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#10
September 12th 03, 12:42 PM
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"Dr. Slick" wrote:
wrote in message ... The tricky part is measuring this correctly, because you would need an SWR meter that is calibrated for the same Z as Zo. It is not nearly that tricky. 'Revised' rho, as you state, predicts 0 Volts across the capacitor. This will be easy to measure with any AC voltmeter that can handle your test frequency. Perhaps, but i'm interested in the forward and reflected waves, which you can only get with directional couplers on a line of the same Z as the Zo, i suspect. So even if you get 0 volts, there are still fwd and rev waves. But what if you do not get zero volts. Sort of messes up the 'revised' rho theory a bit, does it not? I predict, using circuit theory, that if you excite the test circuit with a 1 Volt sinusoid at a frequency that makes the impedances j200 and -j200, that you will measure 4 Volts across the capacitor, not 0. This aligns with the result expected from 'classic' rho. Go ahead and bench test it, and let us know what you find. Rummage. Rummage. Rummage. 2.2 uH +/- 10%, 100 pf tolerance unknown, 33 ohms +/- 5% R = 34 measured L = 2.2 uH C = 100 pF But wait, there will be a scope probe across C, vendor says 15 pF nominal when compenstaed for a 15 pF scope input, but the scope input is 20 pF. Oh well, use 15 pF anyway. So: C = 115 pF f = 10.006 MHz Zres = 34 + j0 Zind = 0 + j138.3 Zcap = 0 - j138.3 But it is always wise to predict the outcome before the measurements... So let's use a 1 Volt sinusoid at 10.006 MHz. From circuit theory: Ires = 0.02941 + j0 A Vcap = 4.067 /_ -90 V From 'classic' rho: Vi = 0.5 V rho = (Zl-Z0)/(Zl+Z0) = ((0 -j138.3)-(34+j138.3))/((0 -j138.3)+(34+j138.3)) = 8.1965/_ -97.0 Vr = Vi * rho = 0.5 * 8.1965/_ -97.0 = 4.09826/_ -97.0 V Vcap = Vload = Vi + Vr = 0.5 + 4.09826/_ -97.0 = 4.067/_ 90.0 V So I expect the magnitude of the voltage to be 4.067 volts. But wait, there are a whole bunch of tolerances so that is unlikely to be the voltage, so what is the expected range? We are not sure of the capacitor tolerance but it is unlikely to be better than 10% and the scope probe is unknown, so let's call it 10%. The resistor was measured at 34 +/- 1 digit + meter error, so 5% is probably good. So if the capacitor is 10% high and resistor is 5% low the error would be 1.1/.95 = 1.16 or about 16%. So if the result is within 16% of 4.067 it will be consistent with expectations. First adjust frequency for resonance f = 10.14 MHz, tolerably close to the predicted 10.006 MHz. And the measured voltage across the capacitor is... Hold on, before revealing the answer.... In the interests of minimizing the wiggle room, perhaps you would be so kind as to provide your prediction for the voltage across the capacitor. Using 'revised' rho, in a previous post I recall you predicted 0 volts. Is this still your expectation? ....Keith |
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