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December 11th 04, 07:39 PM
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In article , Paul Burridge
writes: On 09 Dec 2004 19:12:18 GMT, (Avery Fineman) wrote: Of course the resistor lead contribute some inductance. In fact, most of it. However that inductance is directly calculable based on old, available data. What remains is the resistor body itself. A very quick approximation of that body would be to get a scrap of kitchen aluminum foil and wrap it tightly around the body with the overlap tight around the leads to make contact. [won't make much difference because the body, being larger in diameter than the leads, will have much less inductance than those leads] Sorry, Len, I stand to be corrected (no doubt) but surely this way of shorting the ends together is going to make matters much worse? Aren't you going to end up with a significant amount of capacitance between your outer foil and the inner spiral of resistive film? Isn't that going to just throw another complex variable into the mix and probably completely change the resistors SRF? Paul, don't kick yourself after reading this, but shorting out the body with foil will put a conductor in contact with BOTH ends of the resistor body. :-) I mentioned that only in passing since it isn't necessary to do in order to find out anything significant. Finding a "self resonant frequency" involves doing several measurements of the total R, C, and L of the device, finding the complex R and X at each, then plotting that (a Smith Chart will do it nicely) to see the skew shape of the curve as compared to a perfect resistor (a single point on a Smith Chart). You will have to work out the SRF yourself based on that information; that is going to vary with each specified R value, film type, and the kind of laser trimming (or whatever) is done to get the DC/low-frequency R value precise through a film spiral or gouging or whatever. Not needed. If you just measure the device with a bridge/instrument yielding the complex impedance or admittance, you just apply that to the circuit taking the device and be done with it. You will have to allow for some adjustment in the circuit itself to compensate for the device characteristics (whatever they come out to be). An analytical model of the resistor is an R component in series with an L component (due to any spiral of film, if any, plus the length/diameter of the resistor body), but that has fringing capacity between the ends of the leads inserted into the body...that capacity being in parallel to the series R-L connection. You can estimate that and do a paper exercise to see the effect for jollies...or just skip it, use the device measurements to base the overall model as it applies to the circuit and go on with the project. The effect, if any, is going to be minimal with 1/4 Watt or smaller resistors at VHF on up to low UHF bands. Nothing to worry about provided the leads themselves follow the usual "short as possible" rule. One can go nuts on the intellectual paper exercise and about all it is good for is mumbling-bragging over glasses at the local pub. :-) |
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