I am a big fan of digital modes, as such I needed a TNC or sound card interface. Commercial versions are expensive and most amateur versions use a serial port. So I built my own using the USB port and USB sound card. I am now working on a new version, Its still in development here : http://www.m0zpk.co.uk/datainterface3/ Here is the schematic, its a little big for the page. so might be difficult to read. You can click on it for a bigger version.
Diodes : D1, D2, D3 1N4148
Resistors : R1 47K R2 330K R3 4K7 R4 330 Ohm R5 330 Ohm R6 4K7
10 K Variable Pot X 2
50 K Variable Pot
Capacitors : C1, C2, C3, C5 1 uF ( not critical, could uses 2,2 uF or even 4.7 uF) C4 0.1 uF
Transistors : 3 x 2N3904
IC : NE555 Opto Isolator
Other : Switch Audio Isolation Transformers Plugs / sockets. USB Sound card.
I wanted something that I could build easily but would still be usable in all the software I liked and usable on different operating systems. I also wanted to build it from bits I had in my bits box. So the components are not too critical. Its built up of circuits that have been around a long time, with slight tweaks for this purpose. I have been using it for a year with no problems. A few people have asked me for the schematic, wanting to build their own. So I have put it on here. I have split the circuit up into sections, this makes it easier for people to understand and build.
Isolation This is the beginning of our Interface. It takes audio from a computers speaker output and plays audio from the Radio into the computers microphone or line in. I have added isolation transformers between the computer and circuit. This is to prevent damage to the computer and radio if anything goes wrong. It also offers some impedance balancing. The audio isolation transformers can be brought from ebay or an electronics supplier like maplin for a few pound. You will also notice two potentiometers AUDIO VOL TO Radio and Audio VOL FROM RADIO. These let us control the audio levels going into the Radio and into the PC. I use a usb sound card. I purchased it from ebay costing Â£1.50. It sits within the Interface box. When connected to the computer it comes up as a new sound card. This means we only have one connection to the computer a single USB lead. This provides power and audio.
Building the Circuit The above image is prototyping board, I have built the circuit on it, to help demonstrate. I have drawn coloured boxes to indicate sections of the circuit, that I will cover below. The red box is a common emitter amplifier. The blue box is a rectifier, the yellow box is a transistor switch. The circuit is powered with 5 Volts DC.
Common emitter amplifier
The first part of the circuit is a common emitter amplifier, which has high input impedance and low output Impedance. This does two things for the circuit. It helps to separate the PC from the rest of the circuit and it amplifies the audio. This type of amplifier also changes the phase of the signal. The output will be 180 degrees out of phase. This is because, as the base voltage rises, the collector current increases and the collector voltage decreases and vice versa.
What does a phase change look like ?
The above can be a little difficult to understand at first, especially if you are new to electronics. So I will use an oscilloscope to help show what’s happening. The above image shows the circuit built on prototyping board, there are two test leads ( grey ) the first is at the start of the circuit, the second is at the end of the common emitter amplifier. The above image is the display from my oscilloscope. The red trace is the input. A 2 kHz sine wave from a function generator. The yellow trace is the result of the common emitter amplifier. You can see that the signal has become a square wave. This is because the transistor has been driven over its normal working range. This causes distortion, which consists of harmonics and is normally not desired in an audio amplifier. For us its perfect, because we need as much DC voltage as possible to drive transistors and switch the Radio to TX. You can also see the phase change, when the red trace is high the yellow trace is low and vice versa. The above image shows the two traces over each other. I have done this to help illustrate the phase change that happens with common emitter amplifiers.
So you have built this section, how can you know its working ?
If you have an oscilloscope, you can do the same as I have done above. If not we will have to use a less precise method. For this to work we need an audio input. This can be achieved by playing audio from a pc speaker and connecting the wires to the start of the circuit. We are going to test some voltages to see if our circuit is working correctly. The above image shows a multi Meter. The common ( black ) lead is connected to the circuit ground. The test ( red ) lead is held on the collector of the transistor. This should show a voltage of DC 2.26 Volts.
Rectifier This section of our circuit is an AC rectifier. It starts with a 1 uF capacitor. This is to block DC voltage and only allow the AC audio signal. Following this is two diodes. These change the varying AC waveform into a constant DC voltage. This section finishes with a 1 uF capacitor in parallel, this is to help smooth the DC voltage.
How do we test it ?
Looking at a DC waveform on an oscilloscope is not all that exciting. So I have omitted it. We will check the voltage with a Multi Meter. Again we have the common ( black ) lead on the circuit ground. The red ( test ) lead is placed on the resistor that leads to the next section. The meter should show around 1.57 volts DC.
