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Passive Reflectances

From Eq.(C.75), we have that the reflectance seen at a continuous-time impedance is given for force waves by (C.76)

where is the wave impedance connected to the impedance , and the corresponding velocity reflectance is . As mentioned above, all passive impedances are positive real. As shown in §C.11.2, is positive real if and only if is stable and has magnitude less than or equal to on the axis (and hence over the entire left-half plane, by the maximum modulus theorem), i.e., re (C.77)

In particular, for all radian frequencies . Any stable satisfying Eq.(C.77) may be called a passive reflectance.

If the impedance goes to infinity (becomes rigid), then approaches , a result which agrees with an analysis of rigid string terminations (p. ). Similarly, when the impedance goes to zero, becomes , which agrees with the physics of a string with a free end. In acoustic stringed instruments, bridges are typically quite rigid, so that for all . If a body resonance is strongly coupled through the bridge, can be significantly smaller than 1 at the resonant frequency .

Solving for in Eq.(C.77), we can characterize every impedance in terms of its reflectance: Rewriting Eq.(C.76) in the form we see that the reflectance is determined by the ratio of the new impedance'' to the old'' impedance in which the incoming waves travel. In other words, the incoming waves see the wave impedance step'' from to , which results in a scattering'' of the incident wave into reflected and transmitted components, as discussed in §C.8. The reflection and transmission coefficients depend on frequency when is not constant with respect to .

In the discrete-time case, which may be related to the continuous-time case by the bilinear transform7.3.2), we have the same basic relations, but in the plane:         (C.78)

where denotes admittance, with (C.79)

Mathematically, any stable transfer function having these properties may be called a Schur function. Thus, the discrete-time reflectance of an impedance is a Schur function if and only if the impedance is passive (positive real).

Note that Eq.(C.79) may be obtained from the general formula for scattering at a loaded waveguide junction for the case of a single waveguide ( ) terminated by a lumped load (§C.12).

In the limit as damping goes to zero (all poles of converge to the unit circle), the reflectance becomes a digital allpass filter. Similarly, becomes a continuous-time allpass filter as the poles of approach the axis.

Recalling that a lossless impedance is called a reactance7.1), we can say that every reactance gives rise to an allpass reflectance. Thus, for example, waves reflecting off a mass at the end of a vibrating string will be allpass filtered, because the driving-point impedance of a mass ( ) is a pure reactance. In particular, the force-wave reflectance of a mass terminating an ideal string having wave impedance is , which is a continuous-time allpass filter having a pole at and a zero at .

It is intuitively reasonable that a passive reflection gain cannot exceed at any frequency (i.e., the reflectance is a Schur filter, as defined in Eq.(C.79)). It is also reasonable that lossless reflection would have a gain of 1 (i.e., it is allpass).

Note that reflection filters always have an equal number of poles and zeros, as can be seen from Eq.(C.76) above. This property is preserved by the bilinear transform, so it holds in both the continuous- and discrete-time cases.

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