Yes sometimes things make no sense at all. Typically because you stuffed up in the first place. In this post haste modern world I got my Ying mixed up with my Yang and well things go bonkers when you do that.

So I am still trying to work out why my inductors are not right, but I have finally started to get some results that make sense and fit with that I am seeing with my measurements. So my dilemma has been my low pass filter designed for 7mhz had a center frequency closer to 20mhz, obviously something is wrong. I checked all the cap values as I was building it, they were all correct, I wound the inductors with the correct number of turns for the inductance needed, check them with an LCR meter and they seemed to corroborate. There were no solder bridge’s or cold joints. So the obvious place to look was the inductors.

So i knocked up this test jig, the inductor is in series with a known value resistor. V1 is a signal generator, the frequency is changed until the voltage at V_2 is exactly 50% of the voltage at V_1 and the frequency of the signal generator noted.

So I did this 2 times, the first time above using a 99.7R resistor, that was the measured value of the resistance used and the above was measured on the oscilloscope. The blue trace is the input voltage V_1 and the yellow trace the 50% measured voltage V_2. The frequency on the signal generator was 6.3mhz. This scope if not very accurate.

I repeated the process using 199R resistor and noted its frequency, the measured inductance was 4.3uH and 4.5uH certainly near enough to the method was working as it is meant to and well within the margin of error for this type of ball park measurement technique.

So with the frequency of the 50% voltage we can use this formula to find the inductance. L= R*sqrt(3)/(2*pi*f) which works out to be 4.3uH or near enough

So with my new found inductance value, I simulated the filter this time using 4.4uH for the inductance value mid way between my 2 measured values, and the simulation plot started to look a lot like what I was I was seeing with the bode plots i was making of the filter. So it looks like I have found my culprit. The actual inductance was lower than what would have been expected for the number of turns I had on the toriod. A few more turns and it should be right to go.