Transistor Switch This is the third section of the circuit. It takes the DC voltage from the previous section and switches two transistors on. It starts with a 4K7 resistor. This is to limit the current and prevent damage to the transistors. This value can be reduced but should not be increased. The first transistor turns on a Red LED, to show that the circuit is activated. The second transistor turns on the next section, which is a 555 based timer. I have included a switch, which can be used to prevent the radio being put in PTT.
How do we test it ?
The above image shows this section under test. The common ( black ) lead is on the circuit ground. The red ( test ) lead is on the base of the transistors. It shows 0.65 volts. This value can go higher and a little lower.
NE555 based Timer
The VOX circuit is quite reactive. So it will only trigger the radio into transmit mode while there is audio. As soon as it stops the radio goes back to RX. This can be a problem with some modes. Like JT65, CW etc. Causing the radio to flick in and out of TX mode continuously. So I have added a 555 based Monostable delay. This circuit has an adjustable delay from around 500 ms to 1 second. Achieved by adjusting the 50K potentiometer. It is automatically triggered by the VOX circuit and will keep the radio transmitting while the pc is playing audio and continue once it stops for the pre-set time. It makes the Interface much more useful. This part of the circuit is how we actually put the radio into transmit mode. On most there is a TX line which needs to be pulled to ground. I have used an opto-isolator. This is triggered by the delay circuit. I have also added a PTT inhibit switch. Just in case things go wrong or we want to prevent the TNC from triggering the radio. The above image shows sections 1 through 4. The Red LED should now light when ever audio is played into the circuit. You will also see that as soon as you stop the audio, the Red LED goes out. The 555 timer will continue to keep the radio in TX mode for an amount of time, this is set by adjusting the variable resistor. ( under the yellow wires to the lower right ) If you would like to see me, making my version, see here : http://www.m0zpk.co.uk/category/usb-data/
Connecting this Interface to a PC
I have chosen to use a USB sound card, this enables me to plug it into any operating system that supports USB sound. Windows, Linux, Mac etc, even android and Ipad. if you don’t want to do this, you can create yours with two fly leads that plug into what ever audio source you desire.
Connecting this TNC to a Radio
There are too many different Radio configurations for me to cover them all. I would also advise you to do some research to find out exactly what connections you need to make. I will show some common ones below. First of all, lets look at the connections from our circuit. This can be seen above. We have :- Radio Audio Out : This is the audio that the radio has received on air. Radio Audio IN : This is the Audio from the PC, that we would like the Radio to transmit. Radio TX Pin : This is a Pin that we can pull to ground to make the radio TX. Radio Ground : This is the radio Ground. Icom, Yaesu. 6 Pin Mini-Din
This is a 6 pin Mini-din data jack. It is used on a lot of Radio’s from Icom and Yaesu. The pin out is normally the same but you should check !
What are the connections ?
From the TNC | From the Radio Radio Audio Out | DATA OUT 1200 bps Radio Audio IN | DATA IN Radio TX Pin | PTT Radio Ground | GND On most there are two pins for audio out, 1200 bps and 9600 bps. You should use 1200 bps ! The other one bypasses some internal filtering and limiting, which can cause lots of problems, so if your not sure, stick to 1200 bps.
Powering this Interface
This Interface is powered from the USB port. It draws very little current in standby and use. If your worried about using the USB as a power source. you can replace it with a battery. The battery positive connects to where the USB 5V would connect. The battery negative connects to the Interface ground. a 9V PP3 battery should last a very long time. You could also power it from the radio, most have a 5v or 8v output on the data, microphone or control port. These methods have a bonus of keeping the pc and radio completely separate. This can help to prevent RF getting into the pc and ground loops.
You shouldn’t have any problems. One thing to watch out for is stray RF. If your antenna system has a problem, this will cause the RF to find its way onto the exterior of the feeder and the case of the radio. This can then find its way into the Interface. If this is a problem or you think it might be, add a switch so you can either power it from USB or a 9V battery. Th 9V battery option will stop the RF getting into the computer and causing damage. You may run into this type of problem on field days. Another problem that may arise is a ground loop. This is where the USB ground is at a different potential to the radio ground. We think of ground as 0 V this is often not the case. When we measure voltage. We actually measure potential difference, the difference between the ground and power line. So the ground could easily be 0.5V and the power line be 9.5 V. We would then get a reading of 9V. This is normal. The problems arise when we connect two circuit grounds together. One could be 0V and the other 0.5V or any non identical values. This causes problems. In audio equipment it may present itself as a hum, squeal or hiss. In more complex equipment it can cause problems for micro-controllers or digital electronics. Amplifier biasing etc. If you think a ground loop might be your problem. go with the 9v battery option. I have rushed the description a little and will come back and improve it when I get time. I hope this is of help to people. If you use it. please send me an e-mail ! I cant accept any responsibility for how you build or use this circuit and any damage it may cause accordingly. I built it and have never had a problem. Hopefully yours will be the same.