Soundcard Interface using 2N2222s

A Sound Card Interface

for FM Transceivers

Howard “Skip” Teller, KH6TYW hile looking for a way to enable

more people to use the Narrow Band

Emergency Messaging System (see

www.w1hkj.com/NBEMS/) on 2 meters, it

became clear that the lack of VOX in most

FM-only transceivers was a serious stumbling

block. That’s because NBEMS relies

on sound-card-based software. In addition to

creating the audio signal for transmission, the

software must also have access to a hardware

interface that would allow it to switch the FM

transceiver into transmit and back to receive.

You could use a commercial or home brew

sound card interface for this application, but

many of these interface devices require

computer serial (COM) ports to function. If

your computer lacks a COM port (most new

laptops don’t have them), you’d have to use a

USB port, which, in turn, requires a USB-to-

Serial converter to create a virtual serial port

for the digital software to use.

I decided to try an easier, more elegant

approach, one that would work not only

with NBEMS software, but also with other

 

sound-card modes such as DominoEX with

the free Fldigi software (www.w1hkj.com/

Fldigi.html). DominoEX in particular has

demonstrated intriguing performance when

used on VHF FM, rivaling even “weak signal”

SSB in some instances (see the sidebar

“Try “Weak Signal” Digital FM”). Of course,

you could also use this interface for soundcard

packet radio with AGW Packet Engine

(www.sv2agw.com/ham/agwpe.htm) software.

No COM or USB ports required!

Let the Audio do the Work

Most digital modes work by modulating

the transceiver with an audio tone, and that

tone can also be used to switch the transceiver

in and out of transmit automatically

by using a voice-operated switch (VOX)

circuit.

First the audio tone must be amplified

to get enough signal to detect and switch

a transistor for the transmit/receive line. In

order to amplify the tone, there needs to

be some convenient source of dc voltage

to power the amplifier. A review of the

schematic diagrams for modern transceivers

revealed that most have a voltage on the

push-to-talk (PTT) line that can power the

switching transistor. For those transceivers

that have a DTMF tone generator built into

the microphone, there is also 8 Vdc available

at the microphone jack, and this voltage can

be used to power the necessary amplifier as

shown in Figure 1.

2 June 2009

Try “Weak Signal” Digital FM

Here’s an application for your newly built interface that you and your friends

can try right away. All you need are ordinary 2-meter FM voice transceivers.

Tell everyone to go to www.w1hkj.com/Fldigi.html and download and

install Fldigi. This free multimode software package is available for both

Windows, MacOs and Linux operating systems. Once everyone has their

software running successfully, set up some times to meet on the air. One

suggested frequency is 145.00 MHz, simplex. (When selecting a frequency,

always follow the band plans that are in effect in your area and listen carefully

before transmitting.)

When you’re ready, fire up Fldigi, choose either the DominoEX8 or

DominoEX4 modes and start enjoying keyboard-to-keyboard text conversations.

What you’re likely to discover is that you can span a remarkable range

with this setup, much farther than FM voice alone.

If you really want to push the envelope, use 50 W FM transceivers (or add

“brick” amplifiers to the radios you are using now) and horizontally polarized

Yagi antennas (short 3 or 4 element Yagis will do). Depending on the terrain in

your area, you may find that you can have DominoEX chats over astonishing

distances! — Skip Teller, KH6TY

Figure 3 — A version of the VOX interface using the circuit board provided by

the author.

The transmit audio from the sound card

SPEAKER or LINE OUT jack is fed into a

600:600 Ω isolation transformer to eliminate

the possibility of hum or ground loops

between the computer and transceiver. The

isolated audio signal is passed through C1

and attenuated to microphone levels by R1

before reaching the transceiver microphone

input. A portion of the signal is also coupled

via C2 and R2 to the base of Q1, where it

is amplified to a level of several volts. This

ac voltage is peak detected by D1 and D2,

which form a voltage doubler, generating

enough voltage across C4 to cause the

base of Q2 to go high and switch the PTT

line to ground, thereby switching the transceiver

into transmit mode. When the tone is

stopped by the software, Q2 is turned off and

the transceiver returns to receive state.

Construction

There are only a few parts to the interface,

so one method is to use “ugly bug” construction

and handwire the circuit on a piece of

copper-clad circuit board material. A small

RadioShack project enclosure was used to

hold the interface circuit board, mount the

stereo audio input jack so it is insulated from

the circuit board, and protect the interface

circuitry (see Figure 2).

If you’d prefer a cleaner approach, a

fiberglass circuit board with plated-through

holes, parts legend and solder mask is

available (Figure 3). Send $5 with a selfaddressed,

stamped envelope to: KH6TY,

335 Plantation View Ln, Mount Pleasant,

SC 29464.

The interface terminates in a 4-wire

cable and a microphone connector to match

whatever transceiver is going to be used.

For transceivers using the plastic RJ-45 or

RJ-12 microphone jacks, existing CAT-5 or

telephone jumper cables can be cut in half,

eliminating the need to purchase a crimping

tool.

Installation

Installing the VOX interface couldn’t

be easier. A standard stereo audio patch

cable is connected between the sound card

SPEAKER or LINE OUT jack and the stereo

jack on the interface, and the interface

microphone connector is plugged into the

transceiver microphone jack. For receive

audio, a separate cable must be connected

between the sound card MICROPHONE or

LINE IN jack and the transceiver earphone/

external speaker jack.

Using the Windows Volume Control

panel, the WAVE and VOLUME CONTROL

sliders are adjusted while running the software

in the transmit or calibrate mode until

the transceiver goes into transmit, and then

raised a little higher. This should provide the

right level of audio for the transceiver. If the

level is too low, R1 can be reduced in value

to increase the audio drive to the transceiver.

Although I designed this circuit for digital

operating with FM transceivers on the

VHF and UHF bands, nothing would stop

you from putting this interface to work for

HF digital as well. All you need to do is supply

5 to 14 V dc for the amplifier stage.

Howard (“Skip”) Teller, KH6TY, is an ARRL

member and was first licensed in 1954. He

received his commercial First Class

Radiotelephone license in 1959 and worked his

way through college as chief engineer of several

radio stations. He holds a BS degree in electrical

engineering from the University of South

Carolina and is retired from running a factory in

Taiwan, where he manufactured the weather

alert radio that he originated in 1974 and is still

sold by RadioShack and many other companies.

Skip enjoys developing digital software, such as

DigiPan and NBEMS, designing 2 meter transceivers

and antennas. He is currently studying

the potential of working 2 meter DX on FM using

digital modes. You can contact Skip at 335

Plantation View Ln, Mt Pleasant, SC 29464;

KH6TY@comcast.net.

Done