Patent ID: 6121778
Filing Date: 2000-09-19
Classification: G01R

Abstract:
A method of estimating the component values of an operational pi-section filter having input and output connections, the output connections being connected to a load, comprising the steps of:(a) providing a pi-section low pass filter as a circuit to be tested, the circuit having input and output connections;(b) applying a series of test signals from a source having a source resistance over a range of frequencies to the input connections of the pi-section filter;(c) measuring for each of the range of test signal frequencies the voltage at the inputs of the pi-section filter, thereby obtaining voltage/frequency pairs;(d) estimating the 3 dB frequency, F.sub.3dB ;(e) estimating the load resistance R.sub.L of the load;(f) making a first estimate of the resistance R.sub.p across the input connections when the filter is connected to the source and the load by setting R.sub.e =0 in the following equation: ##EQU28## where R.sub.e defines the loss resistance associated with the inductor of the pi-section low pass filter and R.sub.S defines the source resistance;(g) making a first estimate of the total capacitance of the circuit, C.sub.T, using the equation: ##EQU29## assuming that the capacitances C.sub.1, C.sub.2 are equal and thereby defining the total capacitance of the circuit, C.sub.T, as:C.sub.T =C.sub.1 +C.sub.2 +C.sub.L =C.sub.L +2C.sub.1where C.sub.1 is the capacitance of a first capacitor and C.sub.2 is the capacitance of a second capacitor of the filter; and C.sub.L is the capacitance of the load;(h) determining the inflection points, the first inflection point being due to a minimum in the frequency response at f.sub.min and the second inflection point being due to the maximum in the frequency response at f.sub.max ;(i) estimating the frequency f.sub.b which defines a point on the frequency response of the pi-section filter where the voltage ratio of the measured voltage to the open circuit voltage is 3 dB less than the voltage ratio at the maximum frequency, f.sub.max ;(j) extrapolating the initial slope of the frequency response to obtain an estimate of the frequency, f.sub.a, where the voltage ratio is also 3 dB below the voltage ratio at the maximum frequency, f.sub.max ;f.sub.a =F.sub.3dB (V.sub.low -V.sub.E);where V.sub.low is the voltage measured at the lowest frequency, and V.sub.E is the voltage measured at f.sub.b ;(k) estimating the extrapolated frequency response, k, from frequencies f.sub.a and f.sub.b : ##EQU30## (l) obtaining a first estimate of C.sub.1 =C.sub.2 using the estimate of C.sub.T from step (g) and the extrapolated frequency response k: ##EQU31## (m) improving the estimate of R.sub.e using the linear voltage ratio B=V.sub.low /V.sub.oc, the estimates of R.sub.S and C.sub.1 : ##EQU32## where: ##EQU33## (n) replacing the estimate of R.sub.e with the new estimate from step (m) to obtain a further estimate of R.sub.p ;(o) using the further estimate of R.sub.p to find a further estimate of C.sub.T ;(p) using the further estimate of C.sub.T to find a further estimate of C.sub.1 ; and(q) repeating steps (m) to (p) until subsequent values of C.sub.1 are within a predetermined amount of each other.