Lump Inductors

A project log for Yet Another MW Radio - The Audion

Another super simple medium wave radio. This time a Franklin Audion (synchronous detector).

agp.cooperagp.cooper 01/07/2017 at 05:360 Comments

Lumped Inductors

Often radio circuits have coil taps and/or coil couplings. These be simplified as lumped inductors.

For example a 50% tapped 200 uH inductor is the same as two 100 uH inductors in series with regard to LC oscillation frequency and impedance transformation. The main down side with lumped inductors is that for similar coil construction the total resistance will be higher, that is the Q will be lower (i.e. a 200 uH inductor needs only 41% more turns than a 100 uH inductor).

So my tapped 320 uH coils consists of a 100 uH and 220 uH inductors in series. The simulation suggests a frequency range (using a BB212 varicap diode not the 1SV149 diode) of 1.27 MHz (for an 8 volt control voltage) and 564 kHz (for 1 volt control voltage. Note the series resonance response and "roll-up"!:

This is half way between the 25% and 50% taps shown the the schematic:

Modelling a coupled coil has no series resonance response and a normal "roll-down":

Changing the coupling produces a normal response:

History Repeats

I was rummaging through my junk box looking for some inductors and came accross an old super-regenative tube radio I build in the mid-nineties. I used the lumped inductors then and this receiver worked okay:

Here is the schematic: