HK1091964B - A method and device for automatically calibrating a loop-filter of a phase locked loop - Google Patents

Description

Method and device for automatically calibrating loop filter of phase-locked loop

Technical Field

The invention relates to a method for automatically calibrating a loop filter of a phase locked loop. The invention relates equally to an integrated circuit chip comprising a loop-filter for a phase locked loop and to a unit comprising such an integrated circuit chip.

Background

A Phase Locked Loop (PLL) is a negative feedback loop well known in the art.

The PLL comprises a Voltage Controlled Oscillator (VCO) which generates an output signal of the PLL. The output signal may be used, for example, as a local oscillator signal of a receive mixer of a receiver chain or a transmitter mixer of a transmitter chain in a mobile terminal. The VCO is driven by a loop filter that determines the loop characteristics of the PLL, such as settling time and loop stability. Therefore, the response of the loop filter must be very accurate. An accurate response is generated if the RC product of the RC (resistor capacitor) filter components of the loop filter exactly coincides with the predetermined RC product, so that the desired filter frequency is obtained in each RC filter component.

In addition, in order to reduce the number of external components, it is desirable to use an integrated loop filter in the PLL. The probability of disruptive coupling can also be reduced by means of an integrated loop filter. However, the value of the integrated component varies more than the value of the external precision component due to process variations or environmental influences. For example, an external negative positive zero (NP0) capacitor has very stable values over a wide temperature range (typically between-25 ℃ and +85 ℃).

Therefore, conventional PLLs typically include precise external components for the loop filter. However, when using an integrated loop filter, a complex calibration procedure needs to be employed.

Disclosure of Invention

The invention aims to simply calibrate an integrated loop filter of a PLL.

A method of automatically calibrating a loop-filter of a phase-locked loop is proposed, the loop-filter comprising at least one RC-filter component and being integrated on a chip together with at least one RC-filter component of a further entity different from the phase-locked loop. The proposed method comprises adjusting at least one RC-filter component of the loop-filter based on measurements made on at least one RC-filter component of the further entity.

Furthermore, an Integrated Circuit (IC) chip is proposed, which chip comprises a loop-filter for a phase locked loop, which loop-filter comprises at least one RC-filter component. The proposed chip additionally comprises at least one RC-filter component different from the further entity of the phase locked loop. Furthermore, the proposed chip comprises a calibration component for performing measurements on at least one RC-filter component of the further entity and adjusting at least one RC-filter component of the loop-filter based on such measurements.

Finally, a unit is proposed comprising the proposed IC chip.

The invention proceeds from the consideration that a loop-filter can be calibrated by adjusting at least one RC-filter component of the loop-filter. The invention further proceeds from the consideration that most chips on which loop filters are integrated comprise at least one tunable RC-filter component belonging to some other entity. Further, the present invention derives from the consideration that process variations and environmental effects such as temperature are very similar for all RC filters on one chip. We therefore propose that the RC filter of the loop filter can be adjusted using measurements made to adjust the RC filter of another entity on the same IC chip.

The invention has the advantage that the integrated loop filter can be automatically calibrated without a special complex calibration circuit. Measurements for RC-filter components of further entities integrated on the same chip are usually still required and such measurements can be used quite well for calibrating the loop-filter according to the invention.

The RC-filter component of the further entity may be, for example, the RC-filter component of the baseband filter of the transmitter chain or the RC-filter component of the channel selection filter of the receiver chain, but equally any other tunable RC-filter component as long as it is integrated on the same chip as the loop-filter.

The RC-filter of the loop-filter is adjusted by changing the value of the resistor of the RC-filter component, or the value of the capacitor of the RC-filter component, or both. In order to calibrate the loop filter, the adjustment according to the invention is preferably carried out for all RC-filter components of the loop filter.

The proposed unit may be any unit that requires both a PLL and some entity with adjustable RC-filter components. Such a unit may be, for example, a communication unit such as a mobile terminal including a transmitter chain and a receiver chain.

Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the claims herein. Further, it should be understood that the drawings are not necessarily drawn to scale and that they are merely intended to conceptually illustrate the structures and procedures described herein.

