JP2011211635A - Rf receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an RF reception circuit capable of fully responding to various reception environments different in frequency differences between desired waves and interfering waves, intensity differences between the desired waves and the interfering waves, or the like.SOLUTION: This RF receiver 10 includes: an RF power detector 15 for detecting an RF power detection value α of an RF signal amplified by an RF signal amplifier 11; an IF power detector 16 for detecting an IF power detection value β of an IF signal extracted by a passband filter 13; and an arithmetic circuit 17 for enabling the RF power detection value α only when the RF power detection value α exceeds a third threshold Z and enabling the IF power detection value β only when the IF power detection value β is in the range between a first threshold Yand a second threshold Y, and generating a gain restraining signal γ based on the enabled RF signal detection value α and IF signal detection value β to be supplied to the RF signal amplifier as an external signal.

Description

  The present invention relates to an RF (high frequency) receiver that performs automatic gain control (AGC) in a high frequency amplification stage or an intermediate frequency amplification stage, and more particularly to an RF reception apparatus used in a terrestrial digital broadcast receiver.

  There are several forms of conventional automatic gain control in the RF receiver as shown in FIGS. Each form basically includes a low noise amplifier (LNA) 1, a down conversion mixer (MIX) 2, an LPF or BPF 3, a variable gain amplifier (VGA) 4, and a power detector 5. The role of the power detector in automatic gain control is to prevent degradation of the linearity of the RF receiving circuit by performing control to lower the gain of the RF receiving circuit when a strong signal is received. Of course, it is also possible to increase the dynamic range of the RF receiver circuit itself in order to prevent deterioration of linearity, but it is very difficult to increase the dynamic range in an RF receiver circuit that requires low power consumption.

  In the form shown in FIG. 1A, the IF signal intensity before being input to the LPF or BPF 3 is detected by the power detector 5, and the output of the power detector 5 is fed back to the low noise amplifier 1. As an example in which this form is realized, there is a product (model: MAX2160) provided by Maxim.

  In the form shown in FIG. 1B, the RF signal intensity before down-conversion to the IF signal is detected by the power detector 5, and the output of the power detector 5 is fed back to the low noise amplifier 1. As an example in which this form is realized, there is a product (model: MxL5003S) provided by MAXLINE®.

  In the form shown in FIG. 1C, the IF signal intensity detected at the output of the variable gain amplifier 4 is fed back to the variable gain amplifier 4 in addition to the form shown in FIG. 1A. As an example in which this form is realized, there is a product (model: MT2131) provided by Microtune Corporation.

  However, in any of the above-described forms, there is a problem that the reception characteristics of the RF receiver are deteriorated in an environment where an interference wave stronger than the desired wave is simultaneously input to the RF receiver. Especially when the RF receiver is a terrestrial digital broadcast receiver, the reception band is very wide from 470 MHz to 770 MHz in the UHF band, and in order to obtain stable reception characteristics, the interference wave far from the interference wave close to the reception frequency is obtained. It is necessary to consider the influence of interference waves in a wide frequency band up to. When RF signal strength is used for gain control, the detectable frequency range is relatively wide depending on the high frequency. On the other hand, when the IF signal strength is used for gain control, the detectable frequency range is relatively narrow depending on the low frequency. In particular, in a low intermediate frequency (Low-IF) system often used in terrestrial digital broadcast receivers, the IF frequency is as low as 1 MHz or less, and the detectable frequency range is further narrowed.

  For example, in the form shown in FIG. 1A, since the IF signal intensity before the input of the bandpass filter 3 is used for gain control, the detectable frequency range is somewhat wide, but a strong interference wave far from the desired wave cannot be detected and gain suppression is performed. Therefore, there is a problem that the linearity of the circuit is deteriorated between the low noise amplifier 1 and the bandpass filter 3 and the reception characteristic is also deteriorated.

  In the form shown in FIG. 1B, the detectable frequency range can be widened to detect the RF signal for gain control, but the circuit gain is lowered by a strong interference wave regardless of the intensity of the desired wave. End up. That is, when the intensity of the desired wave is relatively low, there is a problem that the desired wave cannot be satisfactorily received because the gain is excessively reduced. Although this problem can be dealt with by increasing the detection threshold at the time of RF signal detection, on the other hand, when the desired wave intensity increases, the problem is that the linearity of the circuit deteriorates due to the disturbing wave.

