Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
1999-05-28
2001-07-17
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S553000
Reexamination Certificate
active
06262623
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to log-domain filters and in particular to a biasing system for a log-domain filter having a variable dynamic range window.
2. Background
In wireless communications systems, such as those employing the CDMA IS-95A, -95B, -95C standards, there is typically a wide variation in the amplitude of signals transmitted between the mobile units and the handsets thereof caused by various factors such as slow and fast fading. In many cases, the range in amplitude in such systems can exceed 60 dB. Consequently, the filters used in such systems must have sufficient dynamic range to handle the wide variability of signals which are present. Even in systems employing closed loop power control, such as CDMA wireless communications systems, there are circumstances in which a wide dynamic range filter is required.
The problem is that there are limits in the amount of current or voltage swing current active filters can handle and still remain linear. In traditional filters, such as gmC filters, the dynamic range can only be increased through increased power consumption and die area. For example, traditional methods for increasing the dynamic range involve either the introduction of larger degeneration resisters on the emitters of differential pairs, which is wasteful of die area, or higher bias currents, which is wasteful of power. Furthermore, the range is fixed by the design, and cannot be varied to accommodate signals of widely varying amplitude. Consequently, the power consumption in such filters is always set to accommodate worst-case scenarios, making the consumption wasteful at all other times.
One class of filters that have been proposed in response to the demands presented by modem communication systems is log-domain filters. See, e.g., M. Punzenberger & C. Enz, “A Compact Low-Power BiCMOS Log-Domain Filter”,
IEEE J Solid
-
State Circuits,
vol. 33, no. 7, pp. 1123-1129, July, 1998; M. Punzenberger & C. Enz, “A 1.2-V Low-Power BiCMOS Class AB Log-Domain Filter,”
IEEE J Sold
-
State Circuits,
vol. 32, no. 12, December, 1997, both of which are hereby fully incorporated by reference herein as though set forth in full.
Log-domain filters typically comprise a logarithmic integrator sandwiched between a logarithmic compressor at the input, and an exponential expander at the output. An incoming signal is compressed into the log domain, filtered while in the log domain, and then exponentially expanded. Log-domain filters, because they operate on signals in the log domain, can handle signals having large swings in amplitude. Moreover, despite their internal non-linearity, log-domain filters achieve linearity from an overall standpoint.
The problem is that the dynamic range of log-domain filters cannot achieve a dynamic range of greater than about 60-70 dB. The reason is that the noise floor of a log-domain filter, which defines the lower bound of the dynamic range, rises and falls as the compression point, which defines the upper bound of the dynamic range, rises and falls. Consequently, the dynamic range, which is defined as the range between the compression point and the noise floor, is substantially invariant to changes in the compression point. The dynamic range thus remains at its nominal level of about 60-70 dB. Thus, these filters are not suitable for general use in wireless communications systems, particularly wireless communications systems where very wide dynamic range is required.
Switchable RC filters offer a wide dynamic range, but the transfer function thereof is subject to numerous, discontinuous jumps as components switch in and out of the filter during tuning. Such discontinuities render the switchable RC filter unsuitable for general use in wireless communications systems. In addition, such filters consume a large die area.
Other tunable filters comprise gmC filters. However, like log domain filters, gmC filters are limited to a 60-70 dB dynamic range.
Thus, there is a need for a tunable filter which overcomes the disadvantages of the prior art.
SUMMARY OF THE INVENTION
In accordance with the purpose of the invention as broadly described herein there is provided a filter system comprising a parameter detector, adjuster and a variable dynamic range window filter. The parameter detector detects a parameter of a signal, and the adjuster, responsive to the detected parameter, adjusts the dynamic range window, that is, the range of operation defined by the 1 dB compression point and the noise floor, of the filter. The result is to achieve an effective dynamic range for the filter system which is greater than the nominal dynamic range.
In one embodiment, the parameter detector is a peak detector, and the adjuster is a biasing unit. In this embodiment, the peak detector detects a peak of a signal, and the biasing unit, responsive thereto, biases the signal in order to increase the effective dynamic range of the filter system. In one implementation, the signal is a differential mode signal, and the biasing unit biases the common mode component of the signal responsive to the detected peak. In one implementation example, the filter is a log-domain filter, and the biasing unit biases the common mode component of the signal to avoid zero crossings thereof. In this implementation, the logarithm of zero or negative numbers is undefined, and the biasing unit, by biasing the signal to avoid zero crossings thereof, adjusts the compression point of the filter responsive to the detected peak. Since the compression point defines the upper limit to the dynamic range of the filter, the effect is to shift the dynamic range window, that is, the range of operation defined by the 1 dB compression point and the noise floor, to achieve a greater effective dynamic range even though the instantaneous dynamic range, that is, the difference between the 1 dB compression point and the noise floor, is unchanged.
In one implementation, the filter is a current mode filter in contrast to a voltage mode filter. Operation in the current mode is advantageous because, unlike the voltage mode, the power supply voltage does not set an upper limit to the 1 dB compression point.
In one embodiment, the filter comprises a compressor, an integrator, and a decompressor coupled together in a cascade arrangement. In this arrangement, the filter has an overall transfer function which is that of a linear low pass filter. Thus, even though the internal workings of the filter are intentionally non-linear, the overall transfer function of the filter is linear.
In a second embodiment, the filter comprises a plurality of filter blocks, each of which comprises a compressor, an integrator, and a decompressor coupled together in a cascade arrangement. In this embodiment, the filter blocks are configured to achieve a desired transfer function, whether that of a baseband, passband, bandpass, high pass or other filter. In one implementation, in which each of the filter blocks is a log-domain filter, the log-domain outputs of one filter block are provided directly to the integrator of a second filter block in order to avoid unnecessary decompression and recompression operations. In this implementation, the decompressor of the first filter block, and the compressor of the second filter block, are bypassed.
One embodiment of the method embodying the subject invention comprises the steps of detecting a parameter of a signal, biasing the signal responsive to the detected parameter, and then filtering the biased signal in the log domain. In one implementation, the method further comprises detecting a peak of the signal, and biasing the signal responsive thereto. In one example, the method further comprises biasing the common mode component of a differential mode signal responsive to the detected peak to avoid zero crossings thereof.
Other features and advantages of the invention, as well as the structure and operation of particular embodiments of the invention, are described in detail below with reference to the accompanying drawings.
REFERENCES:
patent: 5
Conexant Systems Inc.
Dinh Paul
Lyon & Lyon LLP
Wells Kenneth B.
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