Drawings

FIG. 1 is a block diagram schematically representing an embodiment of an IC chip according to the present invention; and

fig. 2 is a flow chart illustrating an embodiment of a method according to the present invention.

Detailed Description

Fig. 1 schematically shows a preferred embodiment of an IC chip 1 according to the invention. The IC chip 1 is suitable for use in a transceiver 5 of a mobile terminal 10, such as a cellular phone, the mobile terminal 10 being shown in dotted lines in fig. 1. A transceiver 5 comprising a transmitter chain and a receiver chain is indicated by dashed lines in fig. 1.

The IC chip 1 comprises a PLL 2, a calibration component 3 and at least one baseband filter 4 of a transmitter chain of a transceiver 5.

The PLL 2 comprises a phase detector 21, a charge pump 22, a loop filter 23, a VCO 24 and a programmable frequency divider 25 connected to each other in a loop in this order.

The output of the charge pump 22 is therefore connected to the input of the loop filter 23. The input of the loop filter 23 is connected to ground within the loop filter 23 via a first capacitor C1 and in parallel via the series connection of a first resistor R1 and a second capacitor C2. Further, the input of the loop filter 23 is connected to ground within the loop filter 23 via a second resistor R2 and a third capacitor C3. The connection between the resistor R2 and the capacitor C3 constitutes the output of the loop filter 23, which is connected to the input of the VCO 24. The capacitor C2 and the capacitor C3 are tunable capacitors.

The baseband filter 4 of the transmitter chain comprises at least a series connection of a resistor R4 and an adjustable capacitor C4.

The calibration component 3 may control the baseband filter 4. In addition, the calibration component 3 may control the capacitors C2 and C3 of the loop filter 23.

The PLL 2 on the IC chip 1 operates in a well-known manner. The VCO 24 generates a signal whose phase is determined by the applied voltage. The frequency divider 25 divides the frequency of the output VCO signal and forwards the resulting signal to the phase detector 21. In addition, a reference signal Ref having a known frequency is applied to a reference input of the phase detector 21. The phase detector 21 compares the phase of the divided VCO signal with the phase of the reference signal Ref and outputs an error signal. The PLL 2 is locked when the two phases are equal, which means that the frequencies of the compared signals are also equal.

To achieve or maintain the lock-up state, the charge pump 22 generates current pulses Icp, the length of which is controlled by the output signal of the phase detector 21. As its name indicates, the charge pump 22 pumps charge, i.e. provides current. The generated current pulses are filtered by the loop filter 23, and the loop filter 23 provides a corresponding control voltage to the VCO 24, thereby causing the VCO 24 to generate a signal having a desired frequency. By changing the coefficients in the programmable divider 25, the frequency of the signal output by the VCO 24 can be changed. For example, the phase locked VCO signal may be provided as a local oscillator signal for a mixer (not shown) of the transmitter chain.

The baseband filter 4 of the transmitter chain filters signals transmitted by the mobile terminal 10 in a known manner.

The calibration component 3 is used to calibrate the baseband filter 4 and the loop filter 23. The operation of the calibration unit 3 is explained using the flowchart in fig. 2.

The calibration component 3 calibrates the baseband filter 4 of the transmitter chain in a known manner. The calibration component 3 first measures the product of the resistance of the resistor R4 and the capacitance of the capacitor C4. The measured RC product is compared to the RC product required by the desired filter frequency. Based on the comparison, an adjustment value is determined which is used to adjust the RC-product of the baseband filter 4 to the desired RC-product. For example, the adjustment value may be a control word that indicates the coefficient of capacitance that capacitor C4 must increase or decrease in order to achieve the desired RC product. The adjustment is achieved by applying the control word to the control input of capacitor C4, thereby changing the capacitance of capacitor C4.

