Rtty interface (FSK/AFSK) with sound cards
Or how to remove the so called "ground loop" and the
reentry in the RTTY broadcast and make radio and PC
happily coexist One of the
problems that affects our digital broadcasts which make
use of audio cards, as for instance RTTY and PSK 31, is
the reentry of the signal between RTX and the PC. These
reentries often block the keyboard, make it impossible
to get back to RX, send out "dirty" signals and so
on.
A solution in these cases is to lower gradually
the power, until the problem is over; anyhow this is not
the ultimate solution, and in some cases you can put it
into practice only by getting down to few watts. To get
rid of the problem efficiently and successfully, you
should isolate electrically both the RTX and the PC.
Let’s see how:
The minimum kit in order to transmit
digitally is the following:
- A cable connecting the audio output (headphones,
aux, phone-patch...) to the "line in" socket of the
sound blaster
- A cable between the mike connector (or aux socket)
and the "line out" socket of the sound blaster
"line-out"
 Both should
be electrically interrupted by a small audio
transformer, like those used in the am/fm portable radio
sets.
If you want to transmit by FSK and use the PTT
operated via serial port, you should use 2 more cables
between the PC serial and RTX.
The home-site of the
excellent software MMTTY (by JE3HHT) provides the
transistor based diagrams for the interface, which
though do not isolate radio and computer.
In this
case the solution I have adopted to separate RTX/PC is
to use 2 optocouplers instead of transistors:
- The first being used for the PTT commutation
- The second being used for FSK transmission (for
the RTX that allow it)
Here is the complete diagram of the interface I have
realized for the TS940 with the precious
contribution of Vincenzo IW3FOL and Vittorio I3VFJ.
CAUTION: Though the circuit works
perfectly, I don't take up any responsibility about the
possible damages the diagram might cause to the PC,
radio or any other item.
This is the component's list:
AFSK (TX) side
- C1 - 100 nF Capacitor
- P1 - 4k7 ohm Potentiometer
- T1 - Audio transformer (1:1)
FSK (TX) side
- R2 - 560 Ohm 1/4 Watt Resistor
- OK2 - 4N32 Optocoupler
- LED2 - Led Diode
A/FSK (RX) + PTT side
- T2 - Audio transformer (1:1)
- R1 - 560 Ohm 1/4 Watt resistor
- OK1 - 4N32 Optocoupler
- LED1 - Led Diode
PIN's
list
| diagram's Pin |
On generic radio |
On TS940 |
| A |
MIC |
PIN 1 (MIC) |
| B |
GND (MIC) |
PIN 7 (MIC) |
| C |
Radio data output |
PIN 3 (ACC2) |
| D |
PTT |
PIN 13 (ACC2) |
| E |
GND |
PIN 4+12 (ACC2) |
| F |
FSK |
(FSK) |
| G |
FSK |
(FSK) |
| diagram's Pin |
On PC (sound card) |
| H |
Line Out |
| I |
Line Out |
| J |
Line In |
| K |
Line In |
| diagram's Pin |
On PC (COM DB9) |
| L |
PIN 7 (RTS) |
| M |
PIN 5 (GND) |
| N |
PIN 3 (TXD) |
NOTICE: The circuit
described here works perfectly on Kenwood TS940, whose
transmission circuit FSK is reverse operated
compared to the majority of other RTX: for use with
other radio sets you will have to slightly modify (only
for the FSK (TX) part) the circuit by connecting the
OK2’s pin 1 (including R2 and LED2) to the "N" contact
instead of the "M" contact (see diagram) and the pin 2
to the "M" contact of PC’s COM port.
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Versione in Italiano
