Even putting 10 mW into the BPF, the inductors aren't going to see current anywhere near those levels.
If one of the cores goes into saturation, for sure the first one would be the 47uH core. So, for this inductor you have to spend some time finding a good core.
The resonators can have rather high Q ... have you checked (simulated) the actual inductor currents?
^ this occurred to us (myself and coworker) this morning. Simulation shows current through the 47 uH to be quite a bit higher than I though, but not to the level that I'd expect to see significant inductance rolloff... assuming 1V peak input, it was on the order of 10 mA or so... I wouldn't expect to see rolloff until at least twice that.
It may not actually be full blown core saturation, but the BH curve is far from linear, and the dynamic permeability, eddy current loss, and hysteresis loss all change with drive amplitude.
Have you tried using one of the RF powdered iron toroids instead of ferrite ?
The saturation flux density is several times that of ferrite, and with the right powdered iron "mix" the operating Q at 1mHz can be optimized.
**broken link removed**
Funny I see a double tuned filter coupled using two low impedance high Q filters coupled by a higher impedance mid-section to widen the bandwidth., so each Q much higher than 3 with high impedance on the 47uH and low impedance on the 2.2uH chokes resulting in attenuation after the 560pF and then a boost in current again on the 2nd 2.2uH.
IS that what you intended?
so the filter appears to have very high loaded Q resonators and the coils are becoming non-linear.
You could put a transformer at the front, and another one at the rear, to transform the filter match impedance up. Say you designed the BPF for 100 ohms match, the currents flowing in the coils would be similarly lowered. (at this narrow bandwidth, the "transformer" might be a simple L-C network)
Funny I see a double tuned filter coupled using two low impedance high Q filters coupled by a higher impedance mid-section to widen the bandwidth., so each Q much higher than 3 with high impedance on the 47uH and low impedance on the 2.2uH chokes resulting in attenuation after the 560pF and then a boost in current again on the 2nd 2.2uH.
IS that what you intended?
I did simulate this, with tapped capacitor transformers before and after the filter, going from 50 to 75, and back again. As impedance increases, the inductor values increase, which results in a lower saturation threshold, and only modest current reduction.. IE, the series coil went from 47 uH to 79 uH (68% increase), while reducing current from 20 mA to 16 mA (20% decrease). Also, the current in the shunt inductors of the transformers is quite high (300 mA, if my simulation is correct).
I tried the transformation with a tapped inductor xformer instead and I'm getting weird results, so I have to double check the math in that. That said, my calculated values for the tapped inductor were a lot more reasonable... everything was nano level...
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