Saturday, October 1, 2011

Transmit (TX) Audio Cable

Transmit (TX) Audio Cable

Sound Card "Line Out" to Radio TX Audio
This page has advice for those building their own interface.
The TX audio cable can be a little more complicated then the RX audio cable. Normally, the TX audio cable must have a circuit to attenuate the voltage leaving the sound card's LINE OUT jack, otherwise the radio's transmit circuit will be overdriven.
Note: Use the LINE OUT jack, do not use the SPEAKER jack (which is found on some older sound cards). If your card or laptop only has a HEADPHONE jack, it can be used, but you will need to lower the sound card's TX audio volume. Quality will usually not be quite as good as a LINE OUT.
On the sound card, the typical color coding of the LINE OUT jack is light green
The exact attenuation will depend on both your radio and sound card. An approximation is a 100:1 attenuation (40 dB) which will reduce the sound card output level (max. 2 Volt p-p) down to the level your radio normally would expect for microphone  ( 20 -40 milliVolt p-p ). For example, a 50:1 attenuation works better for me, since I use my my radio's data jack which has a maximum input voltage of 40 mV p-p.
Don't Overdrive Your Radio!  If a sound card (or a TNC) overdrives a radio's input circuit (i.e. it is too loud), the radio may distort the true packet tones as it tries to limit the signal to a maximum deviation of approx. 5 kHz. The attenuation circuit described on this page attempts to prevent such overdriving and distortion. Interestingly, a 1200 baud packet station that is properly adjusted with 3.25 kHz of deviation (+/- .25) should sound softer than channel noise!
Here's a schematic for the attenuation circuit:

Option #1: Instead of the two resistors, you could use a 4K7 (4.7K) Ohm variable resistor (potentiometer/pot). The pot would actually give you more control over the attenuation, although a pot is bulkier.
Option #2: Or here's a schematic of a circuit that combines a pot with a fixed resistor to give the pot a less delicate range of settings and make is easier to adjust:

To build your TX cable:
  • Sound card connector: You will need a plug that will fit the sound card LINE OUT jack, probably a 1/8" (3.5mm) stereo 3 conductor male mini-plug  (e.g. Radio Shack part #274-284 -- pkg. of 2). Do not use a mono plug 2 conductor plug.

    Note: The middle conductor (ring) of the sound card plug (right channel) should be left unwired, unless you are building a TX cable for a second radio

  • Radio connector:
    • If you need your radio's "pinout" information, you'll probably find it at or  Buck's packet site (on Buck's site, go to the page for TNC-to-radio cables and find your radio. Click on the "Add to Cart" button for the cable for your radio and this will reveal a schematic of the radio's pin out information.)
      Does your radio have a 6-pin mini-DIN data jack?
      Then you might want to read this page about such jacks.
    • Mobiles or Base Radios: You can use either the microphone connection or preferably the radio's special "data" jack, if it has one. You'll  be using the same connector as the one used for your PTT and RX line, unless you're using the speaker jack for RX audio).

      Handhelds: TX audio will go to the radio Microphone jack, which usually require a 3/32" (2.5mm) stereo (3 conductor) or mono (2 conductor) sub-mini plug. If your radio's user manual says to use a stereo plug instead of a mono plug, be sure you do. You'll also be feeding your PTT line into this plug.

      Check your radio's instruction manual or either of the web sites mentioned above for pin-out information and any resistor and/or capacitor that may be required for the PTT and TX lines of your radio.
    • HF Band Use: Some radios may use different TX audio pins for HF and VHF/UHF. The ICOM 706 is one. Consult your radio's user manual for pin out instructions. This can be the solution if your interface transmits correctly for HF digital modes such as PSK31 but won't transmit audio on VHF/UHF, or vice versa
    • 9600 baud use: The newer 9600-ready radios might have a data jack pin for "9600 baud transmit", although many have both 1200 and 9600 on the same pin. The radios will however most probably have a menu choice for "data speed" that affects the radio's input voltage sensitivity, i.e. the menu selection may amplify or attenuate the TX audio. (Check your radio's user manual.) For example, the "9600" choice allows the pin to accept full 2V maximum, where "1200" accepts only approximately 40mV maximum. That means, to get things right, you will have to experiment with the:
      • radio's menu setting (1200 or 9600)
      • the sound card's playback volume setting
      • any voltage divider circuit or potentiometer that is in the TX audio line to get the right match.

