Patent Application
20120288045
The present invention generally relates to a signal receiving apparatus and in particular it relates to an apparatus that receives and demodulates a signal wirelessly transmitted and also relates to a control method for multiple filters installed in such apparatus.
One of known signal receiving apparatuses operates in the following manner. An IF (intermediate frequency) signal is obtained by converting a received RF (radio frequency) signal into an intermediate frequency band, and this IF signal is subjected to frequency selection processing using an analog band pass filter (hereafter referred to as an analog BPF). Then, the signal after the processing is demodulated so as to restore information and/or data carried on the received RF signal.
The above-mentioned analog BPF is required to have a high Q value in order to increase the interference resistance at the time of signal reception. A filter device for realizing a desired attenuation characteristic and passband is also known. In this filter device, a plurality of filters having different frequency characteristics are connected in series (hereafter referred to as a multi-filter device). Such filter device is, for example, disclosed in FIG. 1 or 3 of Japanese Patent Application Publication (Kokai) No. 8-172338.
In the filter having the above-described constitution, however, a demodulation error may increase due to a group delay of the filter as shown in
In order to eliminate or reduce the demodulation error, a particular digital filter may be provided immediately after the analog BPF, as disclosed in FIG. 1 of Japanese Patent Application Publication No. 2002-141821. Specifically, the group delay characteristic may be offset by the digital filter having an inverse characteristic of group delay characteristic of the analog BPF.
In recent years, an active type filter having an amplifier thereon is used as the analog BPF so as to realize both size reduction and good filter characteristics. In a signal receiving apparatus equipped with the active type filter, drive current of the amplifier of the analog BPF is suppressed such that the apparatus consumes less power and the saturation amplified voltage output is set to a low value (for example, 1.5 V). When the saturation amplified voltage output of the amplifier is reduced, the amplifier operates close to a saturation zone when the level of an RF signal becomes relatively large (e.g., 1.2 V). For example, if a signal receiving apparatus is placed near a signal transmitting apparatus, that is to say, when so-called short-range communications are executed, the level of the RF signal becomes large. As described above, when the amplifier operates close to the saturation zone, the group delay characteristic of the analog BPF changes from the state indicated by the solid line in
When a short-range (or short distance) communication is carried out and the center frequency of a received signal is deviated from a desired center frequency due to a frequency deviation of a signal transmitting apparatus or an initial deviation of a center frequency of a local oscillator on the signal receiving apparatus side, then a signal having a frequency band desired by the BPF cannot be passed and hence accurate data demodulation becomes difficult.
One purpose of the present invention is to provide a signal receiving apparatus that is capable of performing highly accurate demodulation regardless of received signal strength.
Another purpose of the present invention is to provide a control method for multiple filters used in the signal receiving apparatus to attain highly accurate demodulation regardless of received signal strength.
According to a first aspect of the present invention, there is provided a signal receiving apparatus that includes a multi-filter device. The multi-filter device includes a plurality of filters. These filters have different frequency characteristics from each other, and are connected to each other in series. The signal receiving apparatus includes a unit for obtaining a frequency signal from an incoming signal (i.e., received signal). The frequency signal is then subjected to frequency selection processing. The frequency selection processing is carried out by the multi-filter device. The signal receiving apparatus also includes a demodulation part to demodulate a processed signal that has been subjected to the frequency selection processing so as to obtain information and/or data. The information and data may be an output from the signal receiving apparatus. The signal receiving apparatus also includes a received signal intensity determination part to determine whether the received signal intensity is greater than threshold intensity. The signal receiving apparatus also includes a controlling part to bias (or shift) the center frequency of at least one filter of the multi-filter device when the received signal intensity determination part determines that the received signal intensity is greater than the threshold intensity.
According to another aspect of the present invention, there is provided a control method for a multi-filter device included in a signal receiving apparatus. The multi-filter device includes a plurality of filters having different frequency characteristics. These filters are connected to each other in series. A frequency signal is prepared in the signal receiving apparatus upon receiving a transmitted signal. The frequency signal is subjected to frequency selection processing by the multi-filter device. The center frequency of at least one filter of the multi-filter device is biased when the received signal intensity is higher than the threshold intensity.
When a frequency signal derived from the received signal is subjected to the frequency selection processing by the multi-filter device and the received signal intensity is high enough to reach (or almost reach) a saturation zone of an amplifier installed in the multi-filter device, then the center frequency of at least one filter in the multi-filter device is biased. By biasing the filter center frequency toward a direction away from the center frequency of the multi-filter device, the pass band width of the multi-filter device becomes wider, while a changing amount of the group delay is reduced. Therefore, when the changing amount of the group delay of the multi-filter device is large due to high received signal intensity, it is still possible to extract a signal having a desired band to be demodulated, even if the center frequency of the received signal is deviated from a prescribed center frequency. Thus, it is possible perform highly accurate demodulation.
These and other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description when read and understood in conjunction with the appended claims and drawings.
