Sunday, 12 October 2014

Si5351 in the BITX

Having played with the little Si5351 device in the Adafruit breakout board last weekend, I took the opportunity to build it into a VFO system and attach it to a receiver...

The new VFO was functionally a re-work of the dual DDS system I put together for my BITX at the tail end of last year, even sharing the same breadboard and Arduino NANO controller I used in early experiments...


On the breadboard you can see (from left to right) the NANO, a rotary encoder for tuning and the Si5351 breakout board. The LCD reveals that the system is running a modified version of my dual DDS code, which led to the SPRAT article with Pete Juliano, n6qw, "RF Generation for Superhets". This code generates both "BFO" and "VFO" signals - thereby making use of two-of-three of the available oscillators on the Si5351.

The system worked FB - so I hooked it up to my experimental "Breadboard BITX" receiver for a test. Again, everything worked perfectly (once I had set up the code's implementation of the BFO to the correct frequency for the particular 10 MHz IF filter I had mounted at the time).

However, the implementation of the "VFO" system on its own separate solder-less breadboard, seen in the photo above, was all a bit clumsy - so I exploited the simplicity and small physical size offered by the new Si5351 signal generator and tidied things up...


Above you see a complete BITX receiver (that's to say - only the parts required to operate as a receiver are present but all of them are fully bi-directional) on a single plug-in breadboard. Antenna input at "top left", speaker output at the bottom.

An annotated picture will save a thousand further words of description ("BA" = Bidirectional Amplifier)...


As you see, I made a new plug-in tuning module, hosting a rotary encoder. What is not so clear from the picture is that on each side of the tuning knob are push buttons that drive the remainder of the user interface (moving the "cursor" to select tuning rate, navigating the menu structure, etc).

The system seems to work every bit as well as before (as far as I can judge from a few minutes subjective listening in rather noisy band conditions on 40m) with the exception that there is a rather strident "noise" when it is tuned to 7.143 MHz the origin of which I can't explain. Also, I haven't yet done anything to optimise the level of the input to the amplifiers (and / or adjust their gain) I provided for the mixers I just stuck the signals from the Adafruit board straight in and saw what happened. Whilst the system is already working well, I suspect things can only get better.

There will be a release of the "Double DDS" code for the Si5351 - but I'm not yet ready for that. When it is suitable for public disclosure, you'll be the first to know.

This little Si5351 certainly is living up to expectations - I want more.

...-.- de m0xpd


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