To: <amps@contesting.com>
>> Date: Fri, 29 May 1998 15:45:53 +0100
>> From: "Geurts, Gerard" <ggeurts@amp.com>
>
>> > I have never been clear on that point since it appears that a minute
>> > amount of inductance is compensated for by the L that is selected.
>> >
>> Indeed, why would this resistor have to be non-inductive?
>
[Tom Rauch wrote:]
>Think about what we want. We want the resistor to be fully in the
>circuit at the frequency of instability, and fully out of the circuit
>at the operating frequency.
>
which is not possible.
>Now draw the circuit on paper and look at it. If the resistor has
>series inductance, the series inductance decreases the current
>through the resistor at increasing frequency and increases SYSTEM Q
>at higher frequencies.
>
ok
>If the resistor had series capacitance, the opposite would be true.
>The resistor would more rapidly shift into the path with increasing
>frequency and more rapidly shift out at lower frequencies.
>
ok
>> Some people use resistive inductors to improve their surpressors, so why
>> not use inductive resistors?
>
>Well, they "think" they improve the suppressors. What they do is
>increase series resistance at all frequencies from dc up, that's why
>ten meter performance drops.
Is the ESR constant at all frequencies in a resistance-wire suppressor L?
According to Wes' measurements the ESR of the Nichrome-60 inductor
increased from 0.47 ohms @ 10MHz to 4.7 ohms @ 200MHz. Sure, 28MHz
performance drops maybe 1% with a resistance-wire VHF suppressor,
however, thanks to skin-effect, Rp dropped roughly 40% at 100MHz. .
>
>Anything done with a resistive wire can be accomplished with a change
>in lumped resistance.
.... semi-true. Increase L and R proportionately in a suppressor and
performance undoubtedly improves at VHF. The proverbial fly in the
ointment is the power dissipated in R at 28MHz. During the grate
parasitics debate, I repeatedly asked the Rauchian camp to design such a
parasitic suppressor. . . . I'm still waiting.
>
>> From a circuit analysis point of view a
>> non-inductive (I know, they don't exist) resistor in parallel with an
>> inductor is the same as an inductive resistor in parallel with a
>> slightly bigger inductor.
>
>Not at all. The reactive part increases compared to resistance.
>Upper frequency Q, when that component is installed in series with
>the lead from the anode to tank capacitor, increases.
>
>> The same goes for the series resistor we put in the high voltage line to
>> limit the current. I see references to non-inductive resistors for this
>> application all the time, but this resistor is switched in series with
>> the plate RFC. Again, from a circuit analysis point of view a
>> non-inductive resistor in series with an inductor (the plate RFC) is the
>> same as an inductive resistor in series with a slightly smaller RFC. The
>> only thing we need to make sure is that the resistor can handle the
>> energy dumped in it during a major 'event'.
>
>That's correct. The only concern for that component is the ability
>to withstand the momentary impact of a fault. Inductance matters
>little, because it is small compared to the rest of the
>impedance...and actually any inductance would help.
>
agreed
>The most common energy absorbing resistors are non-inductive,
how about "typically have minimal inductance"? As I recall, the Cesiwid
rep told me that such resistors typically have around 9nH per inch of
length.
>but that is side effect of having high voltage ratings rather than a
>goal. RCD manufactures pulse rated wire wound resistors with wide
>turns spacing that are inductive, and work fine in fault
>protection applications.
>
>> The extreme case of this would be if you wind the RFC using resistance
>> wire. This would integrate the RFC and limiting resistor in one
>> component, reducing component count, and would also lower the Q of the
>> RFC making it easier to produce one without resonances. Does anybody
>> know a source of suitably insulated resistance wire so I can give this a
>> try?
>
>You wouldn't want to do this, because choke dissipation would
>greatly increase. The choke stores and returns energy to the anode
>system throughout the RF cycle, and resistance in series with that
>energy storage is a bad idea, especially on the lower end of the
>choke's operating frequency range.
good point. The typical HV-RFC has 1 - 2 A of RF current burden at the
lowest operating freq.
>.......
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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