Adaptive radio frequency (RF) filter
View Patent ↗The invention relates to an adaptive Radio Frequency (RF) filter ( 11 ), which is particularly useful as an RF filter in Wireless Local Area Networks (WLAN's). As greater demands are placed on RF systems, for example in WLAN's in order to increase channel capacity by utilizing available bandwidth, corresponding demands are placed upon performance and tolerance of components used in FR circuits. An adaptive Radio Frequency (RF) filter for filtering first and second RF signals from an OFDM encoded carrier signal is provided, the adaptive RF filter comprises: a low-pass filter ( 102 ) configured to filter first and second RF signals, one from another, so as to provide a first RF output signal; the adaptive RF filter being tunable in response to one or more input signals, the at least one input signal being derived from a comparator ( 37,100 ), which compares the first RF output signal with a desired value for said first RF output signal, and provides a connection factor for varying a characteristic of the filter ( 102 ). An advantage of the invention is that it facilitates filter of two OFDM encoded RF signals, the first typically at 8.1 MHz and the second (unwanted) at 11.9 MHz, from a base-band signal, with a noise floor level of —55 dB or better. Another advantage is that the filter is able to self calibrate and is able to take into account fluctuations which may affect performance, for example thermal drift, and automatically trim its characteristics so as to compensate for these fluctuations.
1. An adaptive filter arrangement for filtering first and second sub-carriers from an OFDM encoded carrier signal, the adaptive filter comprising:
a low-pass filter configured means to filter first and second sub-carriers, one from another, so as to provide a first output signal, and variable capacitors adapted to modify said low pass filter configured means; and
the adaptive filter arrangement comprising:
an OFDM encoder for generating a calibration signal under control of a controller;
a signal path so that the calibration signal can be applied to the low-pass filter configured means;
Fast Fourier Transform means for measuring a response of the first output signal to the calibration signal; and
wherein the adaptive filter arrangement is tunable in response to one or more input signals derived from a comparator operable to compare the response of the first output signal to the calibration signal with a desired value, and in response to provide a correction factor for varying a characteristic of the filter.
2. The adaptive filter arrangement according to claim 1 wherein the correction factor varies at least one capacitive element.
3. The adaptive filter arrangement according to claim 1 and including two low-pass filters for filtering first and second components from a base-band RF signal.
4. A method of calibrating an adaptive filter arrangement including the steps of:
pre-setting an up/down counter;
generating a reference signal in an OFDM encoder;
measuring and storing a filter response corresponding to the reference signal, the measuring including a Fast Fourier Transform;
decrementing or incrementing the up/down counter;
comparing the measured filter response with a desired value; and
varying a capacitance value depending upon the comparison, thereby varying the filter response.
5. The method according to claim 4 in which the reference signal is applied to a filter via a training path whilst decoupling a transmit and receive path.