CN112332803A

CN112332803A - Active low-pass filter bandwidth calibration circuit

Info

Publication number: CN112332803A
Application number: CN202011190650.9A
Authority: CN
Inventors: 姚明; 王友华; 刘万福; 张凯; 张然; 李航标; 赵晓冬; 刘智卿
Original assignee: Southwest Electronic Technology Institute No 10 Institute of Cetc
Current assignee: Southwest Electronic Technology Institute No 10 Institute of Cetc
Priority date: 2020-10-30
Filing date: 2020-10-30
Publication date: 2021-02-05
Anticipated expiration: 2040-10-30
Also published as: CN112332803B

Abstract

The invention discloses a bandwidth calibration circuit of an active low-pass filter, aiming at adjusting the bandwidth in a wider range and keeping the bandwidth accurate and constant. The invention is realized by the following technical scheme: two sinusoidal signals with different amplitudes and frequencies are respectively generated at the input ends of the I/Q two paths of orthogonal channels, wherein the frequency of the I path of signals is the target bandwidth frequency to be calibrated. After the two paths of signals pass through the two paths of active filters, the peak value detection circuit detects amplitude information of the two paths of signals, the amplitude information is sent to the comparator to be compared, and a comparison result is sent to the finite state machine to be processed. The bandwidth of the filter is detected by controlling and changing the capacitance value of an adjustable capacitance unit in the active filter through an algorithm in a finite-state machine. And detecting and iterating the bandwidth of the filter for multiple times according to the result output by the comparator until all the bits of the adjustable capacitor unit traverse, and stopping iteration. A more accurate filter bandwidth that is insensitive to circuit PVT variations may be obtained.

Description

Active low-pass filter bandwidth calibration circuit

Technical Field

The invention belongs to the technical field of filter calibration, and relates to a calibration circuit for an active low-pass filter bandwidth, which can be applied to a System On Chip (SOC) of a wireless radio frequency transceiver.

Background

Low-pass filtering (Low-pass filter) is a filtering method, in which the Low-frequency signal can normally pass through, and the high-frequency signal exceeding a set critical value is blocked and attenuated. But the magnitude of the blocking and attenuation will vary depending on the frequency and filtering procedure (purpose). It is sometimes also called high-cut filter or top-cut filter. There are many types of low-pass filters. Among them, the most common are butterworth filters and chebyshev filters. The active low-pass filter circuit can eliminate high-frequency noise signals, retain direct current and low-frequency response signals, and has stronger high-frequency inhibition capability and higher response speed than an RC filter circuit. The active low pass filter circuit is usually composed of an integrated operational amplifier and passive element resistors and capacitors. It allows signals from zero to some cutoff frequency to pass unattenuated while suppressing signals at other frequencies. The active low-pass filter circuit can be used for filtering high-frequency interference signals and is widely applied to a digital-analog hybrid System On Chip (SOC). In recent years, with the rapid advance of integrated circuit technology, circuit performance is continuously improved, and particularly with the continuous development of a System On Chip (SOC), a digital-analog hybrid circuit and a radio frequency integrated circuit can be integrated on one chip to realize complete transceiving and signal processing functions. At present, the research on the integrated active filter at home and abroad mainly focuses on: wide band filter design, auto-tuned filter design, reconfigurable filters, current-mode filters, and filter designs incorporating several of the above features. In some applications requiring broadband, such as new IEEE communication standard, power consumption becomes a main consideration, a high gain-bandwidth product of an operational amplifier can suppress gain ripple in a passband, and a high gain-bandwidth product means high power consumption. The architecture of the transceiver is selected related to the communication standard and the performance requirement of the transceiver, on the premise of ensuring the performance, the transceiver is required to have the characteristics of low power consumption, high integration level, small size and low price, the radio frequency front end has a decisive effect on the performance of the transceiver, and the filter is generally positioned at the front stage of a post-stage variable gain amplifier of down-conversion and the front stage of a pre-distortion amplifier of an up-conversion mixer. The gain, bandwidth, noise and linearity of the filter directly affect the performance of the transceiver, and different transceivers often require different filter structures.

