1 Introduction

The IRLP Node 6214 operated by AREG and maintained by Adrian VK5ZSN. The IRLP Node allows the AREG repeater VK5RSB to be connected to a large number of other voice repeaters across the world.

The resource requirements of IRLP are low, and older computer equipment can be utilised to run the Node.

For detailed information please visit the IRLP web site, www.irlp.net

2 IRLP Acceptable Use Policy

The IRLP node is free to use by all licensed Amateur Radio operators. The computer hardware, Internet bandwidth, mains power and repeater access are all provided at no charge by AREG members and other interested amateurs.

The only thing we ask in return is the users of IRLP Node 6214 abide by the AREG IRLP acceptable use policy.

Identify before sending DTMF commands.
If VK5RSB repeater is busy, ask before dialling a remote node.
Avoid using IRLP during drive times, 7am - 9am and 5pm - 7pm
Once finished using the IRLP Node, send a disconnect command (73)
Leave around 2 to 3 seconds between overs (allows for delay and lets other join in)

AREG wishes you enjoy using IRLP Node 6214.

3 IRLP Node 6214 DTMF Commands

The DTMF commands are used to control the IRLP node. Some commands are only supported by IRLP Node 6214.

Function	DTMF Command
Current Status of IRLP Node 6214	0
Disconnect an IRLP connection	73
Current Time in Adelaide	11
Dial a remote IRLP Node	xxxx

For extra information about operation and access to the IRLP Node please contact Adrian by email on

4 Installation of Node

The IRLP node was first installed at Adrian VK5ZSN QTH. It was set up using an spare computer using a Version 3 IRLP board. The node was set up as an experiment to see what all this hype was about with VOIP (Voice Over Internet Protocol).

The photo is of Adrian VK5ZSN and Kids with IRLP node 6214 in the back ground just after it was first set up.

Not long after the Node 6214 was operational that AREG found a larger number of people used the node. It was interesting to hear people dial in from all over the world.

During times of JOTA the Scouts use the IRLP Node over the entire weekend. In more recent times AREG has broadcast the audio from NASA TV using the IRLP node and one of the reflectors in the US that relays the audio.

4.1 Dedicated Repeater Link

As the IRLP node was first commissioned, a dedicated link was started being built to connect the IRLP node directly to the voice repeater.

The link radio equipment was modified and put into a couple of boxes. All that was left to do was wire it up. Then the age old problem happened, the members working on the project got too busy with work and other projects and the link is waiting to be finished. One day it will be I am sure.

4.2 Relocation of IRLP Node

The IRLP Node was relocated to Peter VK5TZX house, as he had spare bandwidth to run the IRLP Node. The computer was moved and installed on top of the computer rack in the back room of Peters house.

It was not long before the minister realised another computer had made it's way into the house. It was hoped that the IRLP node could be moved across to a low power computer based around a VIA C3 board.

A number of experiments were conducted by Adrian VK5ZSN and Peter VK5TZX, a problem existed with the IRLP software / audio codec and the VIA sound processor. The IRLP node would play audio fine, but would not put any audio to the internet. It was just a case the driver were not working on the VIA C3.

To cut a long story short, the two were not compatible sound was not reliable on the new computer. Until a new driver could be found the IRLP node would have to run on the old computer.

All that had to be done was wait, as the Fedor team released a new version, and earlier this year we got the VIA C3 board up and going. Now it is just a matter of finding enough time to finish the updated version of the IRLP Interface and install it in the new computer. Then we should be ready to swap the machines over.

5 Update & Repair of IRLP Node

The idea of putting a dedicated link between the voice repeater and the IRLP node had not disappeared. It had just been delayed. What was needed was a way for the IRLP node to be compatible with the AREG standard 15 pin link interface.

The only thing to do was to build an interface board, that would allow the IRLP audio and control lines to be converted to the AREG standard interface. It would not be to hard as all you would need is a couple of operational amplifiers and the balanced line receivers / transmitters.

Then the only problem was the AREG linking / repeater system using a flat audio approach. So the IRLP node would need to have de-emphasis and pre-emphasis built in as well.

The board was designed by Peter VK5TZX and a lot of work was done on the modelling of the de-emphasis and pre-emphasis networks. The frequency response of the shelving amplifiers can be see in the following diagrams.

De - emphasis Circuit Number

Shelving Amplifier Low Pass Design

As this circuit was tested on the IRLP node, it was found to have not enough cut in the audio being sent to other nodes. With some invesitigation I found the audio responce corner frequency should be around 300 Hz not the corner frequency of 3KHz. Changing the 1nF cap for a 10nF capacitor fixed the issue.

Of course checking the TIA/EIA 603 standard would quickly have told me what it should be. The only problem was I would have to purchase the standard to find out, at a cost of $300 AUD. In all I think it is close enought for the moment and maybe one day when I get a copy of the standard I will adjust the corner frequency a bit to fix the issue.

Pre - emphasis Circuits IRLP Interface

Inverting Shelving Amplifier High Pass Design

As you can see, the plots of the two amplifiers are opposite to each other and provide a good 50uS de-emphasis and pre-emphasis. However after some testing, the frequency responce was found to be incorrect, and not enough bost was provided by the filter at frequencies above 300Hz.

Once again I had got the frequencies wrong, the corner frequency should be 300Hz or close to, where I had made it 3KHz. A small change of the 1nF cap to a 10nF cap fixed the problem.

When I get a chance, I will load up P-Spice again and re-model the plots. Just last time I installed the copy I have it crashed my machine in such a way it was a F-Disk and start again from scratch.

