Sunday, 19 May 2013

VFO Pulls No More!

My troubles with the SA602/612-based VFO are over - now she is completely oblivious to large signals, keeping on oscillating at just the same rate as when tickled by weak signals or nothing at all!

The "fix" was motivated by various correspondents (particularly members of the G-QRP Yahoo Group) who pointed to the varicap diode as the most likely origin of my problems. I tried replacing it with a conventional capacitor and - sure enough - the "pull" was gone.

Now - there's nothing intrinsically wrong with varactor / varicap diodes; they have been seen to work as tuning capacitors in many radios before. What was wrong with my experiment turned out to be the physical separation between the diode, the (trimmer) potentiometer used to derive its tuning "bias" voltage and the regulated power supply providing the input to this voltage divider, as seen in the photo below...


I tried generating the control voltage local to the varactor and - sure enough - the oscillator was as solid as when controlled by the "real" capacitor.

I don't know exactly what was causing the problem when the control voltage was remotely generated - John, g8ozh, had suggested ground impedance problems and I'm inclined to agree with his explanation. Whatever the precise cause, I now had a resolution of the issue, which prompted me to modify the ugly circuit close to the tank, adding a socket into which I could plug various capacitors and their tuning means, close to the coil. I also provided a local voltage regulator...

   

With this simple interface socket I could confirm the stability of a "pukka" trimmer and the viability of a locally-controlled varactor. I could also move in my intended direction, adding a digital-to-analog converter to achieve digital control of the oscillator...


The most "complex" of the tuning plug-ins includes an MCP922 12-bit DAC, which is controlled over a serial interface. You can see the whole shebang in action on the bench...

 

The Arduino seen above is running code to generate d.c. voltages on the MCP4922, used as control voltages for the varactor, in response to my inputs on a rotary encoder. I also took the opportunity to add a frequency counter to monitor the oscillator, using the Frequency Counter library provided by Martin Nawrath (whose excellent FFT library I have also used). This allows direct monitoring of the oscillator on 40m (higher bands will need a divider / pre-scaler to operate correctly).

The LCD screen now displays not only the varicap control voltage but also the resulting frequency...


This puts me in the interesting position of being able to "close the loop" and making an automatic controller to regulate the frequency to a desired value.

Can I be bothered - or will I remain seduced by the simplicity of the DDS ?

...-.- de m0xpd

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