Abstract:
A switchable filter for passing one of a plurality of frequency bands has n bandpass filter sections coupled in parallel between a filter input terminal and a filter output terminal. Each of the n bandpass filter sections is arranged to pass a different frequency band. Each of n switches is coupled to a respective one of the n bandpass filter sections, and each of the n switches is operable in either an OFF state for activating a corresponding one of the n bandpass filter sections or an ON state for deactivating the corresponding one of the n bandpass filter sections. The n switches are arranged and controlled so that, when any one of then switches is operated to its off state, the remaining n−1 switches are operated to their ON states and form a series circuit for conducting a common current.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to bandpass filters and is particularly directed to a switchable bandpass filter for processing a wide band RF signal particularly adapted for use in a frequency converter. 
     BACKGROUND OF THE INVENTION 
     An electronic tuner such as used in atypical RF receiver includes up to three tunable circuits, i.e., a tunable circuit between the antenna input and the RF amplifier, a tunable circuit between the RF amplifier and the mixer, and a tunable circuit associated with the oscillator. Each of these tunable circuits is simultaneously tuned to a frequency representing the desired station, or channel. When more than one octave of frequencies is to be covered by the tuner, band switching techniques are frequently used to selectively change some elements of each tunable circuit in order to accommodate the entire desired frequency band. 
     Also, many tuners or frequency converters, such as for example those used with CATV systems, operate without a selective front end or mixer protection. As the desired RF bandwidth in this case increases over an octave of frequencies, the number of intermodulation products and the possibility of crossmodulation products quickly increase, imposing greater demands upon tuner. 
     Prior art approaches to multi-band tuning and mixer protection have generally involved the use of varactor diodes and/or switching diodes in the tuning stages. The varactor diode approach makes use of circuit elements that exhibit capacitances which are functions of the applied voltage. By varying the DC tuning voltage applied to a varactor diode, its capacitance changes, and the frequency to which the resonant circuit is tuned may be thereby adjusted within approximately one octave frequency range as desired. The limitation of the varactor tuning range to approximately one octave is overcome frequently by means of switching diodes forming a tuned and bandswitched network together with the varactor diodes and other mostly inductive components. 
     Another limitation to this varactor diode approach arises from an inherent characteristic of a varactor diode. Specifically, a varactor diode exhibits a nonlinear variation in capacitance as a function of the applied RF voltage. This non-linearity may add more crossmodulation and intermodulation products to that caused by active elements of the tuner. 
     Another problem inherent in varactor tuned frequency converters with a relatively high IF frequency (upconverter) is the difficulty of tracking the frequency between the RF and oscillator tuned circuits. To overcome this problem, complicated analog or digital schemes have been used. 
     On the other hand, a simple switching diode approach to band selection generally involves the use of separate bandpass filter sections that are selectively switched in and out of circuit depending upon the frequency band desired. In this switching diode approach, one bandpass filter section is generally active, while the remaining bandpass filter sections are switched out of circuit and are thus rendered inactive. Thus, the majority of circuit components are not in use at any given time and there is, consequently, a high component count for the various bandpass filter sections. In addition, more than one switching diode is generally required for each frequency band to allow for the switching in and out of circuit of both the input and the output of each bandpass filter section so as to select the circuit for receiving a desired frequency band. 
     In order to reduce the cross- and intermodulation products in wide band CATV tuners and signal converters, double balanced mixers are frequently employed. However, because the CATV band has expanded into the UHF band, the practical limits of double balanced mixer performance within the CATV tuner have essentially been reached in terms of tradeoff between intermodulation and noise performance. With this large RF bandwidth, mixer intermodulation and noise performance determine the dynamic range of the CATV tuner. The noise figure for a typical frequency converter chain with a mixer, which typically exhibits a conversion loss from 6 to 8 dB, is generally in the range of 10 to 13 dB. Placing a feedback type preamplifier with a gain of 10 dB in front of the mixer improves the system&#39;s noise figure to 5 to 7 dB, but decreases the signal converter&#39;s RF processing capability by an associated 10 dB. This decrease of the signal converter&#39;s RF processing capability is generally unacceptable, except where an electronic band switch is also placed in front of the mixer state. However, this capability has not generally been available without employing overly complicated switching arrangements with large numbers of electronic components. 