With the slow development of independent components to the high integration of systems, the integrated active filter is also developed from an independent chip to the high integration of complex systems. Thus, designing a high performance filter suitable for system integration becomes an urgent task. Currently, there are many implementation forms of integrated active filters, and active RC filters are more suitable for modern transceiver systems due to their high linearity and high dynamic range. In modern communication systems, the wanted signal is subject to interference by a large number of out-of-band signals, and therefore the receiver usually requires a channel selection filter to select the wanted signal. The circuit structure of the active RC filter with the characteristic of high dynamic range is selected, and the frog jump type structure is utilized to synthesize the passive filter, so that the sensitivity of the filter characteristic to the feedback resistor and capacitor in the circuit is minimum. In an actual circuit, the phase and amplitude frequency characteristics are often greatly different from the ideal conditions under the influence of the integrated circuit device process, the ambient temperature and the device service life. With the development of CMOS Process and integrated circuit design, the size of the integrated circuit is continuously reduced, and the influence of PVT (Process and performance) fluctuation on the integrated circuit is more and more serious. After the integrated circuit enters deep submicron, the electric leakage of the transistor is increasingly serious, and the widely used portable electronic equipment is in a standby state for most of time. PVT fluctuations arise mainly from the manufacturing flow of the integrated circuit and the actual environment in which the circuit operates, and since PVT fluctuations affect the performance, stability and power consumption of the integrated circuit, they must be taken into account when designing the circuit. The bandwidth of the filter is mainly determined by its time constant RC. If the standard value of the filter design time constant is RC, due to uncertain factors such as process and temperature, the actual RC will deviate due to the influence of chip process deviation (Δ P), power supply voltage fluctuation (Δ V), temperature variation (Δ T), PVT for short, and aging of devices on the chip, which causes the bandwidth of the filter on the chip to deviate from the design requirement. And the offset bandwidth can be compensated and calibrated by adaptively adjusting the value of R or C through a proper algorithm. The bandwidth of the filter can be flexibly and accurately adjusted, and the application range of the transceiver is wider.

The invention discloses a bandwidth calibration circuit of an active low-pass filter, wherein the bandwidth of the filter can be calibrated according to the influence of PVT, and meanwhile, the bandwidth of the filter can be flexibly and accurately adjusted.

Disclosure of Invention

In order to solve the above problems, an object of the present invention is to provide an active low pass filter bandwidth calibration circuit capable of adjusting a bandwidth in a wide range and maintaining a precise constant, in response to PVT influences.

The above object of the present invention can be achieved by the following technical solutions: an active low pass filter bandwidth calibration circuit comprising: I. the bandwidth-adjustable second-order active low-pass filter LPF arranged on the Q orthogonal channel, the I, Q channel two active low-pass filters LPF and the FSM connected in parallel with the two active low-pass filters LPF are characterized in that: two sinusoidal signals with different amplitudes and frequencies are respectively input at the input ends of I, Q two orthogonal channels, the two sinusoidal signals firstly respectively pass through I, Q two bandwidth-adjustable second-order active low-pass filters LPF, the two sinusoidal signals output by the LPF are respectively sent to two peak value detection circuits at the output ends of I, Q two LPFs, the amplitude information of the two sinusoidal signals is detected and sent to a comparator which is commonly connected with the output ends of the two peak value detection circuits for comparison, and the comparator sends the comparison result to a finite state machine FSM connected with the output ends of the comparator for processing; and the FSM controls and changes the capacitance value of the adjustable capacitor unit in the LPF through an algorithm circuit in the FSM according to the comparison result, detects and adjusts the bandwidth of the filter, approaches the amplitude value of the I path signal attenuated by the LPF to the amplitude value of the Q path signal, detects and continuously iterates the bandwidth of the LPF according to the result output by the comparator for many times until all bits of the adjustable capacitor unit are traversed, and stops iterating to obtain more accurate filter bandwidth insensitive to PVT change.

