Archive | February, 2018

uBITX now running on ARM Cortex M4

21 Feb

I bought a $10 ST Micro Nucleo-32 board and got the uBITX code running on it. This is a 70 mHz, 32 bit ARM Cortex M4 processor on a board that’s electrically and mechanically interchangeable with the original Arduino Nano board. This will provide a much larger memory space and much more computing power for the things I have planned for the uBITX. I installed the STM32 hardware support in the Arduino IDE and the code compiled and loaded almost effortlessly!  Later, I’ll move the code over to one of the cheaper ($2.50) generic STM32 boards that are available on EBay.

uBITX code running on Nucleo board

I plan to continue with the Nano until it runs out of steam, but iI can now move on without fear of hitting a brick wall, adding the things I have planned such as the 2.8″ TFT touch-screen graphics display and a separate, smaller color display that will be coded as an analog S meter. I purchased a nice painted aluminum cabinet and have a front panel layout and graphics that will make it look like the old Heathkit SB series equipment. All of that will unfold here as I progress.

 

Enclosure

Prototype touch screen

Nucleo-32 board

 

This is what it will look like with the color graphics display and the color graphics S-meter:

 

Front Panel with touch-screen display and S-meter

I’m planning to add a proper IF AGC circuit using the AGC amplifier described in my earlier posting. The AGC voltage will be derived by digitally sampling the received audio in the microcontroller’s 12-bit A-D converter and generating a control voltage using the microcontroller’s 12-bit D-A converter. An accurate software generated S-meter display will also be driven by the processor.

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My new uBITX Is up and running

18 Feb

I got my UBITX a couple weeks ago, but haven’t had time to turn it on. For your information, the uBITX is an all-band HF SSB/CW QRP transceiver that comes tested and nearly ready to use for only $120 delivered. (http://www.hfsignals.com).

Today, I got a chance to do a preliminary hookup and it seems to receive. Over the next several weeks, I’ll do some instrument testing and report my findings on a regular basis. For now, I can report that it draws about 160 MA and the minimum discernable signal on 40M is about 1uV (-110 dBm). I would have hoped for a little better sensitivity, but this is fine for the lower HF frequencies where atmospheric and external noise predominates. Also, the 50 hZ tunlng steps are bothersome to me and the tuning acceleration feature is annoying. I’ll probably do a firmware mod to get 10 hZ tuning steps with some other way to speed up the tuning. There also appears to be a substantial low frequency rolloff on the received audio, but I don’t have measurements to back that up yet.

……………

Later in the day update:

After some bench probing, I determined that the low end rolloff is because the BFO frequency isn’t set correctly and the lower audio frequencies are way down the crystal filter skirt. This was confirmed after a read of the BITX forum. I’ll calibrate it when I get some time.

Preliminary receiver spurious response testing with an HP8640B signal generator indicates the hard saturation point is about 3 mV (-40 dBm), so total usable dynamic range is about 70 dB. Since there is no IF AGC, this radio will overload on strong in-channel signals. This will be something I’ll want to address in a subsequent posting.

There are numerous spurious responses about 60-70 dB above MDS, including IF feedthrough, images and birdies. The latter are probably due to the well-known many spurious outputs the Si5351 generates when it’s  outputting multiple LOs, as it does in this radio.

These were all very quick and dirty measurents. Every one of these needs to be individually analyzed and dimensioned, but the preliminary results look like this is an “OK” receiver; good enough to have tons of fun, but nothing to write home about.

I hooked it up to an antenna for a quick listen on 40M. Plenty of band noise and signals heard, though the low frequency audio rolloff made listening to SSB difficult.

I see this little rig as a great bargain for what it does and a starting point for many worthwhile improvements.

This should be fun. Stay tuned…