     The invention disclosed in U.S. Pat. No. 4,571,560 overcomes one or more of the aforementioned limitations of the prior art by providing a switchable bandpass filter which is capable of operating over a large bandwidth, which employs a relatively low number of components by using many of the components in each of the inactive, or bandstop, sections of the filter, and which utilizes only a single switching diode in each bandpass filter section. 
     As disclosed in the aforementioned patent, one of the bandpass filter sections is activated by reverse biasing its corresponding switching diode. The other switching diodes are forward biased in order to deactivate their corresponding bandpass filter sections. These forward biased switching diodes, however, conduct in parallel, which causes a relatively large power drain. Unfortunately, in some battery powered applications, such as where tuners are used in laptop computers and in other portable devices, a large power drain shortens the time that the device can operate off of a single battery charge. 
     The present invention reduces the power drain in a bandpass filter by coupling the filter controlling switches in series rather than in parallel to thereby reduce power drain. Furthermore, the elements (e.g., tri-state buffers) that control the switches may be integrated together with other functional blocks of a tuner. For example, it is customary to integrate all sorts of switching means within a frequency synthesizer IC while using the control bus of the latter also for the purpose of bandswitching thus minimizing the cost of the control circuits. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a switchable filter for passing one of a plurality of frequency bands comprises a filter input terminal, a filter output terminal, n bandpass filter sections, and n switches. Each of the n bandpass filter sections is coupled between the filter input terminal and the filter output terminal, and each of the n bandpass filter sections is arranged to pass a different frequency band. Each of the n switches is coupled to a respective one of then bandpass filter sections, and each of the n switches is operable for assuming either an OFF state for activating a corresponding one of the n bandpass filter sections or an ON state for deactivating the corresponding one of the n bandpass filter sections. The n switches are arranged and controlled so that, when any one of the n switches is operated to its OFF state, the remaining n−1 switches are operated to their ON states and form a series circuit for conducting a common current, and n ∃ 3. 
     In accordance with another aspect of the present invention, a switchable filter for passing one of a plurality of frequency bands comprises a filter input terminal, a filter output terminal, at least first, second, and third bandpass filter sections, at least first, second, and third switches, and a controller. Each of the at least first, second, and third bandpass filter sections is coupled between the filter input terminal an the filter output terminal, and each of the at least first, second, and third bandpass filter sections is arranged to pass a different frequency band. The first switch is coupled in controlling relationship to the first bandpass filter section, the second switch is coupled in controlling relationship to the second bandpass filter section, and the third switch is coupled in controlling relationship to the third bandpass filter section. The controller is coupled to the at least first, second, and third switches. The controller controls the first switch to an OFF state so that the first bandpass filter section is activated and controls the second and third switches to ON states so that the second and third switches conduct in series so as to deactivate the second and third bandpass filter sections. The controller controls the second switch to an OFF state so that the second bandpass filter section is activated and controls the first and third switches to ON states so that the first and third switches conduct in series so as to deactivate the first and third bandpass filter sections. The controller controls the third switch to an OFF state so that the third bandpass filter section is activated and controls the first and second switches to ON states so that the fist and second switches conduct in series so as to deactivate the first and second bandpass filter sections. 
