Hi, Stefan, LF,
I suggest you consider the Vackar oscillator. There is a very good
FET/transistor design by G3PDM which you can reach by following the link
below.
It's also important to consider the mechanical design of the oscillator and
its enclosure. I think you'll be disappointed with the results of your
construction - the tinplate box will be quite "flimsy", and the arrangement
with the tuning capacitor in the lid means that you will have quite long
unsupported wires connecting the frequency determining components.
I suggest that you start with a more rigid metal box, say an aluminium
die-cast box, and mount all the rf components, including the coil, close
together and as rigidly as possible. In the past I have used ceramic
stand-off pillars, but strips of fibreglass PC board material soldered to M3
screws works just as well. For the coil, use a styrene or ceramic former
with a dust-iron (not ferrite) tuning slug. Avoid toroids for the oscillator
coil, if possible.
The G3PDM article has some useful information on mechanical constuction.
There was a load of work done by hams and professionals on transistor
oscillators in the 1970s and 1980s, but it's been largely forgotten with the
introduction of PLLs and DDSs. It just needs to be re-discovered!
Good luck!
73, Dave G3WCB IO91RM
http://www.qsl.net/va3diw/vackar.html
>
> The NE602 works very well as oscillator and delivers a clean and low
> noise signal.
> One of the main problem in building VFO's is the mechanical design.
> It can be done, but it is not trivial when you use a tuning capacitor
> and gearbox.
> Mechanical rigidity and freedom from micropfonics requires a solid
> enclosure which with the gearbox need to be mounted on the frontpanel,
> rather than on the chassis.
>
> As the frequency range is small, it will be mechanically much easier
> to use varactor tuning in conjunction with a ten turn potmeter. I have
> been there and done it all.
> I have build a 40 m receiver with varactor tuning and it is rock stable.
>
> Building VFO's is fun!
>
> 73,
> Roelof, pa0rdt
>
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