WO2016190914A2

WO2016190914A2 - Circuits for n-path filters

Info

Publication number: WO2016190914A2
Application number: PCT/US2016/014766
Authority: WO
Inventors: Negar REISKARIMIAN; Harish Krishnaswamy
Original assignee: Reiskarimian Negar; Harish Krishnaswamy
Priority date: 2015-01-25
Filing date: 2016-01-25
Publication date: 2016-12-01
Also published as: WO2016190914A3

Abstract

Filter having filter input, filter output, and filter ground (FG), comprising: first shunt LC section having first port connected to the filter input and ground connected to the FG; series LC section having first port connected to the filter input, second port terminal connected to the filter output, and ground connected to the FG, wherein the series LC section includes first transmission line (FTL), second transmission line (STL), and two-port filter (TPF), first side of the FTL is connected to the first port of the series LC section, second side of the FTL is connected to first port of the TPF, first side of the STL is connected to second port of the TPF, and second side of the STL is connected to the second port of the series LC section; and second shunt LC section having first port connected to the filter output and ground connected to the FG.

Description

CIRCUITS FOR N-PATH FILTERS

Cross Reference to Related Application

[0001] The application claims the benefit of United States Provisional Patent Application No. 62/107,433, filed January 25, 2015, which is hereby incorporated by reference herein in its entirety.

Statement Regarding Government Funded Research

[0002] This invention was made with government support under contract FA8650-14-1-7414 awarded by DARPA. The government has certain rights in the invention.

Background

[0003] Filters play a critical role in radio transceivers, and are used for front-end band- selection and intermediary channel selection in receivers and as output filters in transmitters.

[0004] N-path filters are a promising solution to the need for tunable filters for use in reconfigurable and software-defined radios.

[0005] However, existing N-path filter designs do no provide sufficient performance for many radio transceiver applications, and thus there is a need for improved N-path filters.

Summary

[0006] Circuits for n-path filters are provided.

[0007] In some embodiments, circuits for an N-path filter having a filter input, a filter output, and a filter ground are provided, the circuits comprising: a first shunt LC section having a first port connected to the filter input and a ground connected to the filter ground; a series LC section having a first port connected to the filter input, a second port terminal connected to the filter output, and a ground connected to the filter ground, wherein the series LC section includes a first transmission line, a second transmission line, and a two-port filter, a first side of the first transmission line is connected to the first port of the series LC section, a second side of the first transmission line is connected to a first port of the two-port filter, a first side of the second transmission line is connected to a second port of the two-port filter, and a second side of the second transmission line is connected to the second port of the series LC section; and a second shunt LC section having a first port connected to the filter output and a ground connected to the filter ground.

Brief Description of the Drawings

[0008] FIG. 1 is an example of a schematic of a filter including transmission lines in accordance with some embodiments.

[0009] FIG. 2 is an example of a schematic of a filter including lumped-LC two-port equivalents used to implement transmission lines in accordance with some embodiments.

[0010] FIG. 3 is an example of a schematic of a filter as implemented in an integrated circuit in accordance with some embodiments.

Detailed Description

[0011] Circuits for providing N-path filters are provided.

[0012] Turning to FIG. 1, an example 100 of a filter in accordance with some embodiments is shown. As illustrated, filter 100 includes a shunt LC section 102, a series LC section 104, and a shunt LC section 106. [0013] Shunt LC section 102 can be formed from parallel branches of series connected switches 108 and capacitors 110 in some embodiments. Any suitable number (e.g., sixteen) of branches can be provided in section 102 in some embodiments.

[0014] Series LC section 104 can be formed from quarter-wave transmission lines 112 and 120 and a two-port filter 111, in some embodiments. As shown, two-port filter 111 can be implemented using parallel branches of switches 1 14 and capacitors 116 connected as shown in FIG. 1 in some embodiments. Any suitable number (e.g., sixteen) of branches can be provided in section 104 in some embodiments.

[0015] Shunt LC section 106 can be formed from parallel branches of series connected capacitors 122 and switches 124 in some embodiments. Any suitable number (e.g., sixteen) of branches can be provided in section 106 in some embodiments.