     In accordance with still another aspect of the present invention, a switchable filter for passing one of a plurality of frequency bands comprises a filter input terminal, a filter output terminal, first, second, third, and fourth bandpass filter sections, first, second, third, and fourth switches, and a controllers. The first bandpass filter section is coupled between the filter input terminal and the filter output terminal, and the first bandpass filter section is arranged to pass a first frequency band. The second bandpass filter section is coupled between the filter input terminal and the filter output terminal, and the second bandpass filter section is arranged to pass a second frequency band. The third bandpass filter section is coupled between the filter input terminal and the filter output terminal, and the third bandpass filter section is arranged to pass a third frequency band. The fourth bandpass filter section is coupled between the filter input terminal and the filter output terminal, and the fourth bandpass filter section is arranged to pass a fourth frequency band. The first, second, third, and fourth frequency bands are different. The first switch is coupled to the first bandpass filter section, the second switch is coupled to the second bandpass filter section, the third switch is coupled to the third bandpass filter section, and the fourth switch coupled to the fourth bandpass filter section. The controller is coupled to the first, second, third, and fourth switches. The controller controls the first switch so that the first bandpass filter section is activated and controls the second, third, and fourth switches so that the second, third, and fourth switches conduct current in series to deactivate the second, third, and fourth bandpass filter sections. The controller controls the second switch so that the second bandpass filter section is activated and controls the first, third, and fourth switches so that the first, third, and fourth switches conduct current in series to deactivate the first, third, and fourth bandpass filter sections. The controller controls the third switch so that the third bandpass filter section is activated and controls the first, second, and fourth switches so that the first, second, and fourth switches conduct current in series to deactivate the first, second, and fourth bandpass filter sections. The controller controls the fourth switch so that the fourth bandpass filter section is activated and controls the first, second, and third switches so that the first, second, and third switches conduct current in series to deactivate the first, second, and third bandpass filter sections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which: 
     FIG. 1 illustrates a bridge arrangement for coupling the swtichgn diodes of a switchable bandpass filter in a series relationship; 
     FIG. 2 is a schematic diagram of an exemplary switchable bandpass filter incorporating the bridge coupled switching diodes shown in FIG. 1; and, 
     FIG. 3 shows the four overlapping attenuation curves of the switchable bandpass filter of FIG. 2 as a function of frequency and of the state of the switching diodes. 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 1, switching diodes  10 ,  12 ,  14 , and  16  are coupled in a bridge  18  having four terminals V 1 , V 2 , V 3 , and V 4 . Specifically, a resistor  20  is coupled between the anode of the switching diode  10  and the terminal V 1 , and a resistor  22  is coupled between the cathode of the switching diode  10  and the terminal V 2 . A resistor  24  is coupled between the anode of the switching diode  12  and the terminal V 2 , and a resistor  26  is coupled between the cathode of the switching diode  12  and the terminal V 3 . A resistor  28  is coupled between the anode of the switching diode  14  and the terminal V 3 , and a resistor  30  is coupled between the cathode of the switching diode  14  and the terminal V 4 . Finally, a resistor  32  is coupled between the anode of the switching diode  16  and the terminal V 4 , and a resistor  34  is coupled between the cathode of the switching diode  16  and the terminal V 1 . 
     As explained below, a controller  40  selectively controls the bridge  18  so that one of four bandpass filter sections is active and so that the other three bandpass filter sections are inactive. For example, the controller  40  reverse biases the switching diode  10  in order to activate the bandpass filter section controlled by the switching diode  10  and forward biases the switching diodes  12 ,  14 , and  16  in order to deactivate the bandpass filter sections correspondingly controlled by the switching diodes  12 ,  14 , and  16 . When the switching diodes  12 ,  14 , and  16  are forward biased, they conduct in series between the terminals V 2  and V 1 . Because the switching diodes  12 ,  14 , and  16  conduct in series, they draw less power than the prior art switching diodes, which conduct in parallel. 
     The controller  40  includes eight tri-state buffers  42 ,  44 ,  46 ,  48 ,  50 ,  52 ,  54 , and  56 . The eight tri-state buffers  42 ,  44 ,  46 ,  48 ,  50 ,  52 ,  54 , and  56  operate with one pair being ON and the other three pairs being OFF, as described below, in order to reverse bias a selected one of the diodes  10 ,  12 ,  14 , and  16  and to forward bias the other three of the diodes  10 ,  12 ,  14 , and  16 . When each of the eight tri-state buffers  42 ,  44 ,  46 ,  48 ,  50 ,  52 ,  54 , and  56  is OFF, it should have a very high impedance so that no current flow therethrough. 
