Method and Apparatus for Tunable Current-Mode Filtering

• US 20100066442A1
• Filed: 09/15/2008
• Published: 03/18/2010
• Est. Priority Date: 09/15/2008
• Status: Abandoned Application

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Mu, Fenghao
• Application Number:
  US12/210,483
• Publication Number:
  US 20100066442A1
• Time in Patent Office: Days
• US Class Current: 327/553
• CPC Class Codes: H03H 11/1291 Current or voltage controlled filters

Abstract

According to the teachings presented herein, a tunable current-mode filter is implemented using two or more tunable filter stages in cascade connection. For example, a number of tunable filter stages corresponding to a desired filter order are included in the filter in cascade connection. Use of the current-mode filter simplifies circuit design, particularly in communication transmitter applications, and avoids current-to-voltage conversions needed when voltage-mode filters are used in current-mode signal processing chains. A method and circuit to tune and calibrate the frequency response of the filter are disclosed as well.

Claims

1. A tunable current-mode filter comprising:

a filter input for receiving an input current signal and a corresponding filter output for providing a filtered output current signal; and

at least two cascading tunable filter stages interposed between the filter input and the filter output for filtering the input current signal to thereby obtain the filtered output current signal according to a tunable frequency response;

each said tunable filter stage comprising a current mirror circuit having first and second branches configured to generate a stage output current by mirroring a stage input current, and two passive networks, one coupled to an output of the first branch and the other one coupled to an output of the second branch of the current mirror circuit, and each passive network including a tunable element operative to adjust a frequency response of the tunable filter stage responsive to a control signal, and thereby adjust an overall frequency response of the tunable current-mode filter; and

a control input configured to receive the control signal, such that changing a value of the control signal changes the overall frequency response of the tunable current-mode filter.

2. The tunable current-mode filter of claim 1, wherein the tunable current-mode filter comprises a tunable current-mode low pass filter, and wherein each tunable filter stage is configured to have an adjustable cutoff frequency responsive to the control signal, such that the overall cutoff frequency of the tunable current-mode low pass filter is settable via the control signal.

3. The tunable current-mode filter of claim 2, wherein an overall filter order of the tunable current-mode filter is a function of the number of tunable filter stages in cascade connection between the filter input and the filter output.

4. The tunable current-mode filter of claim 2, wherein the tunable element in each passive network comprises a capacitor circuit having a variable capacitance that is set or otherwise adjusted responsive to the control signal.

5. The tunable current-mode filter of claim 1, wherein the tunable element in each passive network comprises a variable reactance circuit operatively associated with the control input, such that changing the value of the control signal changes the reactance of each variable reactance circuit and thereby adjusts an overall frequency response of the tunable current-mode filter.

6. The tunable current-mode filter of claim 5, wherein the variable reactance circuit comprises a switched capacitor circuit, a varactor circuit, or a combined switched capacitor and varactor circuit.

7. The tunable current-mode filter of claim 1, further comprising a calibration circuit included in or otherwise associated with the tunable current-mode filter, wherein the calibration circuit is configured to adjust the control signal based on detecting the overall frequency response of the tunable current-mode filter in comparison with a desired overall frequency response.

8. The tunable current-mode filter of claim 7, wherein the calibration circuit is configured to detect the overall frequency response of the tunable current-mode filter by directly or indirectly detecting an overall delay time of the tunable current-mode filter.

9. The tunable current-mode filter of claim 7, wherein the calibration circuit comprises a tunable replica circuit operating as a surrogate for the tunable current-mode filter, said tunable replica circuit responsive to the control signal in like manner as the tunable current-mode filter, and a control circuit configured to detect the overall frequency response of the tunable current-mode filter indirectly by detecting an overall frequency response of the tunable replica circuit.

10. The tunable current-mode filter of claim 9, wherein the tunable replica circuit is placed in a feed-back loop to form an oscillator circuit having an oscillation frequency dependent on the overall frequency response of the tunable replica circuit, and wherein the calibration circuit is configured to detect the overall frequency response of the tunable current-mode filter by detecting the oscillation frequency of the oscillator circuit.

11. The tunable current-mode filter of claim 7, wherein the calibration circuit comprises a digital processor configured to detect the overall frequency response of the tunable current-mode filter by detecting an oscillation frequency of an oscillator circuit, where the oscillation frequency is dependent on the overall frequency response of the tunable current-mode filter.