[0016] Any suitable switches, capacitors, and quarter-wave transmission lines can be used for switches 108, 114, and 124, capacitors 110, 116, and 122, and quarter-wave transmission lines 112 and 120 in some embodiments.

[0017] Although filter 100 is shown as having two shunt LC sections and one series LC section, any suitable number of these sections can be used by alternating between shunt LC sections and series LC sections as shown. For example, an additional series LC section can be added by connecting one side of the additional series LC section to VOUT. Continuing this example, an additional shunt LC section can be added to the other side of the additional series LC section to provide a total of three shunt LC sections and two series LC sections.

[0018] The implementation of quarter-wave transmission lines at RF frequencies on chip can be area-hungry and lossy. Consequently, the quarter-wave transmission lines can be replaced in some embodiments with two-port lumped T-sections or Pi-sections that are equivalent at the quarter-wave frequency. Four possible two-port lumped sections exist: CLC T-type, CLC Pi- type, LCL T-type and LCL Pi-type. In some embodiments, it may be easier to implement the filter described herein with fewer inductors. To this end, CLC networks are advantageous (such as the CLC T-type network). The four possible networks differ from each other (and from a true quarter-wave transmission line) in their response at harmonics of the operating frequency, and therefore can be expected to impact the filter performance given its linear periodic time varying (LPTV) nature.

[0019] In some embodiments, quarter-wave transmission lines 1 12 and 120 of FIG. 1 can be implemented as lumped-LC two-port equivalents 212 and 220, respectively, as shown in FIG. 2. As illustrated, lumped-LC two-port equivalents 212 and 220 can be implemented using capacitors CTL 234 and inductors LTL 236 in some embodiments. Any suitable capacitors and inductors can be used for capacitors 234 and 236 in some embodiments.

[0020] As shown and described in connection with FIG. 3, the capacitors of the lumped-LC two-port equivalents can be realized as switched-capacitor banks, which allow partial

reconfiguration of the operating frequency. Tuning one or more of the inductors and the capacitors may be required to maintain characteristic impedance while tuning the quarter-wave frequency.

[0021] As illustrated in FIG. 3, in some embodiments, an N-path filter can be implemented in a circuit 300. As shown, circuit 300 includes a shunt LC 302, a series LC 304, a shunt LC 306, clock circuits 361 , 362, and 363, decoders 364, electrostatic discharge protection circuits 366, and biasing circuit 368.

[0022] Shunt LCs 302 and 306 can be implemented using parallel branches of series connected switches 342 and 358 and capacitors 344 and 360, respectively, in some embodiments. Any suitable switches and capacitors can be used for switches 342 and 358 and capacitors 344 and 360, any suitable number (e.g., sixteen) of branches of switches 342 and capacitors 344 can be used, and any suitable number (e.g., sixteen) of branches of switches 358 and capacitors 360 can be used, in some embodiments. Switches 342 can be controlled by a clock signal CLKi_p0rt_i from clock circuit 361. Switches 358 can be controlled by a clock signal CLKi_p0rt_2 from clock circuit 363.

[0023] As shown in FIG. 3, series LC 304 can be implemented using lumped-LC two-port equivalents 312 and 320 (as quarter-wave transmission lines) and a two-port filter 311.

[0024] Lumped-LC two-port equivalents 312 and 320 can be implemented using capacitors 346 and 352, capacitors 350 and 356, switches 348 and 354, and inductors 370 and 372. Any suitable capacitors, switches, and inductors can be used for capacitors 346 and 352, capacitors 350 and 356, switches 348 and 354, and inductors 370 and 372 in some embodiments. Any suitable number (e.g., four) of branches of switches 348 and capacitors 350, and any suitable number (e.g., four) of branches of switches 354 and capacitors 356, can be used in some embodiments.

[0025] Two-port filter 311 can be implemented using switches 314 and capacitors 316. Any suitable switches and capacitors can be used for switches 314 and capacitors 316 in some embodiments. Any suitable number (e.g., sixteen) of branches of switches 314 and capacitors 316 can be used in some embodiments. Switches 314 can be controlled by a clock signal CLK2port from clock circuit 362.