  Furthermore, in the form shown in FIG. 1C, the IF signal that has passed through the band filter 3 after down-conversion is further used for automatic control, so that the signal strength of the desired wave can be more reflected in automatic gain control. However, this can be realized when the IF frequency is as high as several tens of MHz or more and the detectable frequency range is wide, and is not effective in a digital terrestrial broadcasting receiver.

  As a technique for preventing the linearity of the RF receiving circuit from being deteriorated, there is a technique disclosed in Patent Document 1. Referring to paragraph [0035] of Cited Document 1, a power detector 16 that detects the RF signal intensity amplified by the RF variable gain amplifier 11, and a power detector 17 that detects the IF signal intensity output from the mixer 12; An RF reference level generator 20 for generating an RF reference level that generates an RF reference level according to the signal strengths of both the power detector 16 and the power detector 17, and the digital signal strength obtained by the RF reference level and the ADC 15. An RF receiver including an RF gain controller 21 that controls the gain of the RF variable gain amplifier according to the comparison result is disclosed (see FIG. 4 of cited document 1). With this configuration, even when the received signal power of the broadcast wave component is large in a channel far from the desired channel, saturation of the RF variable gain amplifier 1 due to the far received signal power can be prevented, and the signal component of the desired channel can be prevented. It is said that good reception characteristics can be obtained without distortion.

JP 2009-182859 A

  However, in the technique disclosed in the cited document 1, the RF reference level is determined by the thresholds th1 and th3 set in the power detectors 16 and 17, and the output of the RF variable gain amplifier 11 does not exceed this RF reference level. Control is being made. That is, in the technique disclosed in the cited document 1, the control mode for the gain of the RF gain controller 21 is determined only by the magnitude relationship between the RF reference level and the IF gain control value, and the frequency difference between the desired wave and the interference wave or the desired wave is determined. There is a problem that it cannot fully cope with various reception environments in which the difference in intensity between the interference wave and the interference wave is different.

  An object of the present invention is to provide an RF receiving circuit that can fully cope with various reception environments in which a frequency difference between a desired wave and an interfering wave, an intensity difference between the desired wave and the interfering wave, and the like are different.

An RF receiver circuit according to the present invention includes an RF signal amplifier that amplifies a received RF signal with a gain according to an external signal, a frequency conversion mixer that converts the frequency of the amplified RF signal into an IF signal frequency, and the converted signal. An RF receiver that extracts an IF signal from the received signal, an RF power detector that detects an RF power detection value (α) of the RF signal amplified by the RF signal amplifier, and the band An IF power detector that detects the IF power detection value (β) of the IF signal extracted by the filter, and the RF power detection value only when the RF power detection value (α) exceeds the third threshold (Z). Only when (α) is valid and the IF power detection value (β) is within the first threshold (Y ₁ ) to the second threshold (Y ₂ ), the IF power detection value (β) is valid, Valid R An arithmetic circuit that generates a gain suppression signal (γ) based on the F signal detection value (α) and the IF signal detection value (β) and supplies the gain suppression signal (γ) as the external signal to the RF signal amplifier. And

  According to the RF receiver circuit of the present invention, it is possible to appropriately cope with various reception environments in which the frequency difference between the desired wave and the jamming wave, the intensity difference between the desired wave and the jamming wave, and the like are different, and freedom for distortion suppression The degree of distortion increases, and distortion suppression can be optimized not only according to the interference wave frequency but also according to the desired wave level.

It is a block diagram which shows the structure of the conventional RF receiving circuit. It is a block diagram which shows the structure of the other conventional RF receiving circuit. It is a block diagram which shows the structure of the other conventional RF receiving circuit. It is a figure which shows the 1st Example of this invention and shows the structure of RF receiving circuit. It is a figure which shows the characteristic of RF power detector and IF power detector which were shown by FIG. FIG. 3 is a block diagram showing an internal configuration of an arithmetic circuit shown in FIG. 2. It is a figure explaining the aspect of the automatic gain control obtained by RF receiving circuit. It is a graph which shows the change of RF gain control obtained by RF receiving circuit. It is a graph which shows the change of system NF obtained by RF receiving circuit. It is a graph which shows the change of IM3 obtained by RF receiving circuit. It is a figure which shows the 2nd Example of this invention and shows the structure of RF receiving circuit.

  Embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<First embodiment>
FIG. 2 shows a first embodiment of the present invention and shows a configuration of an RF receiving circuit 10 according to the present invention. The RF receiving circuit 10 includes an RF signal amplifier 11, a frequency conversion mixer 12, a bandpass filter 13, an IF signal amplifier 14, an RF power detector 15, an IF power detector 16, and an arithmetic circuit 17. Assuming that the RF receiver circuit 10 is, for example, a terrestrial digital broadcast receiver, the functions of each unit included in the RF receiver circuit 10 will be described below.

  The RF signal amplifier 11 is a low-noise amplifier, and RF signals broadcast on a plurality of frequency channels are input via an appropriate tuning circuit (not shown). The RF signal amplifier 11 amplifies and outputs the RF signal with a gain (gain) corresponding to the value (γ) of the gain suppression signal. The magnitude of the gain is controlled to be a lower gain as the value of the gain suppression signal (γ) is higher, and is controlled to a higher gain as the value is lower, for example, in the range of −10 dB to +55 dB.

  The frequency conversion mixer 12 mixes the RF signal amplified by the RF signal amplifier 11 and a signal having an appropriate frequency to generate an intermediate frequency signal, that is, an IF signal.

  The band filter 13 extracts an IF signal corresponding to a desired desired wave channel from the IF signal from the frequency conversion mixer 12. The band filter 13 selects a channel of a desired wave, that is, extracts the desired wave, and is realized by one of or a combination of a low pass filter (LPF) and a band pass filter (BPF).

  The IF signal amplifier 14 amplifies and outputs the IF signal extracted by the frequency conversion mixer 12 with a gain corresponding to the external control signal. The signal amplified by the IF signal amplifier 14 is demodulated into a digital video signal or a digital audio signal by a demodulator (not shown) or the like. The external control signal is usually given from a demodulator and adjusted to the IF output amplitude required by the demodulator.

  The RF power detector 15 detects the intensity of the RF signal output from the RF signal amplifier 11, that is, the RF input power, and outputs it as an RF power detection value (α). Since the RF signal has a relatively wide frequency width, the RF power detector 15 can detect not only a desired wave but also an interference wave. That is, the obtained RF power detection value is a value in which desired wave power and interference wave power are superimposed.

  The IF power detector 16 detects the intensity of the IF signal output from the band filter 13, that is, the IF input power, and outputs it as an IF power detection value (β). Since the IF power detector 16 has a narrow frequency band of the extracted IF signal, only the desired wave is detected and the interference wave is not detected.

  The arithmetic circuit 17 takes in the RF power detection value (α) from the RF power detector 15 and also takes in the IF power detection value (β) from the IF power detector 16 and performs a predetermined calculation on these values (value ( A gain suppression signal having γ) is generated and supplied to the RF signal amplifier 11.

  FIG. 3 shows the characteristics of the RF power detector 15 and the IF power detector 16 shown in FIG.

First, looking at the IF power detector output characteristics of (b), the change of the IF power detection value (β) on the vertical axis with respect to the IF input power on the horizontal axis is assumed as a linear change. Therefore, assume the first threshold value _{Y 1} and the second threshold value _{Y 2} at IF power detected value (beta). Then, the first threshold value _{Y 1} to IF power detected value in the second threshold value _{Y 2} a (beta) and the effective range. Here, the effective range means a range that should contribute to the value of the gain suppression signal.

  Next, when viewing the output characteristics of the RF power detector in (a), a change in the detected RF power value (α) with respect to the RF input power on the horizontal axis is assumed as a linear change. Therefore, the third threshold value Z is assumed in the RF power detection value (α). Then, the range of the RF power detection value (α) exceeding the third threshold Z is similarly set as the effective range.

  FIG. 4 shows the internal configuration of the arithmetic circuit 17 shown in FIG. The arithmetic circuit 17 includes a first arithmetic unit 171, a second arithmetic unit 172, and a subtracter 173.

