>Every large inductor has this problem. Even if you short turns at one
end
>(like in a roller) the lowest order series resonance moves at a
different
>rate and direction as the second point of series resonance. The high
order
>can be moving down as the low is moving up!
...
>Designers fight this problem constantly in equipment designs that
cover wide
>frequency ranges and that have somewhat large components, so we must
measure
>such things if we want to do a good design.
>HNY and 73,
>Tom
Tom et al.,
I agree. And a very interesting discussion of choke design. I've
recently had to deal with the same problem in a different context, and
we applied a solution that might be worth talking about here. As you
know, LISNs (Line Impedance Stabilization Networks) are used in the
EMC business to measure conducted emissions on the power leads of
equipment under test (EUTs). Having to test very large UPS systems,
we had to build 3-phase LISNs for 480 Volt 100 Ampere AC service.
In the LISN, each phase is isolated from the mains by an L-network
consisting of a 4 µF 660 VAC capacitor and a 50 µH inductor. The
latter was built using 4 Ga. THHN wire wound on 3.5 inch O.D.
electrical PVC. The inductors are over 17 inches long, so when we did
the electrical evaluation before calibrating the units, we found
self-resonances at around 12 MHz and 21 MHz. These would have made
the attenuation curves for the LISNs very irregular, to say the least.
A solution was suggested by a colleague (W5EMC, BTW), when we found
that placing a hand at just the right place along the inductor would
suppress these resonances. We wrapped a 1"-wide aluminum strap around
each inductor, placed a 56 Ohm resistor across the ends of the strap,
and ran a 470 Ohm resistor to a ground post. When the strap was
positioned correctly on the inductor, the resonances all but
disappeared in our frequency range of interest. End-to-end
attenuation was initially ±10 dB. The strap reduced it to ±1.5 dB.
Unfortunately, I didn't measure the change in inductance of the
inductor, and this might have been interesting to do. It wasn't
necessary because the LISNs behaved properly in the normal ANSI
calibration procedure. The measurement port attenuation was smooth,
and isolation between the Mains and the EUT ports was excellent.
I wonder if the same technique might be useable with chokes in HF
amplifiers. The "Resonance Killer" straps would have to be insulated
well enough to withstand the voltages along the plate choke. I
suspect that if we did this, a continuously-wound choke would look,
electrically, like the multi-section PI chokes that were popular some
time ago.
If it didn't invite parasitic oscillation at the higher resonant
frequencies of the choke segments, this would be good. In the case of
our LISNs, I suspect that we just moved the resonances up and out of
the 150 kHz to 30 MHz band of interest. We didn't make them go away;
we made the choke inductors serve their purpose where we needed them.
HNY es 73,
Brad, KV5V
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