Varactor tuned helical resonator for use with duplex filter

• US 5,717,368 A
• Filed: 11/14/1996
• Issued: 02/10/1998
• Est. Priority Date: 09/10/1993
• Status: Expired due to Fees

• Current Assignee: Filtronic LK Oy
• Original Assignee: Pulse Finland Oy (Yageo Corporation)
• Inventors: Niiranen, Erkki
• Primary Examiner(s): Pascal, Robert
• Assistant Examiner(s): GAMBINO, DARUIS
• Application Number:
  US08/746,675
• Time in Patent Office: 453 Days
• Field of Search: 333/176-180, 333/202-207, 333/174-175, 455/195.1, 455/197.1, 455/340
• US Class Current: 333/202
• CPC Class Codes: H03H 7/12 Bandpass or bandstop filters with adjustable bandwidth and fixed centre frequency H03H7/09 takes precedence; automatic control of bandwidth in amplifiers H03G5/16
• CPC Class Codes: H03H 7/1791 Combined LC in shunt or branch path

Abstract

A radio frequency filter comprising several resonator circuits, such as duplex filter, is provides with a given passband. Signal frequencies generating spurious response, mirror frequencies, etc. can be effectively attenuated so that the filter comprises one more resonator circuit connected as a bandstop circuit. That comprises a transmission line resonator (Res), a series connection of an inductive element (MLIN2) and a capacitance diode (D), one end of the series connection being couples to a coupling point in the transmission line resonator, dividing it into two parts (TLIN1, TLIN2), and the other end being couples to the output connector (1) of the radio frequency filter. In addition, means are provided for carrying the direct voltage (V+) to the cathode of the capacitance diode, whereby the series resonance frequency changes according to the control voltage.

Claims

1. A duplex radio frequency filter having pass bands, comprising:

a receiving part with a plurality of resonators defining a receiving pass band region, said receiving part being connected between a receiving branch of a radio telephone at an output connector and an antenna;

a transmitting part with a transmission pass band region, said transmitting part being connected between a transmitting branch of a radio telephone and the antenna; and

a tunable bandstop filter coupled to the receiving part of the radio telephone at the output connector, the tunable bandstop filter including an electrically tunable resonator which is tunable to attenuate signals in the pass bands of said duplex radio frequency filter, the electrically tunable band stop resonator being disposed in the duplex filter structure at a distance remote from the antenna of the radio telephone such that at least one of said plurality of resonators is situated between said electrically tunable bandstop resonator and the antenna.

2. A duplex radio frequency filter according to claim 1, wherein the resonator of said tunable bandpass filter comprises a plurality of resonators, wherein said resonators are coupled as a series resonance resonator having a series resonance frequency such that an input signal is substantially attenuated, which series resonance frequency can be changed by means of a direct voltage control signal input to said band stop filter, so that at least one value of the control signal causes the series resonance frequency to be located in said receiver pass band.

3. A duplex radio frequency filter according to claim 2, wherein said resonator of said tunable bandstop filter is a transmission line resonator, and filter further comprises:

a series coupled inductive element and a capacitive element comprising a capacitance diode with a cathode, one end of the inductive element being coupled such that the transmission line resonator is divided into two parts and the other end is coupled to the output connector of the radio frequency filter, and

means for applying a direct voltage to the cathode of the capacitance diode.

4. A duplex radio frequency filter according to claim 3, wherein a reactance of the series coupled inductive element and the capacitive element is inductive and greater than a capacitive reactance of the transmission line resonator at a frequency higher than a resonance frequency of the transmission line resonator, whereby the series resonance frequency of the band stop filter is located in an upper region of said receiver pass band.

5. A duplex radio frequency filter according to claim 3, wherein a reactance value of the series coupled inductive element and the capacitive element is capacitive and higher than an inductive reactance of the transmission line resonator when operating at a frequency lower than the resonance frequency of the transmission line resonator, whereby the series resonance frequency of the bandstop filter is located in a lower region of and below the receiver pass band.

6. A duplex radio frequency filter according to claim 2, wherein the direct voltage control signal is coupled to the output connector of the receiving part of said radio frequency filter.

7. A duplex radio frequency filter according to claim 6, wherein the output connector is coupled via a resistor to the cathode of the capacitance diode and the capacitance diode is coupled via a capacitor to the output connector of the filter.

8. A duplex radio frequency filter according to claim 3, wherein the inductive means is a strip line.

9. A duplex radio frequency filter according to claim 8, wherein the series resonance frequency of the resonator of said tunable bandstop filter is at a frequency of a signal causing spurious response in the receiving branch.

10. A duplex radio frequency filter according to claim 1, wherein the series resonance frequency of the resonator of said tunable bandstop filter is substantially the same as a frequency of a local oscillator in the receiving branch.

11. A duplex radio frequency filter according to claim 3, wherein the inductive means is a resonator coil.