The multi-filter device of the present invention has a plurality of filters. These filters have different frequency characteristics from each other, and are connected in series. When a frequency signal derived from a received signal is subjected to frequency selection processing by the multi-filter device and received signal intensity is greater than prescribed threshold intensity, then the center frequency of at least one of the filters in the multi-filter device is biased.
Referring to
In
The BPF 3 applies the frequency selection processing on the intermediate frequency signal IF to allow passage of only a component of a prescribed frequency band with a center frequency fc at the center, and extracts an intermediate frequency signal IFX from which an unnecessary band component is removed. The BPF 3 then supplies the intermediate frequency signal IFX to an intermediate frequency amplifying circuit 4. The BPF 3 is a multiple-filter device that is capable of changing the filter characteristic based on filter control data GD (will be described below).
The intermediate frequency amplifying circuit 4 amplifies the intermediate frequency signal IFX and supplies the resulting signal (i.e., amplified intermediate frequency signal) IFA to both an A/D converter 5 and an RSSI (Received Signal Strength Indicator) circuit 8. The RSSI circuit 8 serves as a received signal intensity measurement circuit. The RSSI circuit 8 rectifies the amplified intermediate frequency signal IFA to measure the intensity of the signal received at the antenna 22 and supplies a received signal intensity signal RS indicating the received signal strength to the A/D converter 5. The A/D converter 5 converts the amplified intermediate frequency signal IFA into a digital value (i.e., intermediate frequency signal IFD) and supplies the intermediate frequency signal IFD to a demodulation circuit 9. The A/D converter 5 also converts the intensity signal RS into a digital value (i.e., received signal intensity signal RSD) and supplies the received signal intensity signal RSD to a received signal level determining circuit 11. The demodulation circuit 9 applies demodulation processing on the intermediate frequency signal IFD to restore the information and/or data wirelessly transmitted to the signal receiving apparatus 20. The demodulation circuit 9 may supply the resulting information and/or data to a signal reception controlling part (not shown) or to another device (not shown).
The received signal level determination circuit 11 determines whether or not the received signal strength indicated by the received signal intensity data RD is greater than threshold intensity RG. When the received signal strength is greater than the threshold value RG, then a strong signal reception detecting flag GF of logical level 1 is supplied to a selector 12. When the received signal strength is not greater than the threshold value RG, the flag GF of logical level 0 is supplied to the selector 12. Since a data error occurs when the operational amplifier (
A control data register for high signal intensity 23 stores, in advance, filtering control data G1 for high received signal intensity. The control data G1 becomes optimal when the received signal intensity is higher than the threshold value RG. The filter control data is used to set (or decide) the filter characteristic of the BPF 3. The control data register for high intensity signal 23 supplies the filtering control data G1 to the selector 12.
The signal receiving apparatus 20 has another control data register 24. This register 24 is provided for low intensity signal, and stores in advance filtering control data G2 for low intensity received signal. The control data G2 becomes optimal when the received signal intensity is not greater than the threshold value RG. The second control data register 24 supplies the filtering control data G2 to the selector 12.
On the basis of the received signal strength flag GF supplied from the received signal level determination circuit 11, the selector 12 selects either the high intensity signal filtering control data G1 or low intensity signal filtering control data G2, and supplies the selected signal to the BPF 3 as the filter control data GD. Specifically, upon receiving the signal strength flag GF of logical level 1, i.e., when the received signal intensity is higher than the threshold intensity RG, then the selector 12 selects the high intensity signal filtering control data G1, and supplies it to the BPF 3 as the filter control data GD. On the other hand, when the signal strength flag GF of logical level 0 is supplied to the selector 12 from the received signal level determining circuit 11, i.e., when the received signal intensity is not higher than the threshold intensity RG, the selector 12 selects the low intensity signal filtering control data G2, and supplies it to the BPF 3 as the filter control data GD.
Referring now to
As illustrated in
Referring to a circuit diagram shown in
In
The second active filter AF2 includes an operational amplifier 41, variable resistances 42-45, and variable capacitors 46, 47. A quadrature-phase component signal Q of the intermediate frequency signal IF supplied from the mixer 2 (“IF(Q)” in
The positive output terminal of the operational amplifier 31 is connected to the positive input terminal of the operational amplifier 41 via a variable resistance 53. The negative output terminal of the operational amplifier 31 is coupled to the negative input terminal of the operational amplifier 41 via a variable resistance 54. The negative output terminal of the operational amplifier 41 is coupled to the positive input terminal of the operational amplifier 31 via a variable resistance 51. The positive output terminal of the operational amplifier 41 is connected to the negative input terminal of the operational amplifier 31 via a variable resistance 52. An output current of each of the operational amplifiers 31, 41 is 10 microampere (μA), for example.
In the first active filter AF1, the in-phase component signal “I” of the intermediate frequency signal IF supplied from the mixer 2 (IF(I)) is subjected to the frequency selection processing with the filter characteristic based on (or decided by) the filter control data GD. As a result, the first active filter AF1 generates an intermediate frequency signal IFX(I). In the second active filter AF2, the quadrature-phase component signal Q of the intermediate frequency signal IF (IF(Q)) is subjected to the frequency selection processing with the filter characteristic based on the filter control data GD. Thus, the second active filter AF2 generates an intermediate frequency signal IFX(Q).
