RF choke: dig SRF
@ 2014/6/17
when i build my version N1TEV 's High Performance Regen receiver, i run into problem while selection a wrong RF choke.
the wrong choke i used:
the correct one:
and i also have some other choke/inductor, does these can be use as a HF band RF choke?
Theory
every book, and internet tell you choke's Self Resonant Frequency should 10 times, at least high than working frequency. then i'm lost here, why my this choke work well? this type of inductor SRF only 800 kHz.
old type HF choke like these:
those choke's SFR even below 2 Mhz. why they actually works? ARRL handbook introduce above the SRF they are capacitors, there must be something i misunderstood, they are might not be a capacitors. I recheck the book, it say(ARRL hand book chapter 5, RF Techniques):
The graph at B shows how distributed capacitance resonates with the inductance. Below resonance,
the reactance is predominantly inductive which increases as frequency increases.
However, above resonance, the reactance becomes predominantly capacitive which
decreases as frequency increases.
ohh, it means "predominately", not a really capacitor. while after DRF if capacitive is not so high, it will still work then. my version choke wind 30 turn, the distributed capacitance might too high, around several tens pf, then after 10 Mhz, it's almost a capacitor..., i wind 30 turns, this might be the problem.
wound toroid with 30 turn, Al should big than 2000, the capacitance is high. the materiel at >10 Mhz can not provide enough impedance is another possible problem. (more like the materiel problem)