G4ALG's QRP Radio Pages

[ Previously GW4ALG (QRT in February 2007) ]

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DDS VFO for the FL-50B Transmitter

The operator of the 1960s Yaesu FL-50B transmitter has the option of using the internal crystal oscillator, or an external VFO.  Yaesu intended that the VFO in the matching FR-50B receiver could be used as the transmitter VFO.  However, VFO stability problems inherent in the design and construction of this receiver's switched-range VFO tend to rule out its use for transmit purposes in the 2020s.

I'm willing to accept and work around the rather unstable VFO in the receiver.  However, in fairness to the operator at the other end of my CW contacts, I have designed and built a DDS VFO for use with the FL-50B transmitter.  My design is based on the work of Paul VK3HN.

The DDS VFO can be seen in the above photo, to the right of the transmitter.

Here are a couple more photos of the VFO (click on the thumbnail image to enlarge):


The program runs on an Arduino 'Nano' board, which controls a Si5351 synthesiser.   You will notice that the VFO enclosure includes several unused components such as switches, sockets, and a spare signal buffer/amplifier that are not used in the current design.  They are there to make expansion easier at a later date.

I'm happy to share the design and software used in this project.  The Arduino code can be downloaded from:

If you would like more information, please send me an email, and I'll update this page accordingly.  If you can see improvements to the design, please let me know!

Digital Board Circuit Diagram

Power Supply Circuit Diagram

Signal Buffer Circuit Diagram



Development Environment

Development of the software was initially carried out using the Arduino Integrated Development Environment (IDE).   This worked perfectly well.  Later, my sons showed me how use Visual Studio Code with the 'Arduino' and Microsoft 'C/C++' extensions.  I love Visual Studio Code!   My sons then showed me how to use Git for code control, and the Xaver Hellauer Clang-Format extension for obtaining consistent program formatting.

With so many wonderful programming tools available, it's a great time to be learning how to program -- even at the age of 68!   My Arduino 'sketch' (program) is available for download from GitHub at:

A hardware development and test environment was specifically made for this DDS VFO project, and similar projects.  Here's a photo of the set-up.  It contains enough switches and expansion potential to permit the development of a multiband transceiver, with full processor-controlled rig control.