     The four tri-state buffers  42 ,  44 ,  46 , and  48  source current to the bridge  18 , and the four tri-state buffers  50 ,  52 ,  54 , and  56  sink current from the bridge  18 . Thus, each of the input terminals of the tri-state buffers  42 ,  44 ,  46 , and  48  is coupled to a positive source  58 , such as a 5 volt source, and each of the input terminals of the tri-state buffers  50 ,  52 ,  54 , and  56  is coupled to a reference potential, such as ground. 
     The eight tri-state buffers  42 ,  44 ,  46 ,  48 ,  50 ,  52 ,  54 , and  56 , for example, may be supplied under part number 74HC126 and may each comprises an input terminal, an output terminal, and an enable terminal. 
     The output terminals of the tri-state buffers  48  and  50  are coupled to the terminal V 1  of the bridge  18 , the output terminals of the tri-state buffers  42  and  52  are coupled to the terminal V 2  of the bridge  18 , the output terminals of the tri-state buffers  44  and  54  are coupled to the terminal V 3  of the bridge  18 , and the output terminals of the tri-state buffers  46  and  56  are coupled to the terminal V 4  of the bridge  18 . 
     The enable terminals of the tri-state buffers  42  and  50  are coupled to a BAND 1  control terminal, the enable terminals of the tri-state buffers  44  and  52  are coupled to a BAND 2  control terminal, the enable terminals of the tri-state buffers  46  and  54  are coupled to a BAND 3  control terminal, and the enable terminals of the tri-state buffers  48  and  56  are coupled to a BAND 4  control terminal. 
     Accordingly, when the BAND 1  control terminal is high, the tri-state buffers  42  and  50  conduct so that the switching diode  10  is reversed biased and so that the switching diodes  12 ,  14 , and  16  are forward biased. When the BAND 2  control terminal is high, the tri-state buffers  44  and  52  conduct so that the switching diode  12  is reversed biased and so that the switching diodes  10 ,  14 , and  16  are forward biased. When the BAND 3  control terminal is high, the tri-state buffers  46  and  54  conduct so that the switching diode  14  is reversed biased and so that the switching diodes  10 ,  12 , and  16  are forward biased. When the BAND 4  control terminal is high, the tri-state buffers  48  and  56  conduct so that the switching diode  16  is reversed biased and the switching diodes  10 ,  2 , and  14  are forward biased. 
     As shown in FIG. 2, a switchable bandpass filter  60  includes four bandpass filter sections  62 ,  64 ,  66  and  68  each of which is coupled to a filter input terminal  70  for selectively filtering RF signals provided to the filter input terminal  70 . In the switchable bandpass filter  60 , the bandpass filter sections  62 ,  64 ,  66  and  68  include the switching diodes  10 ,  12 ,  14 , and  16  and the resistors  20 ,  22 ,  24 ,  26 ,  28 ,  30 ,  32 , and  34  of the bridge  18 . Each of the four bandpass filter sections  62 ,  64 ,  66 , and  68  is tuned to a respective passband and is activated by a corresponding one of the switching diodes  10 ,  12 ,  14 , and  16 . 
     The switching diode  10  is responsive to the voltages at the terminals V 1  and V 2  in order to activate the bandpass filter section  62 . The switching diode  12  is responsive to the voltages at the terminals V 2  and V 3  in order to activate the bandpass filter section  64 . The switching diode  14  is responsive to the voltages at the terminals V 3  and V 4  in order to activate the bandpass filter section  66 . The switching diode  16  is responsive to the voltages at the terminals V 4  and V 1  in order to activate the bandpass filter section  68 . 
     The switchable bandpass filter  60  may be arranged to interface with a 50 to 75 ohm source and load, and can be switched between four states sequentially covering four partially overlapping frequency bands ranging from 50 to about 800 MHz with each band covering approximately one octave. 
