Hello Ha-Jo,
Ten years ago I have been tinkering with passive audio bandpass filters.
In an attemp to reduce the physical size of the inductors, some 9 mm hi-mu
cores were used. The Q of the inductors was measured using a signal
generator loosely coupled to the tuned circuit and reading the frequency at
the - 3 dB points which were measured with a high impedance probe and
oscilloscope. I found that the resonance frequency varied with applied AC
voltage.
In my note book I have the data for a FT-37-43 ferrite core with 200
windings:
The tuned circuit had a 1 uF capacitor in parallel with the core and was
coupled to the audio generator by a 10nF capacitor.
Vpp(mV) F(Hz) L(mH)
=======================
1 1214 17.2
2 1208 17.4
3 1186 18.0
4 1176 18.3
5 1162 18.8
10 1134 19.7
15 1103 20.8
20 1073 22.0
25 1064 22.4
30 1026 24.1
A filter was build with 88mH 9 mm cores with a lot of windings.
I still have it in a direct conversion receiver.
The funny thing is that one can use it as a very crude AGC system when the
signal is tuned to the slope of the filter.
It appeared that larger cores suffered less from this effect and the TOKO
series 10RB magnitically screened inductors did not have it at all.
Best regards,
Roelfo Bakker, pa0rdt
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