Wave transmission lines and networks – Automatically controlled systems
Reexamination Certificate
1999-09-24
2001-10-23
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Automatically controlled systems
C333S174000
Reexamination Certificate
active
06307443
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a system and method for implementing a self tuning filter.
BACKGROUND OF THE INVENTION
Analog circuits, such as filters and oscillators, are increasingly being fabricated as part of integrated circuits (“ICs”). On any particular IC, the frequency response of a circuit is dependent on the values of the capacitors, resistors, transistors, and other circuit components which are used to form the circuit element. Although circuit layouts can be designed very precisely, in practice, the actual size of circuit components across different ICs tends to be highly variable due to cumulative tolerance errors which occur during the manufacturing process. For example, the actual value of a capacitor on an IC can vary by up to ±50% from the nominal design value. Further, the performance is also dependent on the absolute temperature at which the circuit is operating. Thus, it is common practice to provide integrated circuits with various performance characteristics that can be adjusted to compensate for manufacturing variances and operating temperature.
In a conventional arrangement, the performance of bandpass filter circuits and other circuits, such as inductor-capacitance (“LC”) based circuits, are tuned or otherwise manually adjusted using a digital signal processor (“DSP”) which operates by measuring the performance of a representative circuit as it acts on an externally supplied reference signal. For example, in one particular implementation, a multi-stage filter is tuned by first disconnecting one of the filters from the other stages and connecting its input to a DC reference voltage with a predefined and known magnitude. The magnitude of the DC output voltage is then digitized and stored within the DSP. Next, a reference sinusoidal signal is applied to the filter input. The sinusoidal frequency is chosen to be at or close to the desired filter −3 dB frequency and has a peak amplitude which is equal to the magnitude of the reference DC voltage. The power of the filter output is determined and the DSP compares the two measurements and generates a tuning control signal in accordance with predefined data concerning the behavior of the filter at varying tuning signal inputs. (see, Khorramabadi, “Baseband Filters for IS-95 CDMA Receiver Application Featuring Digital Automatic Frequency Tuning”, IEEE International Solid-State Circuits Conference, 1996, page 172-173).
Although such tuning techniques are effective, they depend on the presence of specific reference signals which must be generated off-chip to avoid being compromised by manufacturing variations. Thus, additional circuitry external to the IC is required. In addition, such an arrangement uses valuable chip I/O, which may increase both the cost and complexity of the chip packaging and design.
Accordingly, it would be advantageous to provide a tuning circuit which can automatically tune a component, such as a bandpass filter, without requiring an external reference signal. It would also be advantageous if such a circuit can automatically generate a clock signal for use throughout the chip, which clock signal has a frequency tied to the resident frequency of the tuned circuit.
SUMMARY OF THE INVENTION
A method and system for tuning bandpass filters and other tunable circuit elements includes a tunable filter which receives an input signal having a particular primary frequency component, such as a carrier wave frequency. The input signal can be the same signal as is processed by the circuit during normal operation. The power of the filtered signal is measured and the filter tuning is adjusted until the magnitude of the filtered signal power is at a maximum. In one embodiment, the output signal of a tunable bandpass filter is fed to a series diode-capacitor network. The DC voltage on the capacitor is monitored as the values of one or more tuning elements, such as a tunable capacitor array, are adjusted. For a bandpass filter, the DC voltage on the monitored capacitor indicates the average peak magnitude of the filtered signal. This value will have a maximum peak when the bandpass filter is tuned so that the passband is centered around the input carrier frequency. Once the maximum DC voltage is found, the value of the tuning element is maintained at this value during normal operation of the filter. Advantageously, tuning is accomplished without the need to use an external reference frequency. Rather, the frequency content of the input signal itself is used to tune the filter around the proper frequency. In addition, because components on the same IC will be subject to the same manufacturing and operating conditions, this value can then also be used to tune other circuit components on the IC.
In a particular implementation, and according to a further aspect of the invention, a bandpass filter which is used to receive an input data stream is tuned to a primary frequency component of the input data stream. After the filter is tuned, the derived tuning signal is used to adjust the resonant frequency of an oscillating circuit, such as a tunable LC tank circuit. The oscillator output will then mirror the primary frequency. This output signal can be increased or decreased in frequency as required, i.e., by applying it to a frequency doubling circuit, in order to generate a clock signal suitable for extracting data from the filtered input signal.
REFERENCES:
patent: 3715690 (1973-02-01), Young et al.
patent: 4257018 (1981-03-01), Masdea et al.
patent: 4726071 (1988-02-01), Jachowski
patent: 5227743 (1993-07-01), Yamamoto
patent: 5396195 (1995-03-01), Gabara
patent: 5808527 (1998-09-01), De Los Santos
patent: 6150901 (2000-11-01), Auken
Khorramabadi et al., 1996 IEEE International Solid-State Circuits Conference, Session 10, Paper FA 10.6, pp. 172-173.
Agere Systems Guardian Corp.
Darby & Darby
Jones Stephen E.
Lee Benny
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