radio frequency choke

most component suppliers (like Maplin in the UK) sell ready made RFCs which look like plump resistors with the colour bands indicating inductance in uH Maplin start (I think) about 22uH and go up to 1mH.

Do you really want to *design* the choke rather than just *purchase* one?

In either case, first you need to decide how much inductance you need. Take a look at the small signal impedance of where the inductor touches the transistor. Your want your inductor's impedance at that node, at the frequency you're oscillating at, be be "much higher." 10x is definitely "much higher," although realistically 4x is often plenty.

From that reactance, figure our the inductor size you need (from X=\\omega*L).

At this point, if you can order yourself an inductor from DigiKey or similar -- just make sure it's rated for the DC current you're using for bias.

If you really want to design the choke, if it's a small enough inductance, you can easily make air coils by winding enameled wire around drill bits, pencils or pens, etc. (Any book on RF design or Google will give you the formulas for the dimensions.) If you need more inductance, you'll need to use a core - -folks like Amidon sell them, in common shapes such as bars, toroids, pot cores, etc. This gets a little trickier, though, in that now you're the one stuck with making sure the material doesn't saturate, it's not too horribly lossy at your frequency, etc. -- the Amidon web site and various amateur radio-related web sites have lots of information on this.

What frequency is your oscillator? For someone's first oscillator, I'd stick somewhere in the ones through tens of MHz ballpark... going lower tends to make everything rather bulky, whereas going higher forces you to be a lot more careful with your construction techniques (and you're forced more into the mode of designing for a "ballpark" result and then tweaking rather than being able to get really close first-cut results).

---Joel Kolstad