Common technologies for producing integrated circuits are the so-called Complementary Metal Oxide Semiconductor (CMOS) and Bipolar Complementary Metal Oxide Semiconductor (BiCMOS). CMOS and/or BiCMOS technologies have become the fabrication technologies of choice for a majority of integrated circuits, such as automotive, cellular and wireless applications, due to its flexibility of design, level of integration and manufacturing costs. With (Bi)CMOS technology, however, there are also some limitations such as component tolerances. For example, component tolerances of on-chip passive integrated resistors and capacitors are typically in the range of +/−5% (best case), but more typically in the range of +/−30%. These component tolerances are especially relevant for precision circuits such as RC circuits.
A tolerance sensitive application is for example an automotive radar system. Such radar systems consist of a transmitter (TX) chip having an integrated phase-locked loop (PLL), power amplifiers and a local oscillator output along with one or several multi-channel receivers (RX) that provide the low-noise down-conversion of the high frequency radar signals into an intermediate frequency (IF). These radar systems are, for example, used for blind spot detection, objection detection, stop and go and adaptive cruise control applications. The present invention and the underlying problem will hereinafter be described on the basis of an automotive radar system, however, without restricting the invention to this sort of application.
The receivers within the radar systems typically comprise some chains of high-pass (HP) filters. Traditional HP-filter topologies employ RC-based filters having capacitor (C) and resistor (R) elements. In (Bi)CMOS technology, as already stated above, the C and R elements exhibit high spread in process variation which consequently have an impact on filter properties, such as its the cutoff frequency. Radar applications are sensitive to variations of the high-pass filter cutoff frequency. The cutoff frequency (or corner frequency) is the frequency either above or below which the power output of a circuit, such as an amplifier or an electronic filter, has fallen to a given amplitude. If, for example, the cutoff frequency is lower than expected, the receiver circuit will provide a lower attenuation of low frequency blockers which could be interpreted as erroneous targets at application level. If, on the other hand, the cutoff frequency is higher than expected, the receiver will provide a higher attenuation of useful signals. Consequently, higher in-band gain-ripples will occur.
In order to reduce the adverse effects of component tolerance variations, those (Bi)CMOS based receiver circuits employ RC calibration circuits. Those calibration circuits allow achieving a required filter response with well-controlled cut-off frequency regardless of the process spread on the R and C elements.
U.S. Pat. No. 7,345,490 D2 discloses a known approach for filter calibration. Here, a RSSI (Receive Signal Strength Indicator) is used to first measure the signal strength when the filter is in its bandpass using a first input frequency. Then, a given filter element, for example the R element or C element, is changed and a second input frequency is applied. Then, the signal strength is measured up to the required level, for example the desired attenuation value. Since the calibration circuit disclosed in U.S. Pat. No. 7,345,490 B2 employs a calibration loop and further needs an external pin to inject the calibration signal at the right frequency, this solution is not applicable in so-called stand-alone receivers.
Hence, there is a need to find a simpler, more reliable possibility to correct RC filter parameters for desired filter attenuations, which in particular is applicable in stand-alone receivers.