The first calculation unit 171 obtains a value obtained by removing the value of the third threshold (Z) from the RF power detection value (α). Here, as described with reference to FIG. 3, the RF power detection value (α) is valid only when Z <α, and is output as a zero value otherwise. The second calculation unit 172 obtains a value obtained by multiplying a value obtained by removing the value of the IF power detection value (β) from the first threshold (Y ₁ ) by a predetermined coefficient X. Here, the IF power detection value (β) is valid only when Y ₁ <β <Y ₂ , and is output as a zero value otherwise. X is a positive coefficient and can be arbitrarily set in advance.

  The subtractor 173 subtracts the value obtained by the first computing unit 172 from the value obtained by the first computing unit 171 and outputs the obtained value as the value (γ) of the gain suppression signal. The value (γ) of the gain suppression signal is effective only when 0 ≦ γ, and is output as a zero value otherwise.

  FIG. 5 illustrates an aspect of automatic gain control obtained by the RF receiving circuit 10. As a premise, the RF power detection value (α) of the RF signal including the desired wave and the interference wave is detected. At the same time, the IF power detection value (β) of the IF signal of only the desired wave is detected. This figure assumes a case where the input power of the interference wave is small, that is, a case where the RF power detection value (α) is almost determined by the power of the desired wave. The horizontal axis of this figure is the desired wave input power, and the vertical axis is the IF signal output value output from the IF signal amplifier 14 (see FIG. 2). However, the external control signal of the IF signal amplifier 14 (see FIG. 2) is assumed to be constant.

First, when the IF power detection value (β) of the desired wave is larger than the second threshold Y ₂ (rightward in the drawing from the point B), the IF power detection value (β) is not effective. The gain of 11 is determined only by the detected RF power value (α). Since the control is based only on the RF power detection value (α), the IF signal output value is controlled to be substantially constant regardless of the desired wave input.

From this state, if the IF power detection value (β) of the desired wave is smaller than the second threshold Y ₂ (leftward of the page from the point B), the difference between the second threshold Y ₂ and the IF power detection value (β) is predetermined. The value (γ) of the gain suppression signal is reduced by a value multiplied by the coefficient X. That is, the degree of gain suppression is weakened, and the gain of the RF signal amplifier 11 is larger than that determined only by the RF power detection value (α).

  Point A is a point corresponding to the third threshold Z. At this point A, the two inputs of the subtractor 173 (see FIG. 4) have the same value, and the output of the subtractor 173, that is, the value (γ) of the gain suppression signal becomes zero. When the desired signal power lower than the point A is input, the value (γ) of the gain suppression signal becomes 0 without taking a negative value, so that the gain of the RF signal amplifier 11 is controlled to the maximum, and the desired signal power decreases. The IF signal output value decreases linearly in accordance with (leftward from the point A).

  Even when the power of the interference wave is large, that is, when the RF power detection value (α) is almost determined by the power of the interference wave, the IF power detection value (β) is determined by the power of the desired wave. In the meantime, the same control as in the case where the interference wave power is small is performed. That is, control is performed such that the gain of the RF signal amplifier 11 increases as the desired wave power decreases from point B to point A.

  6, 7 and 8 show the change in the RF gain control, the change in the system NF, and the change in IM3 obtained by the RF receiving circuit 10, respectively. In each of these graphs, the horizontal axis indicates the value of the desired wave input power in common. The system NF is a noise figure and means how much the S / N on the output side deteriorates with respect to the S / N on the input side. The noise figure is usually expressed in decibels. For example, if the S / N on the input side is 20 dB and the S / N on the output side deteriorates to 15 dB, the noise figure of this system is 5 dB. That is, the higher the value, the more the desired wave quality is degraded. IM3 means third-order inter-modulation distortion. Since the third-order intermodulation distortion is generated as the linearity of the circuit with respect to the desired wave input power is worse, the linearity becomes worse as the value of IM3 is larger.

  Also, in each of these graphs, three plots indicated by a dotted line, a broken line, and a solid line are shown. The dotted line indicates a conventional case where the threshold value for the RF power detection value is “−60 dBm”. The broken line indicates a conventional case in which the threshold for the RF power detection value is set to “−50 dBm” and the threshold is relatively higher than the dotted line case. The solid line shows the case according to the invention. Further, it is assumed that the power ratio between the interference wave and the desired wave is 50 dB, and the interference wave is larger than the desired wave. Further, the maximum controllable gain of the RF signal amplifier is 55 dB, and the minimum gain is -10 dB.