Compared with the prior art, the invention has the following beneficial effects:

by the calibration method, the bandwidth of the active low-pass filter can be adjusted in a wider range according to the filtering requirement of the input signal;

the invention can ensure that the design designated bandwidth of the active low-pass filter is not influenced by PVT and keeps accurate and constant.

Drawings

The invention is further described below with reference to the accompanying drawings.

Fig. 1 is a general circuit block diagram of the active low pass filter bandwidth calibration circuit of the present invention.

FIG. 2 is a graph comparing the spectrum of the output signal of the I/Q channel when the actual bandwidth of the LPF of FIG. 1 is equal to the calibration target bandwidth.

FIG. 3 is a graph of a comparison of the spectrum of the output signal of the I/Q channel when the actual bandwidth of the LPF of FIG. 1 is greater than the calibration target bandwidth.

FIG. 4 is a comparison graph of the spectrum of the output signal of the I/Q channel when the actual bandwidth of the LPF of FIG. 1 is less than the calibration target bandwidth.

Fig. 5 is a schematic diagram of the second-order active low-pass filter of the LPF of fig. 1.

Fig. 6 is a schematic diagram of the basic components of the tunable capacitor array of fig. 1.

Fig. 7 is a flow chart of the calibration algorithm of fig. 1.

Detailed Description

See fig. 1. In a preferred embodiment described below, an active low pass filter bandwidth calibration circuit comprises: I. the bandwidth-adjustable second-order active low-pass filter LPF is arranged on the Q orthogonal channel, and the finite state machine FSM is connected with the two active low-pass filters LPF on the I, Q channel in parallel and is terminated. Two sinusoidal signals with different amplitudes and frequencies are respectively input at the input ends of I, Q two orthogonal channels, the two sinusoidal signals firstly respectively pass through I, Q two bandwidth-adjustable second-order active low-pass filters LPF, the two sinusoidal signals output by the LPF are respectively sent to two peak value detection circuits at the output ends of I, Q two LPFs, the amplitude information of the two sinusoidal signals is detected and sent to a comparator which is commonly connected with the output ends of the two peak value detection circuits for comparison, and the comparator sends the comparison result to a finite state machine FSM connected with the output ends of the comparator for processing; and the FSM controls and changes the capacitance value of the adjustable capacitor unit in the LPF through an algorithm circuit in the FSM according to the comparison result, detects and adjusts the bandwidth of the filter, approaches the amplitude value of the I path signal attenuated by the LPF to the amplitude value of the Q path signal, detects and continuously iterates the bandwidth of the LPF according to the result output by the comparator for many times until all bits of the adjustable capacitor unit are traversed, and stops iterating to obtain more accurate filter bandwidth insensitive to PVT change.

The frequency of the sine signal input by the input end of the I-path LPF and the standard bandwidth BW to be calibrated are f_-3dBThe same, the frequency of the sine signal input by the input end of the Q-path LPF is f_-3dBOf amplitude I sinusoidal signal

And (4) doubling. The use of the two inputs of the overall bandwidth calibration circuit I, Q as the calibrated sinusoidal signal may be generated by a digital-to-analog converter (DAC) of the pre-stage circuitry of the LPF. Frequency of the I-path sinusoidal signal and standard bandwidth BW ═ f to be calibrated_-3dBSame, amplitude VI ═ V₁(ii) a The frequency of the Q path sinusoidal signal is f_-3dB/32, amplitude

With reference to FIGS. 2, 3 andfig. 4. After the two paths of I/Q sinusoidal signals pass through the two paths of I/Q LPFs, if the actual bandwidth BW of the LPFs is₀The standard bandwidth f to be calibrated is still not influenced by PVT_-3dBThen, since the LPF has 3dB of gain drop at its 3dB bandwidth, the amplitude VI _ out of the I output signal is attenuated to

The Q path signal has a frequency far lower than f_-3dBThe amplitude VQ _ out will not be attenuated after it passes through the LPF, and will still be

Thus, the I/Q signals are equal in amplitude after passing through the LPF, i.e., VI _ out is equal to VQ _ out, as shown in fig. 2. However, if the LPF is affected by PVT, its actual bandwidth BW₀May be greater or less than f_-3dBThus, the corresponding VI _ out may also be greater or less than

As shown in fig. 3 and 4, respectively.

