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-c 21 \
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tx-fubk
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</pre>
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<hr><center>[<a href="index.html">Back to main page</a>]</center><hr>
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</td></tr></table></center>
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</body>
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</html>
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