[0026] Clock circuits 361, 362, and 363 can be any suitable clock circuits. For example, as illustrated, clock circuits 361, 362, and 363 can output a sequence of sixteen pulses each on one of sixteen different outputs. [0027] Decoders 364 can be any suitable decoders for generating control signals for controlling switches 348 and 354.

[0028] Electrostatic discharge protection circuits 366 can be any suitable circuit for protecting circuit 300 from electrostatic discharge.

[0029] Biasing circuit 368 can be any suitable circuit for biasing any suitable components of circuit 300.

[0030] Sections 102, 106, 302, and 306 in FIGS. 1, 2, and 3 each form a one-port RLC tank of the corresponding filter. Resistance capacitance , and inductance ) of

each tank can be determined using equations 1, 2, and 3, respectively, below:

[0031] In these equations, is the source resistance at the port of the one-port RLC tank,

is the number of branches in the one-port R is equal to is the

capacitance of the capacitor in the one-port RLC tank, and f is the clock frequency of the clock

driving the tank.

[0032] Sections 111 and 311 in FIGS. 1, 2, and 3 each form a two-port RLC tank of the corresponding filter. Resistance , capacitance and inductance ( of each tank

can be determined using equations 4, 5, and 6, respectively, below:

[0033] In these equations, R_s and are the real source and load impedances, A is the

number of branches in the two-port RLC tank, is the clock frequency of the clock driving the

tank, f is equal to

is the capacitance of the capacitor in the tank, and is equal

[0034] The values of C and can be computed using equations 7 and 8 below:

where C_p is the desired overall capacitance of the shunt LC sections, Ro is the reference impedance for the input and output ports of the overall filter (e.g., 50 ohms), C_s is the desired overall capacitance of the series LC sections, Zo is the characteristic impedance of the transmission lines, and /is the quarter-wave frequency of the transmission line. [0035] The L and C values needed for the transmission lines are independent of the nature of the section and can be determined from equations 9 and 10 below:

where Zo is characteristic impedance of the transmission lines and /is the quarter-wave frequency for the transmission lines.

[0036] The various factors that can impact the performance of the N-path filter described herein include: finite number of paths, switch on-resi stance, switch parasitic capacitance, the lumped-LC transmission line equivalents and the loss associated with the inductors in the lumped equivalents. Higher number of paths can lead to more complexity and power consumption in the clock path. Switch on-resistance in the shunt one-port N-path filters can affect filter out of band rejection but may not impact in band loss, while on-resistance in impedance-transformed two-port N-path filter can have the opposite impact. Increasing switch size in the N-path filters can increase clock path power consumption as well as the detrimental impact of switch parasitic capacitance.

[0037] In some embodiments, each filter can be implemented using sixteen paths and separate clock generation can be used to improve symmetry in routing. The CLC sections can be implemented using switched-capacitor banks (to increase the tuning range) and off-chip air-core 6.9nH inductors (part number 0806SQ (typical Q=100) from COILCRAFT of Cary, Illinois).

[0038] The filter described herein can be used in any suitable application. For example, in some embodiments, the filter can be used in applications where sharp filtering and moderate tunability are required, such as frequency-channelized receivers, perhaps preceded by a highly- linear front-end low-noise amplifier.

[0039] Although specific components having specific properties (e.g., resistances, capacitance, sizes, relative sizes, voltages, etc.) are described herein, one or more of the components can be omitted or substituted with one or more alternate components having one or more different properties, in some embodiments.

[0040] The provision of the examples described herein (as well as clauses phrased as "such as," "e.g.," "including," and the like) should not be interpreted as limiting the claimed subject matter to the specific examples; rather, the examples are intended to illustrate only some of many possible aspects.

[0041] Although the invention has been described and illustrated in the foregoing illustrative embodiments, it is understood that the present disclosure has been made only by way of example, and the numerous changes in the details of implementation of the invention can be made without departing from the spirit and scope of the invention, which is only limited by the claims which follow. Features of the disclosed embodiments can be combined and rearranged in various ways.