Next, the amplitude information VI _ out and VQ _ out of the two filter output signals are detected by the peak detection circuit, respectively, and then sent to the comparator for comparison. The comparator outputs 1 or 0 according to the magnitude conditions of VI _ out and VQ _ out, then the comparison result is sent to a finite-state machine for processing, and an algorithm in the finite-state machine controls and changes the capacitance value of an adjustable capacitor array unit in the active low-pass filter according to the comparison result, so that the bandwidth of the filter changes towards the direction of reducing the difference between VI _ out and VQ _ out. Thus, the bandwidth of the filter is continuously subjected to a plurality of detection iterations according to the result output by the comparator until the traversal is finished, so that the VI _ out can be close to the VQ _ out, even if the actual bandwidth BW of the filter₀Approaching the target bandwidth BW to be calibrated can obtain a more accurate bandwidth calibration result. The total number of iterations is equal to the total number of bits of the basic unit of the tunable capacitor array in the filter,

refer to fig. 5 and 6. As shown in fig. 5. The active low pass filter LPF can adopt the second order of an on-chip differential structureAn active Butterworth filter structure. The second-order active butterworth filter is composed of a cascade of two stages of filter segments including transimpedance operational amplifiers OP1 and OP 2. The first stage filter section is symmetrically provided with two basic adjustable capacitor units C connected in parallel₀Form a total capacitance of 2C₀The capacitor array of (1); the second stage filter section is symmetrically provided with a single basic adjustable capacitor unit C₀Formed capacitor array and resistor R_F. The positive and negative output ends of the inverse proportion amplifying circuit of the first stage OP1 pass through two crossed voltage-dividing resistors R_CThe negative and positive input ends of the inverse proportion amplifying circuit of the series second stage OP2 are respectively connected with the input end of the first stage OP1 through two resistors R_AParallel to the output of the second stage OP 2. The 3dB bandwidth of the second order active butterworth filter can be expressed as:

it can be seen that the resistance or C is changed₀The 3dB bandwidth of the filter can be adjusted. In the actual digital-analog mixed chip, the resistance is mostly a fixed value, and C₀Typically tunable, and its internal capacitor array is constructed as shown in fig. 6. The capacitance of the array is increased by a factor of M of 2 (M takes a value from 0 to N), and is divided by a small fixed capacitor C_fixIn addition, the other capacitor arrays are controlled by the switch control signal ADJ<N:0>And (5) controlling. If the switch control signal ADJ<N:0>The corresponding decimal integer is k (k is more than or equal to 0 and less than or equal to 2)^N+1-1), then C₀Can be represented as C_fix+ k Δ C, so that a traversal of the capacitance values in the range of integer k times of the basic capacitance unit Δ C can be realized.

See fig. 7. In the primary calibration process, the calibration algorithm adopts a dichotomy control to change the adjustable capacitor unit according to the value output by the comparator, and the bandwidth of the filter is tested. Firstly, the capacitance value of the adjustable capacitance unit is set as the median value of the maximum capacitance variable range, and a switch control signal ADJ is initialized<N:0>100 … 000, the highest position 1, the remaining bits remain 0. At this time, if the signal amplitude VI _ out output by the I channel LPF and the signal amplitude VQ _ out output by the Q channel LPF satisfy: VI _ out>VQ _ OUT, the comparator output result COMP _ OUT is 1, indicating the current bandwidth of the filterBW₀Greater than the calibrated standard value f_-3dBThus, the capacitance C needs to be increased₀To adjust BW down₀. Therefore, the next detection will be performed in the left half of the median of the maximum capacitance variable range, i.e., the high order region, which corresponds to a range larger than the current capacitance value. Thus, ADJ<N:0>The most significant bit of 1 is determined and the current register is saved. Similarly, if VI _ out<VQ _ OUT, COMP _ OUT equals 0, ADJ<N:0>Will be set to 0 and the current register will be saved. The next highest detection is performed, still first ADJ<N:0>Setting the next highest position as 1, repeating the previous calibration process, judging whether the position is the lowest position, if so, ending the process, and sending a switch control signal ADJ<N:0>Is latched in a register that configures the filter bandwidth, otherwise the switch control signal ADJ is reset<N:0 > current bit 1, repeat the previous calibration procedure until ADJ<N:0>Is determined and then according to ADJ<N:0>Configuring the adjustable capacitor unit by the final code value to determine C₀To complete the calibration process. The values of VI _ out and VQ _ out are now approximately equal within the error tolerance. This error is determined by the decision error of the comparator itself and the total bit of the capacitor array. Finally, the actual bandwidth BW of the filter can be ensured₀And a calibration standard bandwidth f_-3dBAnd the influence of PVT change on the bandwidth of the filter is eliminated. Note that the active low pass filters of the I/Q channels are calibrated at the same time.

