Final thoughts

I'm having lots of fun operating this transceiver. I've worked around North America and even worked a few Europeans. I find it very satisfying to communicate using simple homebrew equipment. I hope it doesn't sound elitist but it definitely has a feel of "real" ham radio about it. The diverse combination of old and new is fun to think about. By that I mean the sophisticated DDS, microprocessor controller, simple direct conversion receiver and of course the ancient thing that makes such simple equipment usable, CW morse code. smiley  I usually send with a K1EL WinKeyer.

I'm sure there's a good chance that if had bought a kit rather than going with the all homebrew approach then I would have spent less time, money and ended up with a much more robust and attractive final result but ... I just wanted to do my own thing. There are some great inexpensive kits available in the QRP community and I encourage anyone to take that route if it feels like the right way to go.

40 meters is a great band for CW. There are the DXers at the bottom of the band, the QRPers up around 7030 and 7040 and then there is another group up around 7110. I think that last group is or at least used to be called the technicians' segment even though US technician class licensees can use 7025 to 7125. Slow CW seems to be the norm up at 7110 so it's a good place for anyone wanting to practice.

The performance of the receiver has exceeded my expectations. I get absolutely no "tunable hum" or other typical direct conversion issues with it. I'm lucky in that I don't have any strong local AM broadcast stations. I thought some shortwave broadcasts may have been a problem with the simple single tuned circuit in the front end. There are some very strong signals in the 31 meter band just below 10 MHz but they don't seem to cause me any problems. Your experience may vary. You might need to improve the front end selectivity if you have strong local signals. A parallel tuned circuit in series with or a series circuit across the input could be used to null out a troublesome local broadcaster if there is one particular station causing problems.

The receiver could probably do with an audio gain control. For most signals, I have the RF gain / attenuator barely above zero to avoid excessive volume in the headphones. At times I think it would be nice to turn the AF gain down and crank the RF gain up a bit. I've heard mention of the TDA7052 audio amplifier chip as a good alternative to the LM386. I think its gain can be controlled by a DC voltage level which sounds like a nice simple way to do it.

At times an audio filter would be nice but I haven't really had a problem with the lack of selectivity. As I get older I guess my ears' high frequency response is fading so I already have at least a low pass filter built-in smiley Of course the beauty of a direct conversion is its simplicity. If you try to improve it with lots of enhancements then there probably comes a point when it makes sense to give up on direct conversion and build a superhet with a IF narrow enough to give "proper" single sided reception.

There is definitely room for improvement with the user interface. The most immediate issue is that we need more buttons. I'm thinking of building a new version of the DDS box. I would use a shift register to drive the display as a serial device. That would free up enough pins to have nine buttons in a 3x3 matrix. The extra buttons would allow such enhancements as:

  • Easy switching back and forward between 100 Hz to 10 Hz steps.  Cycling through all the steps to get from 10 Hz back to 100 Hz is a little tedious.
  • A "reverse shift" mode which allows tuning to the low side of a signal to avoid QRM but still transmit on the correct frequency. I can do this now with the RIT by tuning all the way down through zero beat and up the other side but an instant switch would be nice.

I'm sure there are other enhancements but I'm not sure when I'll get to this next step. For now I'm going to get on the air and enjoy some operating.


Steve G0FUW (not verified)

Sun, 11/21/2010 - 15:15

Great website, well done.
It's amazing how things run in parallel, miles apart. I have also been messing with simple CW transmitters and rediscovered the BD139. I have 2 of them in my version of the Universal MkII, one in a Twofer, and one in the original 'Little Joe' Universal tx. As you say, they are good value replacements.
I have pushed the Twofer up to 20m and it performs just as well as on 40m.
My BD139s have 'live' back plates wired to the collector, I see yours are bolted to the PCB. Interesting!
I have just aquired a DDS from G-QRP Club, a Kanga US module, I think, and have been looking at a MicroMountaineer style trx. Great minds think alike!
Keep up the good work Ross and maybe see you on the air over Christmas/New Year when I get a bit more time to operate.
73, Steve, G0FUW

Thanks Steve. Great stuff. I was a little unsure if the BD139s were really insulated. The back is plastic but it has a somewhat shiny metallic look to it. I tried hard to scratch one as a test but couldn't get any sign conductivity with the collector so I think they are well insulated. A mica washer is probably a wise idea anyway just in case. Do yours have an obvious metal back? Mine are BD139STU if that means anything.


Mine have a metal plate on the back linked to the centre pin, and I have 3 different brands, all much the same, although some are grey in colour, others black. One of our smaller suppliers was offering 10 for £3, about $2, in SPRAT so I stocked up!
73, Steve, G0FUW

Daniel (not verified)

Fri, 12/17/2010 - 09:54

Great Work man!

Please, please, I do not understand all in various schematics...
Post please any complete schema diagram of interconnection between DDS, RX, TX with all materials (transistors, resistors, capacitors....).

I see only:
But I do not exactly where connect to TX, RX,...

This will help very well.
I am sorry for my bad english.

Best regards and thank You,
Daniel :)
Czech Republic

Hi Daniel

Thanks for your comment. I can't really publish the transmitter and receiver design here because they are not my work.

You need to study the transmitter which is the Q2 and Q3 stages of this:

And the receiver which is just the receiver part of this:

Then look at my schematic and my earlier post about the DDS-60.