According to the invention, the calibration component 3 additionally calibrates the loop-filter 23. Since the RC filter including the resistor R4 and the capacitor C4 is integrated on the same IC chip as the RC filter including the resistor R1 and the capacitor C2 and the RC filter including the resistor R2 and the capacitor C3, the deviation of the actual RC product from the desired RC product in the three RC filters is substantially the same. Thus, the calibration component 3 simply applies the control word determined for adjusting the capacitor C4 to the control input of the capacitor C2 and to the control input of the capacitor C3. As a result, the RC filter including the resistor R1 and the capacitor C2 and the RC filter including the resistor R2 and the capacitor C3 are also tuned to a desired frequency. Thus, the loop filter 23 can be calibrated in a simple manner without the need for dedicated complex calibration circuitry. In particular, no additional measurements of the loop filter 23 are necessary.

While there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims (12)

1. A method of automatically calibrating a loop-filter of a phase-locked loop, the loop-filter comprising at least one RC-filter component and being integrated on a chip together with at least one RC-filter component of an entity different from said phase-locked loop and different from a calibration component, said method comprising: adjusting the at least one RC-filter component of the loop-filter by means of the calibration component based on an RC product measured on the at least one RC-filter component of the further entity.

2. The method of claim 1, wherein prior to adjusting the at least one RC-filter component of the loop-filter, measuring an RC-product of the at least one RC-filter component of the further entity and determining an adjustment value to adjust the at least one RC-filter component of the further entity by comparing the measured RC-product with a known RC-product required by the at least one RC-filter component of the further entity, and wherein the adjusting of the at least one RC-filter component of the loop-filter is based on the determined adjustment value.

3. The method of claim 2, wherein said adjustment value is a control word that is applied to said at least one RC-filter component of said loop-filter to adjust said at least one RC-filter component of said loop-filter.

4. The method of claim 1, wherein adjusting the at least one RC-filter component of the loop-filter comprises changing at least a resistance value and/or a capacitance value of the at least one RC-filter component of the loop-filter.

5. An integrated circuit chip, comprising:

a calibration component; and

a loop filter for a phase locked loop, the loop filter comprising at least one RC filter component;

at least one RC-filter component of a different entity than said phase locked loop and than said calibration component;

wherein the calibration component is configured to adjust the at least one RC-filter component of the loop-filter based on an RC product measured on the at least one RC-filter component of the further entity.

6. The integrated circuit chip of claim 5, wherein the calibration component makes the measurement by measuring an RC product of the at least one RC filter component of the additional entity, wherein the calibration component is further to determine an adjustment value to adjust the at least one RC filter component of the additional entity by comparing the measured RC product to a known RC product required by the at least one RC filter component of the additional entity, and wherein the calibration component adjusts the at least one RC filter component of the loop filter based on the determined adjustment value.

7. The integrated circuit chip of claim 6, wherein the calibration component determines a control word as the adjustment value, and wherein the calibration component applies the determined control word to the at least one RC-filter component of the loop filter to adjust the at least one RC-filter component of the loop filter.

8. The integrated circuit chip of claim 5, wherein the at least one RC-filter component comprises at least one of an adjustable resistor and an adjustable capacitor, and wherein the calibration component adjusts the at least one RC-filter component of the loop-filter by at least changing a value of the adjustable resistor and/or a value of the adjustable capacitor of the at least one RC-filter component of the loop-filter.

9. The integrated circuit chip of claim 5, wherein the further entity is a baseband filter of a transmitter chain of a communication unit.

10. The integrated circuit chip of claim 5, wherein the additional entity is a channel selection filter of a receiver chain of a communication unit.

11. A unit cell comprising an integrated circuit chip, the integrated circuit comprising:

a calibration component;

a loop filter for a phase locked loop, the loop filter comprising at least one RC filter component;

at least one RC-filter component of a different entity than said phase locked loop and than said calibration component;

wherein the calibration component is configured to adjust the at least one RC-filter component of the loop-filter based on an RC product measured on the at least one RC-filter component of the further entity.

12. The unit of claim 11, wherein the unit is a communication unit comprising a transmitter chain and a receiver chain, and wherein the further entity is at least one of a baseband filter of the transmitter chain and a channel selection filter of the receiver chain.