The foregoing is directed to the preferred embodiment of the present invention and it is noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design alternative embodiments without departing from the scope of the appended claims. It will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the spirit and substance of the invention, and these modifications and improvements are also considered to be within the scope of the invention.

Claims

1. An active low pass filter bandwidth calibration circuit comprising: I. the bandwidth-adjustable second-order active low-pass filter LPF arranged on the Q orthogonal channel, the I, Q channel two active low-pass filters LPF and the FSM connected in parallel with the two active low-pass filters LPF are characterized in that: two sinusoidal signals with different amplitudes and frequencies are respectively input at the input ends of I, Q two orthogonal channels, the two sinusoidal signals firstly respectively pass through I, Q two bandwidth-adjustable second-order active low-pass filters LPF, the two sinusoidal signals output by the LPF are respectively sent to two peak value detection circuits at the output ends of I, Q two LPFs, the amplitude information of the two sinusoidal signals is detected and sent to a comparator which is commonly connected with the output ends of the two peak value detection circuits for comparison, and the comparator sends the comparison result to a finite state machine FSM connected with the output ends of the comparator for processing; and the FSM controls and changes the capacitance value of the adjustable capacitor unit in the LPF through an algorithm circuit in the FSM according to the comparison result, detects and adjusts the bandwidth of the filter, approaches the amplitude value of the I path signal attenuated by the LPF to the amplitude value of the Q path signal, detects and continuously iterates the bandwidth of the LPF according to the result output by the comparator for many times until all bits of the adjustable capacitor unit of the array are traversed, and stops iterating to obtain more accurate filter bandwidth insensitive to PVT change.

2. The active low pass filter bandwidth calibration circuit of claim 1, wherein: the peak detection circuits are connected to the output of the active low pass filter, the Finite State Machine (FSM) is connected to the output of the comparator, and the two peak detection circuits are connected between the comparators.

3. The active low pass filter bandwidth calibration circuit of claim 1, wherein: the use of the two inputs of the overall bandwidth calibration circuit I, Q as the calibrated sinusoidal signal is generated using a digital-to-analog converter (DAC) of the pre-stage circuitry of the LPF.

4. The active low pass filter bandwidth calibration circuit of claim 1, wherein: frequency of the I-path sinusoidal signal and standard bandwidth BW ═ f to be calibrated_-3dBSame, and amplitude VI ═ V₁(ii) a The frequency of the Q path sinusoidal signal is f_-3dB/32, amplitude

5. The active low pass filter bandwidth calibration circuit of claim 3, wherein: I. the amplitudes of the two Q signals are equal after passing through the LPF, i.e. VI _ out is equal to VQ _ out, and if the LPF is affected by PVT, its actual bandwidth BW is equal to₀Greater or less than f_-3dBThe corresponding VI _ out is also greater or less than

