COMBLINE FILTER
A microstrip combline bandpass filter includes an input port, an output port, and a plurality of resonators each including a microstrip line having a first end and a second end. One of the plurality of resonators is connected to the input port, and another of the plurality of resonators is connected to the output port. The filter also includes a plurality of pairs of series coupled varactors. The first end of each microstrip line is coupled to one of the pairs of varactors, and the second end of each microstrip line is coupled to ground.
1 . A microstrip combline bandpass filter, comprising:
an input port;
an output port;
a plurality of resonators each including a microstrip line having first and second ends, one of the plurality of resonators connected to the input port, another of the plurality of resonators connected to the output port; and
a plurality of pairs of varactors, the varactors in each pair serially coupled, the first end of each microstrip line coupled to one of the pairs of varactors, the second end of each microstrip line coupled to ground.
2 . The filter of claim 1 in which the pairs of varactors are each coupled between the first end of the corresponding microstrip line and ground.
3 . The filter of claim 1 further including a plurality of resistances, in which the second end of each microstrip line is coupled to ground through one of the resistances to provide the filter with greater amplitude response slope as a function of frequency.
4 . The filter of claim 1 in which each pair of varactors includes two diodes coupled together in an anode to anode or cathode to cathode configuration.
5 . The filter of claim 1 , further including a tuning circuit coupled to a junction between each pair of varactors for adjusting the center frequency of the filter.
6 . The filter of claim 1 in which the tuning circuit includes a tuning control terminal and a plurality of inductances and resistances, one of the inductances and one of the resistances each coupled in series between the tuning control terminal and a junction between each pair of varactors.
7 . The filter of claim 1 , further including at least one variable capacitor coupled between the input port and the output port for providing a bandreject notch.
8 . The filter of claim 7 , in which the at least one variable capacitor includes two varactors coupled in series between the input port and the output port.
9 . The filter of claim 7 , in which the at least one variable capacitor includes two pairs of series coupled varactors coupled in series between the input port and the output port.
10 . The filter of claim 9 , further including a bandreject notch control circuit coupled to a junction between each pair of varactors for adjusting the frequency of the bandreject notch.
11 . The filter of claim 1 in which the filter is implemented on a Monolithic Microwave Integrated Circuit (MMIC) die.
12 . The filter of claim 11 in which a low pass filter is also implemented on the Monolithic Microwave Integrated Circuit (MMIC) die.
13 . The filter of claim 12 in which the low pass filter is tunable.
14 . The filter of claim 1 in which the filter is implemented on a planar monolithic substrate.
15 . The filter of claim 14 in which the monolithic substrate is selected from the group of GaAs or SiGe.
16 . The filter of claim 14 in which the monolithic substrate is mounted in a surface-mount package.
17 . The filter of claim 1 in which each varactor includes a p-n junction.
18 . The filter of claim 1 in which each varactor includes a field effect transistor (FET) and uses a capacitance between a gate and a source of the FET.
19 . The filter of claim 1 in which each varactor includes a ferroelectric based capacitor.
20 . The filter of claim 1 in which each varactor includes a MEMS-based capacitor.
21 . A microstrip combline bandpass filter, comprising:
an input port;
an output port;
a plurality of resonators each including a microstrip line having a first and second ends, the second end coupled to ground through a corresponding resistance, one of the plurality of resonators connected to the input port, another of the plurality of resonators connected to the output port; and
a plurality of pairs of electrically tunable varactors, the varactors of each pair serially coupled and coupled between the first end of a corresponding microstrip line and ground.
22 . The filter of claim 21 , further including a tuning circuit coupled to a junction between each pair of varactors for adjusting the center frequency of the filter.
23 . The filter of claim 22 , in which the tuning circuit includes a tuning control terminal and a plurality of inductances and resistances, one of the inductances and one of the resistances each coupled in series between the tuning control terminal and a junction between each pair of varactors.