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This commit is contained in:
Andreas Eversberg
2016-05-28 19:40:19 +02:00
parent 1d20119d85
commit 4e0e13cb2d
3 changed files with 31 additions and 12 deletions
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docs/c-netz.html
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@@ -59,10 +59,25 @@ We need to calibrate the speed of our transmit signal (and receive signal as wel
First of all, we need a reference clock.
Since most systems have standard crystals, we cannot use our system clock as reference without any help.
I decided to use the NTP daemon.
After running the system for one day, the clock speed should be accurate enough for our needs.
After running the system for several days, the clock speed should be accurate enough for our needs.
Be sure to choose a place with constant temperature.
</p>
<p>
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:
</p>
<pre>
server ntp1.ptb.de
server ntp2.ptb.de
server ntp3.ptb.de
</pre>
<p>
This clock provides time base for European radio clocks and most German clocks.
It is perfect for this project, so please use this clock.
</p>
<pre>
netbook ~ # ntptime
@@ -96,7 +111,7 @@ To minimize this effect, don't use extension cables.
<p>
Since we have precise system clock now, we can measure the speed of our sound adapter.
Later we will use the measurement result to adjust our signal processor.
Be sure to have your machine running for at least 24 hours (with your sound adapter attached).
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).
The machine must have network connection to the NTP server.
I suggest to use 'ntp1.ptb.de' and 'ntp2.ptb.de' as reference.
Use command line option '-M' or '--measure' to measure my sound adapter.
@@ -123,8 +138,7 @@ dsp.c:218 notice : Clock: RX=19.44 TX=19.49; Signal: RX=19.35 TX=19.43 ppm
One can see that the measured clock from the sound card (sample rate) differs about +19.45 PPM.
Because I did not do any correction to the signal processing, the same error applies to the signal.
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.
Let it run again for one hour and you will see that signal is now below 1 PPM.
(Or close to it.)
Let it run again for one hour and you will see that clock deviation is below 1 PPM.
</p>
<p>
@@ -142,13 +156,13 @@ Let it run again for one hour and you will see that signal is now below 1 PPM.
</p>
<p>
<font color="red">Importaint: We need to transmit correct polarity!</font>
<font color="red">Importaint: We need to transmit correct polarity, use auto polarity detection always!</font>
</p>
<center><img src="c-netz-burst_part.png"/></center>
<p>
C-Netz uses FSK (Frequency Shift Keying) by directly modulating the carrier, rather than modulating a transmitted tone.
C-Netz uses FSK (Frequency Shift Keying) by directly modulating the carrier, rather than modulating a tone, which is then transmitted via FM.
The advantage is much higher data rate.
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".
(The mobile station transmits reversed polarity.)
@@ -158,7 +172,7 @@ In the picture above, the polarity is wrong.
(The logical "1" points down and the logical "0" points up.)
You must check, with what polarity your transmitter is doing a positive frequency deviation.
Also you must check your sound card's polarity on positive value beeing sent.
To change polarty, use command line option '-F' or '--flip-polarity'.
To change polarty, use command line option '-F yes' or '--flip-polarity yes'.
(Note: don't confuse with '-P'!)
</p>