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Andreas Eversberg
2016-12-06 17:45:48 +01:00
parent 27c6e3d860
commit eed826387a
5 changed files with 79 additions and 12 deletions
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@@ -32,6 +32,7 @@ Dr. Martin Cooper produced the first handheld phone "DynaTAC8000x", the famous B
</p>
<center><img src="amps-dynatac.jpg"/></center>
<center><a href="https://en.wikipedia.org/wiki/Motorola_DynaTAC">Image taken from Wikipedia</a></center>
<p>
<ul>
@@ -57,11 +58,10 @@ Dr. Martin Cooper produced the first handheld phone "DynaTAC8000x", the famous B
</p>
<p>
The detailed communication between base station and mobile station is described in <a href="TIA_EIA_553_A.compressed.pdf">TIA/EIA-553 standard</a>. Click for reading the PDF file. <i>Reproduced under written permission from Telecommunications Industry Association.</i>
</p>
<p>
*TBD*
The detailed communication between base station and mobile station is described in <a href="TIA_EIA_553_A.compressed.pdf">TIA/EIA-553 standard</a>.
Click for reading the PDF file.
Thanx to TIA for allowing me to add the standard document to this project's documentation.
<i>Reproduced under written permission from Telecommunications Industry Association.</i>
</p>
<p>
@@ -81,7 +81,43 @@ Setup of a base station
</p>
<p>
*TBD*
<font color="red">Importaint: We need to connect to PLL and discriminator!</font>
</p>
<p>
AMPS does not use a modulated tone to carry bits of information, instead it modulates the radio carrier directly.
This image shows a signal generated by this software, which is similar to a received signal from the phone:
</p>
<center><img src="amps-burst.png"/></center>
<p>
Generally a receiver will do de-emphasis and filtering after demodulating the signal.
We don't want that.
It may work, but be sure to use a receiver with discriminator output.
This output gives direct signal of the frequency deviation of the received signal.
The signal on the sound card represents the frequency shift of the received signal.
Many receivers can be modified, so search the Web for more info.
</p>
<p>
On the transmitter we don't want to have filtering and pre-emphasis.
It may work, but be sure to use a transmitter that allows to disable emphasis.
I took the schematics of my transmitter and connected the sound card directly to the PLL.
The transmitter will now shift frequency according the the level of the sound card.
</p>
<p>
<font color="red">Importaint: We need to transmit correct polarity, use auto polarity detection always!</font>
</p>
<p>
In my case, my sound card is connected to a transmitter, where a positive sample value causes a negative frequency deviation.
This is why I need to flip the polarity of the signal, so I generate a negative sample value that causes a positive frequency deviation.
I use the '-f yes' option or '--flip-polarity yes'.
Also it is essential to use a transmitter with direct access to the modulator and and a receiver with direct access to the disciminator.
I use '-p -d' or '--pre-emphasis --de-emphasis' to process emphasis for audio signal in software.
I use the calling channel (control channel) 334, which could also be used as traffic channel (voice channel).
</p>
<pre>
@@ -113,6 +149,13 @@ The actual level is not yet relevant.
(You may check the quality using '-l 2' or '--loopback 2' command line option and build a radio loop by tuning the receiver to the transmitter.)
</p>
<p>
If the polarity is correct and the volume of the signal (frequency deviation) is about right, you will make the phone register.
The phone will transmit on the uplink.
If not, try other polarity.
This is how my phone registers to my network:
</p>
<pre>
frame.c:3630 info : RX RECC: DCC=0