     The operation and configuration of the bandpass filter section  62  within the switchable bandpass filter  60  is described herein, it being understood that the remaining three bandpass filter sections  64 ,  66 , and  68  operate in a similar manner with different component values for the different frequency passbands. In addition, it should be understood that, while four bandpass filter sections coupled in parallel between the filter input terminal  70  and a filter output terminal  72  are shown in FIG. 2, the present invention is not limited to this number of bandpass filter sections and may instead include fewer or additional bandpass filter sections as desired or needed. 
     As can be seen from FIG. 2, the switchable bandpass filter  60  relies on some of the reactive components of all inactive bandpass filter sections in order to improve the stop-band suppression of the active bandpass filter section. The result of this arrangement is an efficient band switching network with good in-band matching and attenuation and steep out-of-band slopes. 
     Each of the bandpass filter sections  62 ,  64 ,  66 , and  68  within the switchable bandpass filter  60  includes an input and an output series resonant circuit as well as a parallel resonant circuit. In the case of the bandpass filter section  62 , the input series resonant circuit is comprised of an inductor  74  and capacitors  76  and  78 . Similarly, the output series resonant circuit of the bandpass filter section  62  is comprised of capacitors  80  and  82  and an inductor  84 . The parallel resonant circuit is comprised of an inductor  86  and the capacitors  76 ,  78 ,  80  and  82 . A capacitor  88  provides an RF ground for one end of the inductor  86 , and a capacitor  90  provides an RF ground for the cathode of the switching diode  10 . 
     If it is desired to tune the switchable passband filter  60  to the passband BAND 1 , the BAND 1  terminal of the controller  40  is driven high which causes the tri-state buffers  42  and  50  to conduct so as to drive the terminal V 2  to the potential of the positive source  58  and to drive the terminal V 1  to the reference potential. Accordingly, the switching diode  10  is reversed biased and the switching diodes  12 ,  14 , and  16  are forward biased so that the switching diode  10  does not conduct and so that the switching diodes  12 ,  14 , and  16  conduct. With the switching diodes  12 ,  14 , and  16  conducting, the bandpass filter sections  64 ,  66 , and  68  are deactivated. 
     On the other hand, with the switching diode  10  of the bandpass filter section  62  non-conducting, the bandpass filter section  62  is activated. Accordingly, the capacitors  76  and  78  form an input series resonant circuit with the inductor  74 , the capacitors  80  and  82  form an output series resonant circuit with the inductor  84 , and the capacitors  76 ,  78 ,  80 , and  82  form a parallel resonant circuit with the inductor  86 . The values of the components of these series and parallel resonant circuits are selected such that the bandpass filter section  62  is tuned to the center frequency of the desired passband BAND 1 . 
     With the bandpass filter sections  64 ,  66 , and  68  thus rendered inactive by the switching diodes  12 ,  14 , and  16 , the parallel resonant circuits of the bandpass filter sections  64 ,  66 , and  68  are thus removed from the bandpass filter sections  64 ,  66 , and  68 . Therefore, the frequency bands to which the inactive bandpass filter sections  64 ,  66 , and  68  are tuned are not provided to the filter output terminal  72 . However, the input and output series resonant circuits of the inactive bandpass filter sections  64 ,  66 , and  68  remain in circuit with the active bandpass filter section  62 . Each of the series resonant circuits of the respective inactive bandpass filter sections  64 ,  66 , and  68  is tuned to a notch frequency within the stopband of the active bandpass filter section  62 . 
     The effect that the resonant circuits of the deactivated bandpass filter sections have on the activated bandpass filter section can be further understood by considering the series resonant filters of the switchable bandpass filter  60  when the bandpass filter section  64  is activated and the bandpass filter sections  62 ,  66 , and  68  are deactivated. Thus, if it is desired to tune the switchable passband filter  60  to the passband BAND 2 , the BAND 2  terminal of the controller  40  is driven high which cause the tri-state buffers  44  and  52  to conduct so as to drive the terminal V 3  to the potential of the positive source  58  and to drive the terminal V 2  to the reference potential. Accordingly, the switching diode  12  is reversed biased and the switching diodes  10 ,  14 , and  16  are forward biased so that the switching diode  12  does not conduct and so that the switching diodes  10 ,  14 , and  16  conduct. With the switching diodes  10 ,  14 , and  16  conducting, the bandpass filter sections  62 ,  66 , and  68  are deactivated. 
