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"Standard parts" for rf amps?
RST Engineering (jw) wrote:
Agreed, John, 150 MHz is damned near DC for a lot of us, but as yet I have no answer from the "spice" folks for the 1 nf capacitor question. A lot of fancydancing but no answers. Jim I didn't realize that this question was addressed to all "'spice' folks", but was directed only to the OP. That's probably why the "'spice' folks" haven't leapt to respond. Anyone who uses SPICE professionally, and indeed a great number of amateur users should be able to answer the question easily. At 150 MHz, a capacitor lead can generally be handled as a single lumped inductance. The amount depends on the wire's diameter and environment, but around 7 nH is a good working number for a half inch lead. The inductance of the capacitor body itself might have to be included in the model if high accuracy is important. Capacitance to ground, from both the leads and the capacitor body, might also be important if the impedance of the circuit to ground is high. If so, it can be included. The way I'd approach inclusion of the C would be to calculate the impedance and length of the transmission line comprised of the lead and ground plane or capacitor body and ground plane, then convert those values to a single series L and shunt C rather than just directly using a transmission line model -- very short transmission lines in a model can greatly slow SPICE calculations unless there are also other very short time constants involved. On the other hand, if the "leads" are PC traces over a ground plane, shunt capacitance will be higher, and the approach I mentioned with the transmission line is the way I'd always do it. The model for the leads would include both L and C. As an example, if the "lead" is a .010" trace on .032" FR4 material (er ~ 5) over a ground plane, it makes a transmission line of Z0 = 105 ohms, velocity factor 0.55. The equivalent L and C of a half inch of this line are 8 nH and 0.73 pF respectively. Other characteristics of the capacitor such as ESR might have to be included in the model depending on the application. You gave a capacitor value in your question -- an ideal capacitor of that value would of course be the other part of the model. SPICE is used daily, as it has been for decades, by professionals and produces strikingly good results in the hands of someone who is skilled at modeling and has a good understanding of the circuitry being modeled. I've personally used SPICE for modeling linear to highly nonlinear circuits up to 50 GHz, where even tiny SMT components were often modeled as transmission lines and every pad and solder blob is significant and included. The results were used in the design of products which have been successfully produced by the thousands and sold for years. Roy Lewallen, W7EL |
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