Steve said:
. . .
I'm not so dumb as to
realise that a choke passes dc but not the high frequncy RF. I guess
it boils down to this: how did the designer arrive at the given value
of 0.4uH for this inductor? Why not 4nH? Why not 40uH? Why not 100mH??
What's the deal with this value and since I've only got a 1uH in my
junk box, will that be okay instead?
I'm guessing you're not experienced enough to realize that inductors are
far from ideal. Among their many imperfections, the most problematic in
an application like this is shunt capacitance. This resonates with the
inductance to create a parallel resonant circuit. At resonance, the
impedance is very high -- higher than that of the inductance alone. But
above the self-resonant frequency, the impedance drops, and at some
point becomes less, then very much less, than the impedance of just the
inductance. Well above self-resonance, all you see is the self
capacitance -- it looks like a capacitor, not an inductor.
That's why a designer doesn't just use 100 mH for everything. Just about
any 100 mH inductor looks like a capacitor at 145 MHz, with a very low
impedance, much lower than an inductor with smaller inductance value.(*)
The trick is to choose an inductor that's below or at its self resonant
point while still having enough impedance so it doesn't disturb the
circuit it's across. A good rule of thumb is an inductor whose reactance
is about 5 - 10 times the impedance it's across. (In your case, this
might not be easy to determine. You might be able to get it either from
knowing someting about the previous stage, or from the S parameter
specfication of the transistor.) 5 - 10 times is usually enough, and if
you try for too much, it won't work any better and you run the risk of
being above the self-resonant frequency. Of course, an individual
inductor can be measured to make sure its impedance is high enough at
the frequency of use, if you have the equipment to make the measurement.
So, now, is your 1 uH ok? It depends on its shunt C, which depends on
its construction. If you can't measure it, just try it. The worst that's
likely to happen is that it'll kill the signal (due to low impedance).
At only 2-1/2 times the value of the original, there's a good chance
it'll work ok. If it doesn't, and if your 1 uH inductor isn't potted,
you can get the value down to 0.4 uH by unwinding about 1/3 of the turns.
(*) Even this explanation is highly simplified. At higher frequencies,
the inductor will exhibit additional series and parallel resonances due
to various parasitic capacitances and inductances and transmission line
effects.
Roy Lewallen, W7EL