Other aspects of the design also included electrical isolation between the computer and the radio of the IRLP interface. This was achieved by using opto couples and 600ohm to 600ohm transformers. The radio side of the IRLP interface is powered by the radio power supply, where as the computer side of the IRLP interface is powered by the computer power supply.

The 9 pin D type connector on the IRLP interface has be configured to match the 9 pin D type on the IRLP Version 3.0 Board, thus allowing a straight through cable to be used.

As most IBM compatible personal computers have at least some space inside the case. The IRLP interface can be mounted inside the case and wired to the back panel of the computer. One day when I get time I may get around to designing a surface mount PCB for this board. As it is a lot easier to make and not drill as many holes.

The following photo shows the IRLP interface PCB installed in side IRLP Node 6214 computer.

The power cable and the AREG Standard Interface cable run from the right hand side of the IRLP interface PCB to the rear of the computer.

The power cable has a 2 pin Ultilux male connector installed in the rear of the computer, a spare 12Volt DC feed from the radio power supply is plugged in. The 15 pin D type female is mounted close to the IRLP radio side DC power connector, for detail refer to the following picture.

The picture on the left shows the outside of the computer where as the picture on the right shows the inside of the computer.

The DTMF audio, SQL, PTT signals are normally feed into a Male 9 pin D type on the rear of the computer which connects to the IRLP 10 pin header. As the IRLP interface was being installed inside the computer. A new longer cable was manufactured using the same pin outs as the shorter cable and a direct connection between the 10 pin header on the IRLP Version 3.0 board to the IRLP Interface PCB.

The radio to be used for the link is a Motorola Syntrx. The radio required some modification to get it to work correctly on the required part of the 70cm spectrum.

The installation of a DX Interface PCB inside the Syntrx radio provides the AREG standard interface levels and control signals. The following picture shows detail on where the DX Interface was installed.

As the syntrx was a link type of radio, it came with a 15 pin D type female connector all ready installed on the front. A simple modification to the radio involving two 240 ohm resistors in parallel, turns on the radio with requiring a microphone plugged in.

5.1 Alignment of the DX Interface PCB

The DX interface PCB has four pot on the board. three volume controls on the board.

The radio alignment can take a bit of fiddling depending on what type of test equipment you can use.

The first step is to set up the receiver maximum audio level. Using a RF signal generator or service monitor ( or other equipment as required) generate a 5KHz deviated RF signal on the radio Rx freqency and modulate with a 1KHz tone.

Using suitable audio balanced line power meter (or dual channel oscilliscope) terminate the DX interface output into a 600 ohm load. The suitable audio blanced line power meter may have this function built in. Otherwise the dual chanel oscilliscope and two 1K2 1% resistors in parallel will do the job.

Adjust VR1 so that the audio level is 0dBm, that is 1mW into 600 ohms. Once this is set you can adjust the frequency of the audio tone and variy it from 50 Hz to 4KHz. Adjust VR2 for best flatness accross this freqency band. While adjusting VR2, you may need to re-adjust VR1 to ensure the audio power level at 1KHz is set at 0dBm.

Set up an balanced audio signal generator to generate a 0dBm signal terminated into 600ohms at 1KHz.

Once the audio signal is set up. Feed the audio signal into the TX audio port of the DX interface and make the radio Transmit. Using a RF deviation meter or service monitor measure the RF signal deviation.

Adjust VR3 so the RF signal has 5KHz of deviation. Then adjust the audio signal generator frequency and sweep the audio signal from 50Hz to 4KHz. Adjust VR4 for best flatness over the 50Hz to 4KHz freqency range.

The audio alignment of the radio is now complete. The maximum audio / deviation of the RF link radio has now been set up. Usually most audio will cause the radio to only deviate 3.5Khz. which equates to about -6dBm on the balanced audio system.

5.2 Alignment of the IRLP Interface PCB

The audio alignment of the IRLP node radio has all ready been done. This is required as you can either use the radio in circuit and measure deviation and generate a suitable signal, or you can use a suitable balanced line signal generator.

Set the audio level adjustment of the audio output from the sound card of the IRLP node to the middle of it's adjustment.

Configure the balanced audio power meter or dual chanel oscilliscope to measure the IRLP Interface output level. The test wav has voice and a sine wave signal on it. The part to measure is the sine wave signal. Adjust VR1 so when the sine wave signal is being played by the IRLP node the meaured output of the IRLP Interface is 0dBm.

This once again sets the maximum audio level of the system. While playing the audio test file you will notice the voice is around -6dBm in level with respect to the sine wave at 0dBm.

Set the audio level adjustment of the sound card for input audio to the middle of the adjustment range.

Using the IRLP Echo Reflector, and a balanced audio signal generator, input into the IRLP interface a 1KHz audio tone with a level of 0dBm.

Adjust VR2 so that the same level is played back by the node as is being input into it. It may take some stuffing around but you can get the node to play the same level back as is going into the node.

Once this is complete the audio level are set up on the node, the only thing left to do is adjust the DTMF audio level.

Using a DTMF tone generator on a radio tuned to the RX freqency of the link radio. Generate a 2K8 deviated RF signal and adjust the VR3 so the DTMF led lights on the IRLP I/O interface. With a lot of experimenting if the DTMF tone decoder starts to work at about 2K8 for the RF deviated signal, you will find that most people's DTMF will work fine. If you set the DTMF level too high, you can limit the head room you have avaiable in the DTMF decoder.

By now you should have the IRLP interface and link radio all alinged. Happy playing with IRLP.