Entire DAPNet transmitter

After a lot of planning, collecting, drilling and sawing it’s finally operational: a POCSAG transmitter for DAPNet, with integrated powersupply, Raspberry and amplifier in a 19″ rackmount.

 

Power supply

To keep everything compact I’ve decided to integrate a switched power supply in the 19″ enclosure. Which means I have to get the 220V AC into the case in a safe way. I’ve did this by using a C-14 bulkhead socket with integrated powerswitch and a fuse holder. Using short 1.5mm2 wires with connectors the incoming mains is led to the bipolar switch. The power supply then is connected to the switch by similar wires. This power supply is a small 12V / 10A piece you can buy almost anywhere of little money. Finally the 12V lead contains a 6A fuse before it reaches the power rails.

In the 19″ enclosure a couple of components need 12V:

  • fan
  • data transmitter
  • amplifier
  • Raspberry power supply

These components get their 12V via two power rails to keep the wiring nice and tidy.

As the Raspberry Pi runs on 5V I’ve found a power converter (online) to transform 12V into the required voltage, with a currant of max. 3A. Really sufficient for the RPi.

Raspberry Pi

The RPi has been built into the rackmount without own enclosure. As explained above the power is provided by a 12V to 5V converter with a micro USB connector attached to the power lead. The ethernet port has been led to a bulkhead pass through using a short patch cable.

There are several POCSAG encoder options. The easiest solution is to use the sound interface of the RPi. For my transmitter I’ve choosen to use the ‘C9000-interface‘ based on an Atmel chip. The DAPNet team in DL has developped this board to keep using the old Ericsson C9000-zenders. Nevertheless this encoder can be used with other FSK transmitters after minimal modification.

A small modification is needed for the transistor circuit which keys the FSK signal. As this switches between 0V and 5V, the data transceiver I’ve used as way too much deviation. Therefore I’ve put a 100 Ohm resistor between emitter and collector. This voltage devider then limits the deviation of the RF signal. The best solution of course is to use a trimpot.

The SD card for the RPi will first get a fresh Raspbian image and then:

  • activation of SSH and eventually VNC is done via raspi-config;
  • all updates have to be ran;
  • ‘git’ should be installed;
  • ‘unipager’ needs to be installed (description can be found here);

Important:

The second last paragraph of the installation instructions contains two crucial steps:

  • file /boot/config.txt –> add: enable_uart=1
  • file /boot/cmdline.txt –> remove: console=ttyXXX,115200
  • reboot

In case you skip these steps, the transmitter can start uncontrolled transmissions. This does not apply in case you use the Raspager v1 or audio interface.

RF part

The transmitter used is a Chinese Friendcom FC301/D data transceiver. With a PC frequencies and output power can be programmed. Via a DB9 connector the power supply, PTT and data signal are fed. Because the pins for the 12V power supply are really close to the data pins, it’s a good idea to isolate both power pins with some heatshrink.

The data transceiver has a max of 5W output power, enough for local usage. Nevertheless I’ve attached an amplifier by DF2FQ, which delivers about 40W output getting 5W in. The amplifier has been mounted on a huge cooling block, because this kind of amps is also likely to be used for packet radio. But because the POCSAG transmissions have a very limited duty cycle, this block is way too large.

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