  Referring to the change in RF gain control in FIG. 6, at point A on the horizontal axis, the desired wave is weak and the desired wave input power is −100 dBm. Since the power ratio between the disturbing wave and the desired wave is 50 dB, the RF power superimposed on the disturbing wave and the desired wave is almost determined by the disturbing wave input power and is −50 dBm. At this point A, in the case of the dotted line, the threshold value for the RF power detection value is low, so that gain control based on the RF power detection value is effective, the RF gain is lowered by 10 dB, and the desired wave cannot be received sufficiently. On the other hand, in the case of the broken line, since the threshold value for the RF power is high, the RF power detection value becomes invalid, the RF gain is maintained at the maximum (10 dB increase from the dotted line case), and the desired wave can be received. Also in the case of the solid line of the present invention, the desired wave can be received while the RF gain is maintained at the maximum as in the case of the broken line.

  At point B on the horizontal axis, the desired wave is relatively strong and the desired wave input power is −60 dBm. The RF power is −10 dBm from the relationship between the interference wave and the desired wave. At this point B, in the case of the broken line, the RF gain is 15 dB, and the gain is too large, so that the desired wave cannot be satisfactorily received due to distortion. On the other hand, in the case of the dotted line, the RF gain is 5 dB (10 dB lower than in the case of the broken line), and the RF gain is appropriately suppressed so that the desired wave can be received satisfactorily without distortion. In the case of the solid line of the present invention, the RF gain is appropriately suppressed and the desired wave can be received satisfactorily without distortion as in the case of the dotted line.

  At point C on the horizontal axis, the desired wave is strong and the desired wave input power is −50 dBm. The RF power is 0 dBm from the relationship between the interference wave and the desired wave. At this point C, in the case of the broken line, the RF gain is 5 dB, and the RF gain is too large, so that the desired wave cannot be received satisfactorily due to distortion. On the other hand, in the case of the solid line of the present invention, the RF gain is suppressed to -10 dB. That is, in the case of the solid line according to the present invention, the RF gain is controlled to be reduced by the gain reduction (65 dB) -NF degradation (50 dB (see FIG. 7)) = 15 dB compared to the case of the broken line. The Such an aspect is realized by appropriately selecting the predetermined coefficient X. Thereby, the desired wave can be received satisfactorily without distortion.

  Referring to the change of the system NF in FIG. 7, the solid line of the case of the present invention is the best value of 2 dB of NF in the region where the desired wave with the desired wave input power of −110 to −100 dBm is weak as in the case of the broken line. Is maintained. As the desired wave input power becomes higher than -100 dBm, the RF gain is reduced by the gain control, and the system NF deteriorates by the reduced amount. As described with reference to FIG. 6, in the case of the solid line of the present invention, 50 dB is generated as the degradation of NF when the desired wave input power is −50 dBm.

  Referring to the change of IM3 in FIG. 8, the case of the solid line of the present invention suppresses the increase of IMP3 in the wide range of the desired wave with a desired wave input power of −110 to −40 dBm, as in the case of the dotted line. Yes. In the case of the present invention, the worst value of IIP3 (3rd order intercept point) indicating the degree of IM3 is set to −10 dBm.

  As can be seen from FIGS. 6 to 8 above, it can be seen that the two conventional cases (dotted line and broken line) are in a trade-off relationship with each other. On the other hand, when the case of the present invention, that is, the RF receiver according to the present invention is applied, the plot changes so as to transition between two plots of the conventional case. That is, in the case of the present invention, the desired wave input power is in a wide range of −110 dBm to −40 dBm, avoiding the disadvantages caused by either the dotted line or the broken line in the conventional case, and gain control is performed well so that the reception characteristics are obtained. Improvements are being made.

<Second embodiment>
FIG. 9 shows the configuration of the RF receiver circuit 10 according to the second embodiment of the present invention. In addition to the configuration in the first embodiment (see FIG. 2), the RF receiving circuit 10 in the second embodiment includes a pre-band filter 18 at a stage subsequent to the frequency conversion mixer 12, that is, a position in front of the band filter 13. Has been inserted. The pre-band filter 18 is a low-pass filter, and does not require an attenuation characteristic equivalent to that of the band filter 13, but requires a certain degree of adjacent channel removal characteristic. The IF power detector 16 acquires an IF power detection value from the output of the pre-band filter 18. Further, the gains of the band filter 13 and the IF signal amplifier 14 are controlled in accordance with the external control signal. For example, the external control signal is supplied from a demodulator (not shown) at the subsequent stage. The demodulator detects the input amplitude range and distortion characteristics of the IF signal supplied from the band filter 13 and adjusts the value of the external control signal.