The operation to change the filter characteristic of the BPF 3 will be described below.
When the received signal strength of the signal receiving apparatus 20 is weaker than the threshold intensity RG, the received signal strength determination circuit 11 supplies the flag GF of the logical level 0 indicating low signal strength to the selector 12. Accordingly, the selector 12 selects and supplies the low intensity signal filtering control data G2 from the low intensity signal control data register 24 to the BPF 3 as the filter control data GD.
The low intensity signal filtering control data G2 specifies resistance values of the variable resistances 32-35, 42-45, 51-54 and capacitance values of the variable capacitors 36, 37, 46, 47 for each of the variable characteristic filters F1-F5 illustrated in
As shown in
On the other hand, when the received signal strength of the signal receiving apparatus 20 is stronger than the threshold intensity RG, the received signal strength determining circuit 11 supplies the flag GF of the logical level 1 indicating arrival of high intensity signal to the selector 12. Accordingly, the selector 12 selects and supplies the high intensity signal filtering control data G1 from the high intensity signal control data register 23 to the BPF 3 as the filter control data GD. Similar to the low intensity signal filtering control data G2, the high intensity signal filtering control data G1 specifies the filter characteristic parameters of the variable characteristic filters F1-F5 individually.
With respect to the filter characteristic parameters for the variable characteristic filters F1-F3 and F5, the high intensity signal filtering control data G1 is the same as the low intensity signal filtering control data G2. The filter characteristic parameter for the variable characteristic filter F4 is only different in this embodiment. The filter characteristic parameter in the control data G1 for the filter F4 includes a set of values that cause the filter F4 to function as the BPF that has the frequency characteristic Q6 illustrated in
As shown in
Therefore, the pass band of the BPF 3 in the high received signal intensity mode, which is indicated by the solid line in
In short, when the received signal intensity RS is greater than the threshold intensity RG, the center frequency of at least one of the filters F1-F5 constituting the BPF 3 (i.e., the multi-filter device) is biased or shifted. By biasing the center frequency toward a direction away from the center frequency fc of the multi-filter device 3, the pass band width of the BPF becomes wider while the change amount of the group delay becomes smaller. Accordingly, even if the change of the group delay of the BPF becomes large due to the high received signal intensity and the center frequency of the received signal is deviated from the prescribed center frequency due to the frequency deviation on the signal transmitting apparatus side, or the initial deviation, temperature deviation or the like of the center frequency of the local oscillator on the signal receiving apparatus side, it is still possible to extract a signal IFX having a desired band for demodulation.
Therefore, by applying the signal receiving apparatus 20 of
When the BPF 3 is switched from the low received signal intensity mode to the high received signal intensity mode in the illustrated embodiment, the center frequency of only the filter F4 is biased among the five filters F1-F5. It should be noted, however, that the center frequencies of two or more of the five filters F1-F5 may be biased or changed. By individually biasing the center frequencies of the two (or more) filters among the filters F1-F5, it becomes possible to further widen (expand) the pass band width of the BPF 3, or flatten the frequency characteristic. Also, the Q values (frequency band characteristics) of the respective variable characteristic filters F1-F5 may be changed in addition to biasing of the center frequency as described above. By individually changing the bands in addition to shifting of the center frequencies of some filters among the filters F1-F5, it becomes possible to obtain the desired filter characteristics more accurately.
If the received signal intensity of the RSSI circuit 8 does not have good reproducibility upon (or after) switching the BPF 3 to the low received signal intensity mode from the high received signal intensity mode, then the received signal intensity determination circuit 11 may be designed to have a hysteresis characteristic such that the determination circuit 11 can determine whether the received signal intensity changes from higher-than-the-threshold-intensity RG to lower-than-the-threshold-intensity RG.
In the above-described embodiment, the changes of the filter characteristics of the filters F1-F5 are realized by controlling the variable resistances 32-35, 42-45 and 51-54, and the variable capacitors 36, 37, 46 and 47. It should be noted, however, that the present invention is not limited to this configuration. For example, the variable resistances 32-35, 42-45 and 51-54 shown in
Other changes and modifications may also be made to the illustrated embodiment. For example, although the BPF 3 is used to switch between the high received signal intensity mode and low received signal intensity mode in the embodiment, a direct conversion type low-pass filter may be employed instead of the band pass filter. More specifically, when the multi-filter device is a low-pass filter, and the received signal intensity RS is higher than the threshold value RG, then the center frequency of at least one filter among the multiple filters contained in the low-pass filter may be shifted away from the cut-off frequency of the low-pass filter toward a direction of the high range. Accordingly, when the received signal intensity is high, such biasing widens (or broadens) the pass band width of the low-pass filter and reduces the variations of the group delay.
This application is based on Japanese Patent Application No. 2011-106373 filed on May 11, 2011 and the entire disclosure thereof is incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-106373 | May 2011 | JP | national |
