TNC and Modems ruled the world of digital communications until
recently. With the advent of high speed computing and digital signal
processing (DSP) all this has changed. If you own a home computer
(PC) with a sound card, all you need to do is download software and
obtain an interface between the PC and your radio.
The interface provides the audio and digital signal level translation
between the computer and the radio. Due to lack of locally available
interfaces many hams imported them for large sums of money. It
occurred to me that I could build an interface easily.
Typically being lazy, I needed a push to get started. A swift kick in
the rear would be a more appropriate term. This was provided by
Miku, VU2WAP. He offered me his Rigblaster Nomic for reverse
engineering.
Looking inside I found a 600 ohm 1:1 audio isolation transformer
for PC to transceiver mic input and presets for level control An
optocoupler was used for PTT keying.
On the outside there was an 8 pin mic connector or RJ45 optionally.
Stereo earphone (EP) sockets for soundcard connections. A DB9
connector for RS232 PC COM port connection. To adapt to various
brands they provided internal jumpers.
Now to make the VU version..
Fig 1. Circuit Diagram
The isolation transformer was the most critical part of the
equipment. It was the hot topic of discussion with my friends on
14.130Mhz including Zal VU2DK, VU2DAD Dev VU2NTT Neeraj,
VU2WAP Miku. This transformer is not commercially available in the
electronics market. Initially I used one salvaged from modem cards
but this was not acceptable if many units were to be made. I
designed one taking many parameters into consideration and had it
fabricated locally.
If you want to make your own it is simple, buy a driver transformer
used in 6V L type transistor radio plate. Dissemble and strip its
windings. Wind 1500 turns primary and 1500 turns secondary with
44 SWG wire. Assemble it back. Purist may also add a layer
between the two windings and ground one end to make a Faraday
shield.
The next item was the opto coupler 4N33 with Darlington pair
transistors in its output. (The 4N35 is more commonly available, it
can be used with an additional 2N2222 to boost the output current
and gain.)
Considering that we are ham fisted and the VU market has flimsy EP
connectors, I decided on using commercially available 6 x RCA
sockets used in VCD players. The 8 pin Mic connector was replaced
with a more readily available PC keyboard connector 5 pin DIN. The
9 pin D shell is common so I retained it. Instead of PCB jumpers, I
left it to the home brewer to hardwire the connections with shielded
wire. i.e.
1. Mic and Ground
2. PTT and Ground .
With shield going to respective grounds. As I feel this method is
simpler and every home brewer will at least have this much
information about his rig.
Do not be tempted in using keyboard lead with 5 pin DIN connector
molded , these are prone to Hum pickup. Use two lengths of single
core shielded wire for respective connections.
Finally the PCB..
Fig 2.PCB design
The drawing has all the details of PCB, schematics and component layout. Assemble it according to details provided. Take care with
polarized components like diodes optocoupler etc. Mount all sockets
on the PCB. Finally dont forget to put the PCB in a good looking box
if you want everyone to appreciate your handy work.
Fig 3. Components lay out
You do not need this interface to receive. Simply hook up a lead from your rig to PC sound card and start receiving.
Connect the line in on Interface to out of sound card. Connect PC
speakers to line out on interface. Optionally you may also connect
your headphones to phones socket on interface. Keep Mic level
control at 75% level with some spare margin to adjust.
Check all your work carefully, hook up the interface to your rig and
PC. Install the software switch on and tune, adjust the sound levels
on PC as well as the interface for clean signal. Monitor various
QSOs till you get the hang of things. Join the fun.
With the unit supporting so many modes like PSK31, MFSK16,
MT16, Hellschriber, SSTV, RTTY,AMTOR, PACTOR, PACKET, CW and
over the net Echolink and Echostation.
Its time to conquer new digital domains for The world is not
enough!