  The operation of the RF receiving circuit 10 in the second embodiment is basically the same as that of the first embodiment. However, in the second embodiment, since the IF power detector 16 acquires the IP power detection value from the output of the pre-band filter 18, the degree of freedom is selected by selecting the frequency range of the IF power detection value (β). It becomes easier to handle various reception environments. Further, since the gain of the band filter 13 can be controlled, restrictions on the input amplitude range and distortion characteristics of the subsequent demodulator (not shown) can be relaxed. For example, when a stronger signal is received, the occurrence of distortion can be reduced by lowering the gain of the band-pass filter 13. Further, when receiving a smaller signal, it is possible to reduce input conversion noise by increasing the gain.

  In the plurality of embodiments described above, an RF power detection circuit and an IF power detection circuit are provided in an RF reception circuit such as a terrestrial digital broadcast receiver, and two power detection values are calculated and used as a gain suppression signal of the RF circuit. Value. As a result, it is possible to cope with various reception environments in which the frequency difference between the desired wave and the jamming wave and the intensity difference between the desired wave and the jamming wave are different, increasing the degree of freedom for distortion suppression and the frequency of the jamming wave. It is possible to optimize distortion suppression according to the desired wave level.

  Although the present invention has been mainly described in the form of receiving digital terrestrial broadcasting, other than this, a direct conversion system (also referred to as Zero-IF system) or an extremely low intermediate frequency (Low-IF) The present invention can also be applied to an RF receiver using the method.

DESCRIPTION OF SYMBOLS 10 RF receiver circuit 11 RF signal amplifier 12 Frequency conversion mixer 13 Band filter 14 IF signal amplifier 15 RF power detector 16 IF power detector 17 Arithmetic circuit 18 Pre-band filter α RF power detected value β IF power detected value γ Gain suppression Signal value X Predetermined coefficient Y ₁ First threshold Y ₂ Second threshold Z Third threshold

Claims (5)

An RF signal amplifier that amplifies the received RF signal with a gain according to an external signal, a frequency conversion mixer that converts the frequency of the amplified RF signal into an IF signal frequency, and an IF signal is extracted from the converted signal An RF receiver including a bandpass filter,
An RF power detector for detecting an RF power detection value (α) of the RF signal amplified by the RF signal amplifier;
An IF power detector for detecting an IF power detection value (β) of the IF signal extracted by the bandpass filter;
Only when the RF power detection value (α) exceeds the third threshold value (Z), the RF power detection value (α) is made valid and the IF power detection value (β) is set to the first threshold value (Y ₁ ) to The IF power detection value (β) is valid only when it is within the second threshold (Y ₂ ), and the gain suppression signal is based on the valid RF signal detection value (α) and IF signal detection value (β). An arithmetic circuit that generates (γ) and supplies it to the RF signal amplifier as the external signal;
An RF receiving apparatus comprising: The arithmetic circuit removes the value of the IF power detection value (β) from the first threshold (Y ₁ ) from the value obtained by removing the value of the third threshold (Z) from the RF power detection value (α). 2. The RF receiver according to claim 1, further comprising: a subtracter that generates a value obtained by subtracting a value obtained by multiplying the obtained value by a predetermined coefficient (X) as the value of the gain suppression signal (γ). .   The IF power detector further includes a pre-band filter inserted between the frequency conversion mixer and the band filter, and the IF power detector is extracted by the pre-band filter instead of the IF signal extracted by the band filter. The RF receiving circuit according to claim 1, wherein an IF power detection value (β) of the IF signal is detected.   An IF signal amplifier that amplifies the IF signal with an externally controllable gain, and a demodulator that demodulates the amplified IF signal, wherein the demodulator controls the gain of the IF signal amplifier. The RF receiver circuit according to claim 1 or 2.   5. The RF receiving circuit according to claim 4, wherein the gain of the band filter is externally controllable, and the demodulator controls the gain of the band filter.

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