12. The tunable current-mode filter of claim 11, further comprising one or more isolation circuits operable by the digital processor and configured to place the tunable current-mode filter during a first time into a calibration mode wherein it operates as part of the oscillator circuit for detection of the overall frequency response of the tunable current-mode filter and corresponding adjustment of the control signal, and to place the tunable current-mode filter during a second time into an operation mode wherein it provides current-mode filtering as part of a signal processing chain.

13. The tunable current-mode filter of claim 1, wherein each passive network in comprises m reactive components, where m is an integer ≧

1, and m−

1 resistive components, that are operative in conjunction with transistor output impedances in a respective branch of the current mirror circuit to form an m-order low pass filter.

14. The tunable current-mode filter of claim 1, further comprising a DC offset detection circuit configured to generate an offset reduction control signal responsive to detecting DC offset in at least one of the tunable filter stages of the tunable current-mode filter, and further comprising an offset reduction transistor in the at least one of the tunable filter stages, said offset reduction transistor configured to reduce DC offset in the at least one of the tunable filter stages responsive to the offset reduction control signal.

15. A wireless communication transmitter comprising:

a current-mode mixer configured to generate a transmit signal by modulating a carrier frequency signal according to a filtered current-mode information signal;

a tunable current-mode filter configured to generate the filtered current-mode information signal by filtering a current-mode information signal, wherein an overall frequency response of the tunable current-mode filter is tunable responsive to a control signal;

a current-mode digital-to-analog converter (DAC) configured to generate the current-mode information signal from a digital baseband information signal; and

a control circuit configured to adjust the control signal according to a desired frequency response for the tunable current-mode filter;

said tunable current-mode filter comprising;

a filter input for receiving an input current signal and a corresponding filter output for providing a filtered output current signal; and

at least two cascading tunable filter stages interposed between the filter input and the filter output for filtering the input current signal to thereby obtain the filtered output current signal according to a tunable frequency response;

each said tunable filter stage comprising a current mirror circuit having first and second branches configured to generate a stage output current by mirroring a stage input current, and two passive networks, one coupled to an output of the first branch and the other one coupled to an output of the second branch, and each passive network including a tunable element operative to adjust a frequency response of the tunable filter stage responsive to a control signal, and thereby adjust an overall frequency response of the tunable current-mode filter; and

a control input configured to receive the control signal, such that changing a value of the control signal changes the overall frequency response of the tunable current-mode filter.

16. A method of adjusting frequency response in a signal processing chain that includes a tunable current-mode filter having at least two cascaded tunable filter stages and having an overall frequency response adjustable as a function of a control signal applied to the tunable current-mode filter, each tunable filter stage comprising a current mirror circuit having first and second branches and two passive networks, each one coupled to an output of a respective one of the branches and each one including a tunable element responsive to the control signal, the method comprising:

detecting an overall frequency response of the tunable current-mode filter; and

adjusting the control signal as needed according to a desired overall frequency response.

17. The method of claim 16, wherein detecting an overall frequency response of the tunable current-mode filter comprises directly or indirectly detecting an overall delay time of the tunable current-mode filter.

18. The method of claim 16, wherein detecting an overall frequency response of the tunable current-mode filter comprises detecting an overall frequency response of a tunable replica circuit operating as a surrogate for the tunable current-mode filter, said tunable replica circuit responsive to the control signal in like manner as the tunable current-mode filter.

19. The method of claim 18, wherein detecting an overall frequency response of a tunable replica circuit comprises placing the tunable replica circuit in a feedback loop to form an oscillator circuit and detecting an oscillation frequency of the oscillator circuit that depends on the overall frequency response of the tunable replica circuit.

20. The method of claim 16, wherein detecting an overall frequency response of the tunable current-mode filter comprises temporarily placing the tunable current-mode filter into a calibration mode where it operates as part of an oscillator circuit and therein detecting an oscillation frequency that is dependent on the overall frequency response of the tunable current-mode filter and correspondingly adjusting the control signal according to a desired overall frequency response.

21. The method of claim 20, further comprising detecting the overall frequency response of the tunable current-mode filter via a digital signal processor included in the signal processing chain and configured to generate or otherwise control generation of an analog current-mode signal that is filtered by the tunable current-mode filter, at least when the tunable current-mode filter is not operating in the calibration mode.

22. The method of claim 16, further comprising detecting DC offset in at least one of the tunable filter stages and generating a corresponding offset reduction control signal to control an offset reduction transistor included in the tunable filter stage, wherein the offset reduction transistor is configured to reduce DC offset responsive to the offset reduction control signal.