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Browse Prior Art Database

Method for Tuning Cascaded Filters

IP.com Disclosure Number: IPCOM000114383D
Original Publication Date: 1994-Dec-01
Included in the Prior Art Database: 2005-Mar-28
Document File: 2 page(s) / 72K

Publishing Venue

IBM

Related People

Liu, K: AUTHOR [+3]

Abstract

A common approach to increasing tunable filter selectivity and range is to cascade several tunable filters. The resulting frequency response is the product of the individual filter frequency responses. Typically, a control voltage is used to shift filter response, but the control is often complicated by nonlinearities, drift, hysteresis and environmental sensitivity. As a result control of a cascade is much more difficult because the tuning signals required are not simple deterministic functions of each other, but instead are independent and may require separate active feedback. The overall system becomes particularly complicated if there is no simple way to monitor the response of each individual filter separately. This is often the case in optical systems where power is limited.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Method for Tuning Cascaded Filters

      A common approach to increasing tunable filter selectivity and
range is to cascade several tunable filters.  The resulting frequency
response is the product of the individual filter frequency responses.
Typically, a control voltage is used to shift filter response, but
the control is often complicated by nonlinearities, drift, hysteresis
and environmental sensitivity.  As a result control of a cascade is
much more difficult because the tuning signals required are not
simple deterministic functions of each other, but instead are
independent and may require separate active feedback.  The overall
system becomes particularly complicated if there is no simple way to
monitor the response of each individual filter separately.  This is
often the case in optical systems where power is limited.

      The present invention describes a method for controlling a
cascade of two filters FL1 and FL2 as shown in Fig. 1.  The key is to
add a low-level Amplitude Modulated (AM) White Noise (WN) source to
the input signals coming into FL2.  The requirement on WN is that its
output is distributed over the frequencies to be filtered by the
cascade so that some part of it is always passed by the first filter
FL2.  The AM at frequency f1 is a tag to distinguish the WN from the
externally input signals E.  For an optical system, the white noise
source could be a broadband optical source such as a Light-Emitting
Diode (LED) for example.  The modulation rate of the noise is chosen
to be sufficiently below any modulation rates fs (for modulated
systems) or above any integration constants (for DC systems) for the
primary system so that it can be detected independently with
negligible interference.

      The master control signal to t...