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Receive (RX) Audio Cable
Radio RX Audio to Sound Card "Line In" or
Microphone Jack
For Receive (RX) audio from the radio, normally you will want to make
a cable to plug into the sound card's LINE IN jack. It can handle a radio's typical RX
audio voltage. If you need to use the sound card's more sensitive MIC jack, see below for a
schematic that adds in a voltage divider circuit.
In this cable, I recommend the use of an in-line transformer to
electrically separate the radio and computer grounds for two reasons:
 | to eliminate any potential hum on the signal |
| to reduce the risk of any damage to the devices from any difference in
voltage potential that might occur in the future |
I also recommend the use of shields on the cables to reduce the risk
of any RFI (Radio Frequency Interference) getting on the RX signal.
Here's the schematic:
 <-- click to
enlarge
To build your RX cable:
| 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. The shield can
act as ground line if you are using a transformer. If you aren't using a transformer,
attach the shield at the radio ground only, and not at the computer/sound card ground.
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| Sound card connector: You will need a plug that will fit the sound
card LINE IN/Microphone jack, probably a 1/8" (3.5mm) stereo
3 conductor male mini-plug (e.g. Radio Shack part #274-284). Do not use a mono 2
conductor plug.
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| Radio connector:
 | If you need your radio's "pinout" information, you'll
probably find it at Buck's packet site.
Remember that in addition to the RX line, you will also need to attach the TX and PTT lines to the radio, so all
three inputs could be sharing the same plug.
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| Mobiles or Base Radios: You can use
either the microphone connection or preferably the radio's special "data" plug
jack, if it has one. Or you can use the speaker jacks.
For Receive (RX) audio it's best to use the pin on the radio's data jack, since it
provides a constant audio output level and -- usually -- no squelch (AGWPE needs to hear
an un-squelched signal) . If you use the radio's microphone or speaker jack instead,
remember the audio output level will be affected by both the radio's "volume"
and "squelch" settings and not having these set correctly is often a source of
problems for AGWPE.
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| Handhelds: Most HTs have a 'Speaker' jack
that requires a 1/8" (3.5mm) mono (2 conductor) mini-plug. If your radio's user
manual says to use a stereo plug instead of a mono plug, be sure you do.
Check either Buck's packet site or Gloria's KA5ZTX site for
pinouts and any necessary resistor and capacitor required for the PTT and TX lines into
your HT.
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| Transformer: The type of transformer is determined by the source of
the RX audio. If you take it from the radio's microphone or data jack, you'll use a 600
to 600 Ohm (1:1) transformer. If you take it from the radio's speaker jack (8 Ohm
impedance), you'll need a 1000:8 transformer.
Isolation or audio transformers can be obtained from most electronics parts stores, e.g.
Radio Shack #273-1374 for a 1:1 or #273-1380 for a 1000:8. (Expert hint: for 1:1
transformers, Peter PE1MHO/G7ECN says another source is old internal modems that a
computer store might be glad to give you.)
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 plug. The other lead from the primary
coil will connect to the sleeve (outer conductor) of the sound card plug.
In this circuit, it doesn't matter which lead connects to the tip and which connects to
the sleeve, so long as they are from the same coil. (If you are using a 1000:8
transformer, it is important to connect the 1000 Ohm primary coil of the transformer to
the sound card and not the radio.)
On the secondary coil of the transformer, one lead will
attach to the RX pin of the radio. The other lead
will attach to radio ground. Again, it doesn't matter which
of the connects to the radio RX and which to radio ground, so long as they are from the
same coil. (Again, if you are using a 1000:8 transformer, it is important to connect the 8
Ohm secondary coil of the transformer to the radio and not the sound card.)
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| The middle conductor (ring) of the sound
card plug should be left unwired, unless you are building a RX cable for a second
radio.
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| When you're done, tape a "Line In" label on the sound card
end of the RX audio cable so that you don't confuse the RX and TX cables.
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If you want or need to use the sound card's Microphone jack instead
of the LINE IN jack, then you will probably need to add an attenuation circuit on the RX
audio line so that you do not overdrive the sound card. The exact attenuation will depend
on the maximum output voltage of your radio RX line and the
maximum input voltage on the MIC jack of your sound card. (The Sound Blaster card I have
has a maximum of 200 mVpp on the Microphone jack, where the Line In jack has a maximum of
2 Vp-p.)
Here's a schematic for a 10:1 attenuation circuit that should work fairly well in many setups. The 1:1 audio
transformer provides an electrical separation of the computer from the radio.
 <-- Click to enlarge
When you're done, tape a "Microphone" label on the sound
card end of this RX audio cable so you don't confuse it with the "Line in" or
"Line out" cables.
Also note this hint from George SV2AGW: If you use the MIC input, do
not use the BOOST function if your sound card offers it. You
can look for this in the Volume Control program
(double click on the  icon); look for an Advanced button under the
Microphone volume slider.
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Sound Card Packet