6. The active low pass filter bandwidth calibration circuit of claim 1, wherein: the peak detection circuit detects amplitude information VI _ out and VQ _ out of output signals of the two filters, the amplitude information VI _ out and VQ _ out are sent to the comparator to be compared, the comparator outputs 1 or 0 according to the magnitude of the VI _ out and the VQ _ out, the comparison result is sent to the finite state machine FSM to be processed, an algorithm in the finite state machine FSM controls and changes the capacitance value of an adjustable capacitor array unit in the active low-pass filter according to the comparison result, so that the bandwidth of the filter changes towards the direction of reducing the difference between the VI _ out and the VQ _ out, the FSM continues to carry out detection iteration for multiple times according to the result output by the comparator on the bandwidth of the filter until the VI _ out approaches the VQ _ out, and the actual bandwidth BW of the filter approaches₀And approaching the target bandwidth BW to be calibrated until the traversal is finished, and obtaining a more accurate bandwidth calibration result, wherein the total number of iterations is equal to the total bit number of basic units of the adjustable capacitor array in the filter.

7. The active low pass filter bandwidth calibration circuit of claim 1, wherein: the capacitance of the array is increased by a multiple of 2 to the power of M, and M takes a value from 0 to N, and is divided by a fixed capacitor C_fixIn addition, the other capacitor arrays are controlled by the switch control signal ADJ<N:0>Control if onOff control signal ADJ<N:0>The corresponding decimal integer is k (k is more than or equal to 0 and less than or equal to 2)^N+1-1), then C₀Is represented as C_fix+ k Δ C, thereby enabling the traversal of the capacitance values within an integer k times of the basic capacitance unit Δ C.

8. The active low pass filter bandwidth calibration circuit of claim 1, wherein: the active low pass filter LPF adopts a differential structure of a second-order active Butterworth filter which is formed by cascading two stages of filter sections including a transimpedance operational amplifier OP1 and an OP2, wherein the first stage of filter sections are symmetrically provided with basic adjustable capacitor units C which are connected in parallel in pairs₀And a total capacitance of 2C₀The capacitor array of (1); the second stage filter section is symmetrically provided with a single basic adjustable capacitor unit C₀Formed capacitor array and resistor R_F。

9. The active low pass filter bandwidth calibration circuit of claim 8, wherein: the positive and negative output ends of the inverse proportion amplifying circuit of the first stage OP1 pass through two crossed voltage-dividing resistors R_CThe negative and positive input ends of the inverse proportion amplifying circuit of the series second stage OP2 are respectively connected with the input end of the first stage OP1 through two resistors R_AIn parallel with the output of the second stage OP2, the 3dB bandwidth of the second order active butterworth filter is expressed as:

10. the active low pass filter bandwidth calibration circuit of claim 6, wherein: in the primary calibration process, the calibration algorithm adopts a dichotomy control to change the adjustable capacitor unit according to the value output by the comparator, the bandwidth of the filter is tested, firstly, the capacitance value of the adjustable capacitor unit is set as the median value of the maximum capacitance variable range, and a switch control signal ADJ is initialized<N:0>100 … 000, i.e. the highest position 1, the remaining bits are kept at 0, and the signal amplitude VI _ out output by the I channel LPF and the signal amplitude VQ _ out output by the Q channel LPF if: VI _ out>VQ _ OUT, the comparator output COMP _ OUT is 1, and the next detection will be performed in the left half of the maximum capacitance variable range, i.e. the high-order region, which corresponds to a range larger than the current capacitance value, ADJ<N:0>Is determined to be 1, the current register is saved, and likewise, if VI _ out<VQ _ OUT, COMP _ OUT equals 0, ADJ<N:0>The highest bit of the register is set to be 0, and the current register is saved; the next highest detection is performed, still first ADJ<N:0>Setting the next highest position as 1, repeating the previous calibration process, judging whether the position is the lowest position, if so, ending the process, and sending a switch control signal ADJ<N:0>Is latched in a register that configures the filter bandwidth, otherwise the switch control signal ADJ is reset<N:0>The previous calibration procedure is repeated until ADJ, with the current bit being 1<N:0>Is determined and then according to ADJ<N:0>Configuring the adjustable capacitor unit by the final code value to determine C₀To complete the calibration process.