It must have been in the early '70s that I became interested in data transmission over radio. My school for Electrial Engineering in Amsterdam (the ETS, a great school) supported my enthusiastic and gave me two Siemens T37 telex machines. Those things weighed a ton, and it was a hell of a job to get them going again, but after a lot of fiddling with audio filters and discriminators, I finally knew the converting alternating tones on the shortwaveinto legible writing. And vice versa! "RYRYRYRYRYRYRY CQ CQ CQ de PE1BOS" A little later I got access to a beautiful sky-blue Siemens T100S Telex, complete with punched tape writer/reader. After that it became a green teletype video display unit and a IBM EBCDIC keyboard that I converted to ASCII and - later still - the Apple II+ with its plug-in cards and various TNCs for AMTOR, FEQ and AX25.
We are now about forty years later and have picked up the idea to put things together again for the digital modes of WSJT-x, FT4 and FT8. The possibilities for designing and having a neat print produced have improved enormously in the meantime. Looking for a design on the internet for an audio interface to my old TS430, TS440 and TS50 Kenwood transceivers I ended up with the CM108 and CM119 chips from C-Media. So I ordered a few of those things from Aliexpress for a few euros each and designed a little PCB for them. And I have to say..., I'm very happy with the result. The thing presents itself neatly as an audio adapter in Windows 10 without the need for an additional driver and WSJT can also handle it very well.
I have set a few requirements for the design:
- Powered from the PC
- Completely galvanically isolated, so audio transformers in the transmission and reception path and an optocoupler for the (PTT) activation of the transmitter.
- Do not want to be dependent on the VOX function of the transmitter, but also no exotic drivers on the PC to address the GPIO outputs of the CM108/CM119. Instead, audio detection on the interface controlling the PTT.
- Some possibilities to adjust the level of the audio signals.
- Dimensions suitable to fit in an aluminum diecast box which I had available.
- A robust USB Type B female Jack, directly on the PCB on one side of the PCB, and a header on the other side to keep any RFI measures outside the PCB. It later turned out that there was no question of RFI.
In addition to the ICs, I also ordered a handful of 1:1 - 600 Ohm EL14 audio transformers from AliExpress (€ 2.80/10pcs)
The result can be seen below. The implementation did not cause any problems.
Some comments
For the audio detection I used a simple single rail dual opamp, a diode and N-Channel Mosfet. The time constants are chosen to be approximately 100uSec for switching on and 100msec for switching off. it doesn't need to be all that accurate.
The CM108/CM119 has the option to save the settings in an EEPROM. I have prepared the layout for this, but there is actually nothing to store and I did not have to place that part on the PCB for this application. But whoever wants it can!
Application as duotone generator for transmitter intermodulation measurements
Initially I only used the left audio channel to present the audio to the transmitter (R1 0 ohm and R2 omitted) but later it turned out that it was very useful to use both channels because it turned out to be a very handy tool to duo-tone transmitter intermodulation measurements. There are several freeware dual audio generator programs available and also MP3 files that produce two tones of equal amplitude and different pitches but all require stereo. By merging both channels in the interface (with R1 and R2 both 1k) it is a very nice way to adjust SSB exiters. A good reason in itself to assemble such a thing.
Schematic
