By the way, I found another surplus toroid with somewhat better
characteristics....higher permeability and higher Q, as measured around
200 kHz. That has now been incorporated in a small switching power
supply operating at about 30 kHz. It works, though not as well as is
predicted by a SPICE model. Must be some of those vices that Reg
mentions ! My SPICE model does not take into account the variation of
the permeability/inductance with DC current, so this may be at least
part of the difference. One of these years I'll break down and get an
oscilloscope so I can figure out what non-microwave circuits are really
doing, and maybe a signal generator that works below 150 kHz.

It is adequate (barely) for what I need, however, so it has now been
incorporated as a bias supply in my 3.4 GHz transverter. I'm crossing
my fingers that it keeps working over the temperature range it will
encounter in portable operation.

The Q-measurement technique I have been using involves connecting a
signal generator through a 50 ohm attenuator (to set the output
impedance) to a 50-ohm input microwattmeter. The inductor and a
capacitor are connected as a series-tuned resonant circuit and inserted
either in series between the pad and the meter or shunted across the
meter input. The inductance is obtained by finding the resonant
frequency and working backward through the formula, given the known
capacitance. The tuned circuit is then replaced with a resistor which
is adjusted (by substitution) to give the same power on the meter as
the tuned circuit at resonance. This resistance is equal to the
equivalent series resistance of the tuned circuit, from which the Q can
be determined.

As yet the Q results I obtain with the series and shunt connections
tend to be somewhat different, so my techniques certainly have room for
improvement (there are quite obvious stray-coupling issues, even at
LF), but it gives me a rough idea, anyway.

73,
Steve VE3SMA