New and Improved Sound Card Packet Interface ver2
now with CW  and tested with PSK31, and APRS

Most of the digital communications used in the amateur bands use some form of tone translation.  One tone will represent a high and another tone will represent a low, and by shifting between  tones data can be transmitted and received.  The data is then fed to a computer program that decodes the data in to understandable language or pictures.  Early on the tone decoding was done by a device called a TNC (terminal node controller), which is synonymous with a computer modem used on a wire line or telephone network.  In fact the early TNC were modified computer modems.  Now that most computers come with very advanced soundcards it is possible to connect your radio directly to your PC without a TNC by using the computers sound card and additional software.    The following circuit is a description of the circuit and software needed to make the connections between your computer and your radio.  I built this circuit to do APRS (automatic positioning and reporting system) on 144.390 MHz, and most recently have used it for PSK31 on 20 and 40 meters. Please refer to for a very good and complete discussion on sound card packet radio.

Please refer to the diagrams for the following procedure.

  1. Place the components on the circuit as shown in the “Parts Layout” diagram.  Make sure that the LEDs; capacitor, and transformers are positioned as shown.  The primary side of the transformers should be facing the top of the board.
  2. There are enough solider pads for each of the wires coming from the 9 pin serial port computer cable, however only 3 are needed (pin 7 RTS , pin 4 DTR and pin 5 ground). 
  3. There are 8 solder pads for terminating the plug that will connect to the radio’s microphone jack.  If your radio has fewer pins on the microphone plug then not all of the solder pads need to be used.  Be sure and connect the wires sequentially with the microphone plug pin 1 connected to the circuit board pin 1, etc.
  4. You will need to solder jumper wires between the place where the microphone plug cable solders to the circuit board and the solder pads for PTT, radio ground, audio in, and audio ground, which are marked on the circuit board.  These jumpers could be different for different radio microphone plugs and cables.  Check with your radio’s manual to identify which pins correspond to PTT, ground, audio in and audio ground.
  5. Optionally you can install a chassis mounted microphone jack if you want to be able to use the microphone without having to unplug the sound card interface.
  6.  Optionally you can install a jack for headphones.  This will allow you to listen to the packets as they are received and also allow you to use the radio for voice communications when not doing packet without having to unplug the sound card interface.  Your computer will need to be on for this to work.
  7. Set  R5 to its center position.  

 Setup and testing 


  1. Download “Sound Card Packet” document from  This is the comprehensive document on the subject of sound card packet.  It will walk you through the setup and use of the sound card.  It is also, a great resource for sound card packet software.
  2. Generally speaking this is the process you will go through to install, setup and use the sound card interface.  It is covered in great detail in the document mentioned above.   What follows is a summary:
    1. Download AGWPE from
    2. Install and setup AGWPE according to the instructions mentioned above.  This software is the packet sound card engine required by UI-View APRS software and some other sound card software.  It must be running before UI-View or other software that requires it is opened.
    3. If you want to do APRS packet the best software is UI-View16 (for computers <100 MHz) or UI-View32 (for computers >100 MHz).  Free download from
  1. Install the sound card interface
  2. Turn on the computer
  3. After the computer boots then turn on the radio.  Tune to an active digital mode frequency (144.390 for APRS)
  4. Open AGWPE if you are using a AGWPE dependent software like UI-View
  5. Open your software
  6. Read the help files…your done!  

PSK 31  (using MixW software)

    1. Download MixW software with the instructions from
    2. Install MixW software on your computer.
    3. Connect the soundcard interface between your computer and your radio. Both LEDs should light when the computer is "on".
    4. Turn on your computer and start the MixW software.
    5. Click on the speaker icon (bottom right corner of screen) and set your sound card levels.
    6. Set the "Line In" and "Master Volume" to mid-range.
    7. From the "Master Volume" page select "Options/Prosperities/Record" 
    8. Set the "Line Out" to minimum level. You can further adjust your audio-out using your radio volume control.
    9. Set your radio to display "ALC" (auto-limiting-control).
    10. Adjust R5 when you transmit so that ALC is "0". (you will need to readjust when you change frequencies).

Don't forget to read the MixW instructions!!

Foil side view
(3  13/16" x 2")

Reverse foil side view

Sound Card Packet Interface Parts List
  Circuit Components   Mouser   Ocean State
T1 Audio transformer (800 ohm - 8 ohm) 1 42TL08    
T2 Isolation transformer (800 ohm 600 ohm) 1 42TL016    
IC1 OPTO Coupler 1 551-MCT9001    
P1, P2, P3, P4 Stereo plug 3.5 mm 3 171-2035   30-488
P5 9 pin D connector 1 571-7479042   DS9F
R5 10K potentiometer 1 317-2080F-10k    
  hood for 9 pin D connector 1 156-2009   DE9
R1, R2, R3 resistor  1K ohm 1/4 W   1 271-1K    
R4 resistor 100 ohm 1/4 W 1 271-100    
M1 8 pin female mobile connector (microphone plug)** 1 specific to your radio   T700
M2 8 pin male mobile connector (microphone plug) chassis mount** 1 specific to your radio   P700
LED1 LED yellow 1 638-HLMP4719     
LED2 LED green 1 638-HLMP4740    
C1 4.7 mf capacitor   1 647-UVR1H4R7MDD     CER4.7-25  
J1 stereo jack 1 161-3504      
PCB printed circuit board 1 make using foil pattern    
  2 conductor cable ft 9     SMC222
  8 conductor cable ft 3     SMC822
  Misc mounting hardware        
  PCB making supplies        
  etailed information on sound card packet hookup and use    
  AGW Packet Engine (AGWPE.ZIP)      
  AGW Monitor (AGWMONITOR.ZIP)    
  AGWTerminal (AGWTERM.ZIP)    
  UI-View 16     Use if your PC is <100 MHz  
  UI-View-32     Use if your PC is >100 MHz  
  MixW software    
  ** specific to your radio the plugs shown are for an ICOM 1020A          

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Copyright ©,Jerry Baumeister