  • Cable material: Use a cable with a single insulated wire and a braided shield. Small coaxial cable like RG-174U should work well; even RG-58U would work.
  • Capacitor: The value of capacitor C1, if used, is often specified in the radio's user manual in the section on MIC/Speaker accessory plug wiring. [The value will depend on the input type of the radio... resistive, capacitive, or direct coupling. Resistive will require a higher value cap (4 to 10 uF), while capacitive a bit less. Direct I/O are best coupled at 1 uF, as RF might become a problem if they go too high in capacitance.]
  • Transformers:  1:1 audio transformer can be obtained from either parts stores (for example, Radio Shack #273-1374) or Peter PE1MHO/G7ECN  suggests another source is old internal telephone modems that a computer store might be glad to give you.
    Note for 9600 baud packet: There's a general consensus that that the RX and TX audio lines in your interface must NOT have audio isolation transformers for 9600 operations. In general, the available small and inexpensive audio or power transformers do not have the correct frequency response range. For example, Radio Shack's #273-1374 600 ohm 1:1 audio line transformer has a 300 Hz to 5 kHz frequency response range which is not low enough.

    Until someone sends me an isolation circuit with suitable components (please!), take these precautions when running without isolation:
    • make sure the computer and and the transceiver's power supply are connected to the same AC power circuit (i.e. sharing the same 110/220 V ground).
    • connect/disconnect the sound card interface to the transceiver or computer only when both the transceiver and computer are unplugged from the AC power circuit.
    Attaching the transformer: Be sure you use the proper lead wires on the audio transformer as identified by any accompanying schematic for the transformer or by using an Ohm meter to determine the leads for the same coil (windings) of the transformer. Note that if the transformer has 6 leads, two will be center tap wires that won't be used. (Don't cut them until you know you have the 4 other leads correctly wired.)
One lead from the primary coil of the transformer will connect to the center conductor (tip) of the sound card LINE OUT plug by way of the pot/voltage divider circuit. The other lead from the primary coil will connect to the sleeve (outer conductor) of the sound card LINE OUT plug. In this circuit, it doesn't matter which transformer lead connects to the sound card LINE OUT and which connects to sound card ground, so long as they are from the same coil/winding of the transformer.
On the secondary coil of the transformer, one lead will attach to the TX pin of the radio.  The other lead will attach to radio ground.  Again, it doesn't matter which of the connects to the radio TX and which to radio ground, so long as they are from the same coil/winding of the transformer.
 I see that Buxcomm now offers a simple add-on isolation cable if you don't want to build your own. You simply plug in the Buxcomm isolation cable between your sound card and your audio in and your audio out lines. The item is called a ISOL8R and you can order it for $9.97 ($17.97 for 2) at
  • When you're done, tape a "Line Out " label on the sound card end of the TX audio cable so that you don't confuse the RX and TX cables.

Handheld Radios: Did you know?Most handheld radios use the microphone jack to feed both the TX audio signal (AC) and the PTT signal (DC) into the radio.
To make sure the TX audio signal goes into the radio and not back out the PTT line, a resistor is often put in the PTT line(AC will follow the path of least resistance).
And to make sure DC power doesn't go back up the TX audio line to ground, a capacitor is often put in the TX audio line(capacitors block the DC but allow AC to pass). If your handheld radio goes into constant PTT when the plug is put in its MIC jack, the lack of the capacitor is often the answer. (Note that the schematic above includes such a capacitor for just that reason. It's a good precaution to take to block any DC on the TX line.)
These components still need to be used in AGWPE cables for handheld radios.

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