In a wireless radio receiver, an incoming high-frequency radio signal, such as an FM radio signal, is converted into a signal with an intermediate frequency (IF), which is then amplified and passed to a demodulator which retrieves information, such as baseband audio, from the radio signal.
Modern integrated FM receivers preferably realize channel selectivity at low IF frequencies. As an example, in FIG. 1 the back-end of such an integrated FM radio receiver is shown. This back-end consists of the IF part and the demodulator of such an integrated radio receiver. The FM receiver back-end 10 comprises a complex IF filter 12 which receives a complex input signal having separate in-phase (I) and quadrature (Q) component signals and which is used for adequate image suppression. In other words, the filter 12 only has a pass band at the positive IF frequency or the negative IF frequency.
In combination with an image-rejecting complex mixer in front of this filter 12, the image frequency caused by the mixer will be suppressed by the selectivity curve of the complex filter. Since the image channel is very close to the desired channel for low-IF receivers, the image rejection is merely realized in the combination of the image-rejecting mixer with the complex filter.
Respective IF limiting amplifiers 14, 16 are provided for each of the I and Q signals output from the complex IF filter 12, which either linearly or non-linearly amplify the signals. The I and Q signals from the IF limiters 14, 16 are then used to drive a complex FM demodulator 18. The complex demodulator 18 (using I and Q input signals) is preferred for its suppression of spurious responses at 2ωIF, where ωIF represents the radian IF centre frequency.
In many cases RSSI information is required in the receiver. For instance, the RSSI information can be used in the channel-search algorithm in the tuning system of the receiver. The RSSI information is derived from each of the I and Q signals from the respective IF limiters 14, 16 and provided at an output-RSSI-out 20.
The complex demodulator 18 comprises a complex demodulator filter 22, respective phase detectors 24, 26 for the I and Q signals output from the complex demodulator filter 22, and a summing block 28 for combining the outputs of the two phase detectors 24, 26.
After the demodulator 18, a low-pass filter 30 is provided to prevent spurious signals around harmonics of 38 kHz being mixed down to audible “birdies” in a subsequent stereo decoder.
However, the requirement for respective IF limiters for each of the in-phase and quadrature signals increases the on-chip area needed to implement the integrated radio receiver.
Where an active real or complex filter in a radio receiver has large tolerances as a result of the components in the filter, it is desirable to be able to accurately identify and fix the filter response.
It is known to achieve this by using a reference signal that has an accurate radian frequency ωref which serves as a basis to tune all time constants in the filter to their desired values. For this purpose the time constants in the filter need to be tunable with a control signal Xs, for instance by regulating the transconductances (resistors) in the filter or by tuning the capacitors in the filter by using varactors. The filter to be fixed is referred to as the main filter.
In order to obtain the control signal to tune all of the time constants in the main filter, a master filter is used which closely matches the time constants in the main filter. Commonly used master filters use a low-order filter like a single pole complex filter or a two pole real filter. In a control loop, the centre frequency of the master filter is tuned to be equal to the accurate reference radian frequency (ωref, thereby generating a control signal Xs. This control signal will also be used in the main filter to accurately tune all of the time constants to their desired values, thereby realizing an accurate filter response.
However, due to parasitic effects in the main filter, the time constants may not linearly scale to the master filter. This is especially true in the general situation where the pole positions of the main filter are located at different positions compared to the low-order master filter. This non-linear scaling will result in an error in the pole positions and gain of the main filter.
It is therefore an object of the invention to accurately fix the filter response of an active real- or complex-filter, even when there are large tolerances in the filter components.