Claims

What is claimed is:

1. A circuit for an N-path filter having a filter input, a filter output, and a filter ground, the circuit comprising:

a first shunt LC section having a first port connected to the filter input and a ground connected to the filter ground;

a series LC section having a first port connected to the filter input, a second port terminal connected to the filter output, and a ground connected to the filter ground, wherein the series LC section includes a first transmission line, a second transmission line, and a two-port filter, a first side of the first transmission line is connected to the first port of the series LC section, a second side of the first transmission line is connected to a first port of the two-port filter, a first side of the second transmission line is connected to a second port of the two-port filter, and a second side of the second transmission line is connected to the second port of the series LC section; and

a second shunt LC section having a first port connected to the filter output and a ground connected to the filter ground.

2. The circuit of claim 1, wherein the first transmission line is formed from a two-port equivalent comprising: a first capacitor having a first side forming the first side of the first transmission line and having a second side; an inductor having a first side connected to the second side of the first capacitor and having a second side connected to the filter ground; and a second capacitor having a first side connected to the second side of the first capacitor and having a second side forming the second side of the first transmission line.

3. The circuit of claim 2, wherein the second transmission line is formed from a two- port equivalent comprising: a first capacitor having a first side forming the first side of the second transmission line and having a second side; an inductor having a first side connected to the second side of the first capacitor and having a second side connected to the filter ground; and a second capacitor having a first side connected to the second side of the first capacitor and having a second side forming the second side of the second transmission line.

4. The circuit of claim 1, wherein the first shunt LC section is formed from a plurality of parallel branches each having a series coupling of a capacitor and a switch between the first port of the first shunt LC section and the ground of the first shunt LC section.

5. The circuit of claim 4, wherein each switch of the first shunt LC section is controlled by a different clock signal.

6. The circuit of claim 4, wherein the second shunt LC section is formed from a plurality of parallel branches each having a series coupling of a capacitor and a switch between the first port of the second shunt LC section and the ground of the second shunt LC section.

7. The circuit of claim 6, wherein each switch of the second shunt LC section is controlled by a different clock signal.

8. The circuit of claim 1, wherein the two-port filter is formed from a plurality of parallel branches formed between the first port of the two-port filter and the second port of the two-port filter, wherein each branch includes a first switch having a first side connected to the first port of the two-port filter and having a second side, a capacitor having a first side connected to the second side of the first switch and having a second side connected to the filter ground, and a second switch having a first side connected to the second side of the first switch and having a second side connected to the second port of the two-port filter.

9. The circuit of claim 8, wherein each switch of the two-port filter is controlled by a different clock signal.

10. The circuit of claim 1, wherein the first transmission line comprises:

a first side node forming the first side of the first transmission line;

a second side node forming the second side of the first transmission line; an inductor having a first side and having a second side connected to the filter ground; and

a plurality of branches each comprising:

a first switch having a first side connected to the first side node and having a second side;

a first capacitor having a first side connected to the second side of the first switch and having a second side connected to the first side of the inductor;

a second capacitor have a first side connected to the first side of the inductor and having a second side; and

a second switch having a first side connected to the second side of the second capacitor and having a second side connected to the second side node.

11 The circuit of claim 10, wherein the first transmission line further comprises:

a first capacitor having a first side connected to the first side node and having a second side connected to the first side of the inductor; and

a second capacitor having a first side connected to the first side of the inductor and having a second side connected to the second side node.

12. The circuit of claim 10, wherein the first switch and the second switch of each branch are controlled by a control signal to select the capacitance of the first transmission line.

13. The circuit of claim 10, wherein the second transmission line comprises:

a first side node forming the first side of the second transmission line;

a second side node forming the second side of the second transmission line; an inductor having a first side and having a second side connected to the filter ground; and

a plurality of branches each comprising:

a second capacitor have a first side connected to the first side of the inductor and having a second side; and a second switch having a first side connected to the second side of the second capacitor and having a second side connected to the second side node.

14. The circuit of claim 13, wherein the second transmission line further comprises:

15. The circuit of claim 13, wherein the first switch and the second switch of each branch are controlled by a control signal to select the capacitance of the second transmission line.