Filter apparatus including slave gm-C filter with frequency characteristics automatically tuned by master circuit

• US 7,180,364 B2
• Filed: 01/31/2005
• Issued: 02/20/2007
• Est. Priority Date: 02/03/2004
• Status: Expired due to Fees

• Current Assignee: Renesas Electronics Corporation
• Original Assignee: NEC Electronics Corporation (Renesas Electronics Corporation)
• Inventors: Kimura, Katsuji
• Primary Examiner(s): LE, DINH THANH
• Application Number:
  US11/045,358
• Publication Number:
  US 20050168274A1
• Time in Patent Office: 750 Days
• Field of Search: 327552-559, 330/305
• US Class Current: 327/553
• CPC Class Codes: H03H 11/0472 Current or voltage controlled filters
• CPC Class Codes: H03L 7/0805 the loop being adapted to provide an additional control signal for use outside the loop
• CPC Class Codes: H03L 7/0816 the controlled phase shifter and the frequency- or phase-detection arrangement being connected to a common input

Abstract

In a filter apparatus includes a master circuit receiving a reference frequency to generate a control voltage, and a slave gm-C filter for receiving an input voltage to generate an output voltage. The slave gm-C filter is controlled by the control voltage to adjust the cut-off frequency or center frequency of the slave gm-C filter. The master circuit is a PLL circuit including a phase shifter receiving a reference frequency signal to change the phase of the reference frequency signal, a phase comparator to generate a phase error signal, and a loop filter adapted for excluding an AC component signal from the phase error signal to generate a DC component thereof. As a result, this DC component signal is supplied as the control voltage to the phase shifter and the slave gm-C filter.

Claims

1. A filter apparatus comprising:

a master circuit for receiving a reference frequency signal having a reference frequency to generate a control voltage; and

a slave gm-C filter comprising at least one first operational transconductance amplifier and at least one first capacitor, the operational transconductance amplifier of said slave gm-C filter being controlled by said control voltage to tune one of a cut-off frequency and a center frequency of said slave gm-C filter,

said master circuit comprising;

a phase shifter comprising a second order am-C filter including at least two operational transconductance amplifiers forming an inductance, said phase shifter being adapted to receive said reference frequency signal and change a phase of said reference frequency signal in accordance with said control voltage;

a phase comparator connected to said phase shifter, said phase comparator being adapted to compare a phase of an output signal of said phase shifter with said phase of said reference frequency signal to generate a phase error signal; and

a loop filter, connected to said phase comparator, said loop filter being adapted to exclude an AC component from said phase error signal to generate a DC component thereof as said control voltage,

a phase locked loop of said master circuit being operated so that a loop gain thereof is maximum.

2. The filter apparatus as set forth in claim 1, wherein an operating frequency band of said phase shifter is different from an operating frequency band of said slave gm-C filter.

3. The filter apparatus as set forth in claim 2, wherein said phase shifter comprises:

a lower-order gm-C filter than said slave gm-C filter.

4. The filter apparatus as set forth in claim 1, wherein said phase shifter further comprises:

an input resistor connected to said second order gm-C filter.

5. The filter apparatus as set forth in claim 4, wherein said second-order gm-C filter comprises:

a second-order gm-C high pass filter.

6. The filter apparatus as set forth in claim 4, wherein said second-order gm-C filter comprises:

a second-order gm-C band pass filter.

7. The filter apparatus as set forth in claim 4, wherein said input resistor comprises:

two operational transconductance amplifiers.

8. The filter apparatus as set forth in claim 4, wherein said phase shifter further comprises:

a terminal resistor connected to said second-order gm-C filter.

9. The filter apparatus as set forth in claim 8, wherein said terminal resistor comprises:

an operational transconductance amplifier.

10. The filter apparatus as set forth in claim 1, wherein the phase comparator comprises:

a two-input analog multiplexer.

11. The filter apparatus as set forth in claim 1, wherein the phase comparator comprises:

a two-input exclusive OR circuit.

12. The filter apparatus as set forth in claim 1, wherein said phase shifter comprises:

an input resistor; and

a capacitor connected to said input resistor.

13. The filter apparatus as set forth in claim 12, wherein said input resistor comprises:

two cascaded first-order high pass filters.

14. The filter apparatus as set forth in claim 13, further comprising:

two terminal resistors each connected to one of said first-order high pass filters.

15. The filter apparatus as set forth in claim 1, wherein said second-order gm-C filter comprises:

a first capacitor; and

a second capacitor.

16. A filter apparatus for receiving a reference frequency signal having a reference frequency to generate a control voltage to tune one of a cut-off frequency and a center frequency of a slave gm-C filter, said filter apparatus comprising:

a phase shifter comprising two operational transconductance amplifiers forming an inductance and at least one capacitor, said phase shifter being adapted to receive said reference frequency signal and change a phase of said reference frequency signal in accordance with said control voltage;

a phase comparator connected to said phase shifter, said phase comparator being adapted to compare a phase of an output signal of said phase shifter with said phase of said reference frequency signal to generate a phase error signal; and

a loop filter, connected to said phase comparator, said loop filter being adapted to exclude an AC component from said phase error signal to generate a DC component thereof as said control voltage, and

a phase locked loop of said master circuit being operated so that a loop gain thereof is maximum.