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Friday, 11 December 2020

Ham radio digital modes interface.

Many moons ago I built an interface to connect my computer to my IC-706IIG transceiver so I could operate a digital mode, PSK31.
I don't do much operating really, but quite fancy coupling my new Raspberry Pi400 up to a radio and having a go with another digital mode, FT8 (I know, I'm a bit late to the party.)

So, the plan is to re-jig my interface and up-grade it a bit. 

First things first, is to work out what I did originally !

It's all a bit of a lash up of parts. 

There's a stipped out USB hub, which has 2 USB to serial adaptors plugged in, and a really cheap USB sound card connected. 

The sound card mic and headphone sockets feed two pots on the front panel to control RX and TX level..



... and onto this PCB, where the audio is isolated by two transformers and connected to the transceiver via a 9-Pin D.










One of the USB to serial converters is just gunged to the back panel. It's the socket on the right. This was originally interfaced to the transceiver to control it's frequency. It's redundant now, as I have USB CAT control leads. 





The other USB to serial converter feeds two optoisolators on the PCB, which are used as KEY or PTT to tell the transmitter to go into transmit. Why two lines to do the same task? Practically speaking, there's no difference between KEY and PTT, except KEY is traditionally meant to mean a morse code or telegraph key, and PTT or "Push to talk" for voice modes. Software generally requires one or the other, rarely (if ever) both, and is generally configurable in your chosen software package anyway, so the RTS or DTR of the serial port can be specified to trigger transmit, as either PTT or KEY. 

A decision is made to rebuild. I want to upgrade the little USB soundcard in there, as it's not a great performer. After doing a bit of research, it seems the Sabrant USB soundcard rates well, and is only £9.99 from Amazon. As pointed out above, there's little point in having two PTT and or KEY isolators, so I'll make one, and make it switchable between a DTR and RTS, and output as either PTT or KEY to two separate jacks.

Both the serial converters in the box can go. I will use an FTDI board to control the PTT, which has the added benefit of only having a 5v output, rather than +/-15V (although I somehow doubt the original RS232 converters actually did this, I may be wrong!). Also it will provide convenient access to a +5V supply to run a level monitoring circuit.

The FTDI board is wired up to give a 5V supply, GND, RTS and DTR outputs.

After a couple of false starts, it turns out that the RTS and DTR signals are inverted, active low, but this was resolved by re-wiring the opto isolator, using the FTDI to sink the current. Whilst this seemed like a good idea at the time, something rang alarm bells with me... a check of the datasheet shows I can only sink 6mA, so I use a couple of BC547 transistors to invert the output and drive the optocoupler. 

More false starts... lord only knows what the isolation transformers I have are, but they're not the right thing at all  (did the original box ever work at all? I'm damned if I can remember!). 


I order some small 1:1 600 ohm isolation transformers from eBay








and conjure up a schematic in Kicad ...
I've changed the BC547 transistors to their smd brothers, to maximise the available space. It actually turned out a bit smaller than needed, but that's no bad thing...



Because (this time) our transformers are 1:1, we may well need to add a bit of attenuation into the signal. This is provided by the four SIT (suit in test) resistors R1, R5 ( transmit audio) and R11, R12 for received audio. RV2 is going to be mounted on the front panel, so it connects via a three pin header. I also plan to connect another front panel pot before the input at J1, it's the same deal as RV2. You may notice the RTS and DTR lines are coupled together. I'll feed one or the other from a change over switch. 

May as well design a board ...















I do love Kicad's 3d modelling. It's not just pretty, its great for polishing the layout of labels etc..






The gerbers are sent to JLC PCB for manufacture, and after a little wait for delivery, arrive. 


















A little while is spent mounting the components. 
The four SIT resistors are set for no attenuation to start with. (R11 & R1 S/C, R12 & R5 O/C)









... and as usual, an error is spotted... thankfully it's just on the legend for the FTDI, the 5V and GND are reversed! 








It's all wired up into the enclosure... I'll tidy it up later, after we're happy with the performance...










WSJT-X is loaded up onto the Raspberry Pi400, and set up, the interface connected to my FT-817, and we're decoding!







The two trimmers are adjusted on the PCB, to give an indication of level on the LED indicators...

Incidentally, my friend Pyers 3D printed the front and rear panels for me.













There's an issue though... the FT-817 won't transmit :( 

The PTT is working just fine, but there's no modulation.




A fair amount of time went into checking and re-checking cables, and audio levels, then caution was thrown to the wind, and the FT-817 gets it's first ever factory reset in the many years I've owned it... after resetting menu 26 to USER-U , it bursts into life and transmits. 

A few minutes later and our first QSO on 2m (144MHz) is complete, with Stuart, G0LGS a few miles down the road in Cheltenham.  

PSK reporter also reports my tiny signal is being heard just outside of Cork, in Ireland, and, although I can't hear him, that's not bad for 5W into a discone antenna!

I've got a number of boards left over... if you'd be interested in one, drop me a line.

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