     With the switching diode  12  rendered nonconductive, the electrical configuration and operation of the bandpass filter section  64  is identical to that previously described with respect to the bandpass filter section  62 . With the bandpass filter section  62  rendered deactivated by the switching diode  10 , the capacitors  76  and  78  are arranged in parallel and together with the inductor  74  form a series resonant circuit across the input terminals. Similarly, the capacitors  80  and  82  are arranged in parallel and, in combination with the inductor  84 , form a series resonant circuit across the output terminals. The parallel resonant circuit of the bandpass filter section  62  is removed from the bandpass filter section  62  when it is rendered inactive. The frequency band to which the bandpass filter section  62  is tuned is thus no longer provided to the filter output terminal  72 , and its series resonant circuits, together with the series resonant circuits of the remaining inactive bandpass filter sections  66  and  68 , remain in circuit with the active bandpass filter section  64 . Each of the series resonant circuits of the respective inactive bandpass filter sections  62 ,  66 , and  68  in tuned to a notch frequency within the stopband of the active bandpass filter section  64 . 
     FIG. 3 shows a graphical representation of the response of the switched bandpass filter  60  over a range of frequencies. The frequency range shown in FIG. 3 is for a typical CATV converter whose frequency range extends from approximately 50 to more than 500 MHz. As shown in FIG. 3, the frequency response for each of the bandpass filter sections  62 ,  64 ,  66 , and  68  is a separate response curve. For example, f 1 , f 2 , f 3  and f 4  represent the respective frequency responses of the bandpass filter sections  62 ,  64 ,  66 , and  68 . It can be seen that each of the f 2 , f 3 , and f 4  response curves exhibits a notch frequency within the passband of f 1 . Similarly, the passband of f 2  coincides with notch frequencies of the f 1 , f 3  and f 4  frequency response curves. The same is true of the response curves f 3  and f 4  which coincide with notch frequencies of the remaining response curves. 
     From the frequency response curves of FIG. 3, it can be seen that the series resonant circuit portions of the inactive bandpass sections cooperate with the active bandpass section to form notch circuits outside of the active band for improved stop-band suppression of the active bandpass section. This improved filtering of the received RF signals is made possible by using approximately two-thirds of the components of all of the inactive bandpass section in circuit with the active bandpass section to provide improved selective filtering of the received signal. 
     The filter input terminal  70  in FIG. 2 is shunted to ground by a parallel resonant circuit  100 . Similarly, the filter output terminal  72  in FIG. 2 is shunted to ground by a parallel resonant circuit  102 . The parallel resonant circuits  100  and  102  are identically tuned to a frequency between the lowest and the highest passband frequencies. The purpose of the parallel resonant circuits  100  and  102  is to improve the passband ripple for the highest and for the lowest passbands of the switchable bandpass filter  60 . 
     There has thus been shown a switchable bandpass filter having a plurality of parallel bandpass filter sections each including input and output series resonant circuits as well as a parallel resonant circuit coupled therebetween. Each bandpass filter section further includes a switching diode coupled across the parallel resonant circuit for grounding the parallel resonant circuit so as to render the corresponding bandpass filter section inactive. Only one of the bandpass filter sections is active for passing a given frequency band, with the input and output series resonant circuit of the inactive bandpass filter sections complementing the active bandpass filter section by serving as notch filters outside of the active band for improving the stop-band suppression of the active bandpass filter section. 
     Certain modifications of the present invention have been discussed above. Other modifications will occur to those practicing in the art of the present invention. For example, as described above, the switching diodes  10 ,  12 ,  14 , and  16  are used to activate the deactive the bandpass filter sections  62 ,  64 ,  66 , and  68 . However, other suitable forms to switches may be used instead of diodes for this purpose. 
     Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.