JP2008244551A - Fm detection circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an FM detection circuit for turning off an output detection signal caused by a received signal in which a C/N ratio is not ensured by fixing level control by AGC if a reception power level is low to detect a reception signal power level. <P>SOLUTION: The FM detection circuit is provided with an amplifier, a variable attenuator, a mixer, a phase comparator for outputting a detecting signal, a switching means provided on a transmission path, an RSSI circuit for generating a reception signal strength index signal, an AGC circuit for receiving one output from the RSSI circuit, and a determination device for receiving the other output from the RSSI circuit. If the level of a reception signal strength index signal is lower than a threshold voltage of the AGC circuit, the AGC circuit maintains the attenuation quantity of the variable attenuator at the minimum value, and if the level of the reception signal strength index signal is lower than a threshold voltage set in the determination device within a minimum value maintaining range of the variable attenuator, the switching means is opened. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an FM detection circuit used in a receiving device for receiving an FM modulated signal.

  In a radio frequency (RF) receiver such as mobile communication or television, the level of an interference wave such as a distorted wave must be suppressed until the band is limited to only a desired wave by a frequency filter. In particular, in a system in which signals of three or more waves with equal frequency intervals are input, a part of the third-order intermodulation distortion wave caused by a signal other than the desired one becomes the same frequency as the desired wave and is difficult to remove with a frequency filter It is. Further, in the quadrature FM detection circuit, it is necessary to input to the phase comparator with a sufficient amplitude. For example, Japanese Patent Application Laid-Open No. 2004-282464, FIG. 1 (see Patent Document 1) discloses an AGC (Auto Gain Control) that suppresses intermodulation distortion and reduces intermodulation interference. .

JP 2004-282464 A (FIG. 1)

  However, in Patent Document 1, although the output signal is made constant by performing AGC, the intermodulation distortion wave is suppressed and the intermodulation interference is reduced, the reception power level of the received signal is low because AGC is always performed. In this case, there is a problem that a detection signal based on a reception signal including a noise signal is output even though the C / N ratio (CNR) is small.

  The present invention has been made to solve the above-described problems. When the received power level is low, AGC level control is fixed so that the received signal power level can be detected, and the C / N ratio is An object of the present invention is to provide an FM detection circuit that turns off an output detection signal based on a reception signal that is not secured.

  An FM detector circuit according to a first aspect of the present invention includes an amplifier that amplifies a high-frequency signal, a variable attenuator that varies the output of the amplifier, and a mixer that down-converts the high-frequency signal that has passed through the variable attenuator to an intermediate frequency. A first signal line that branches the intermediate frequency and has one branch output passed through a first amplitude limiting amplifier via a phase shifter and a second signal line that has passed through a second amplitude limiting amplifier A phase comparator that outputs a detection signal proportional to the phase difference between the first signal line and the second signal line, and the first signal line and the second signal line. The switching means provided in the transmission path of the one branch output excluding the RSSI circuit that generates a received signal strength indicator signal from the other branch output of the intermediate frequency, and gain adjustment to which one output of the RSSI circuit is input Comparison for When the level of the received signal strength indicator signal is lower than the threshold voltage provided in the AGC circuit, the AGC circuit When the attenuation level of the variable attenuator is maintained at a minimum value, and the level of the received signal strength indicator signal is lower than the threshold voltage provided in the determination unit within the minimum value maintenance range of the variable attenuator, the Transmission of the detection signal is interrupted by opening the switching means, and when the level of the received signal strength index signal is higher than the threshold voltage provided in the determination unit, transmission of the detection signal is performed by closing the switching means. If the level of the received signal strength index signal is higher than the threshold voltage provided in the AGC circuit, the AGC circuit increases or decreases the attenuation amount of the variable attenuator, and the intermediate frequency Is an output level which was kept constant.

  As described above, according to the first aspect of the present invention, when the received power level is low, the AGC level control is fixed, and the detection of the received power level is maintained, but the C / N ratio is not secured. Although the detection signal to be output is turned off with respect to the reception signal, AGC is performed over a wide reception power range, and a detection output with little distortion can be obtained and noise is reduced when the C / N ratio is small. Since a detection signal including a signal is not transmitted, an FM detection circuit in which the quality of the detection signal is stable can be obtained.

Embodiment 1 FIG.
Embodiment 1 of the present invention will be described below with reference to FIG. FIG. 1 is a block configuration diagram of an FM detection circuit according to the first embodiment. In FIG. 1, 1 is a high-frequency amplifier (amplifier) that amplifies a high-frequency reception signal, and 2 is a variable that increases or decreases the amplified signal amplified by the amplifier 1. Attenuator 3 is a down mixer (mixer) that down-converts the amplified signal to an intermediate frequency (IF), 4 is an intermediate frequency amplifier (IF amplifier) that amplifies the IF-converted intermediate frequency received signal, and 5 is an intermediate A band-pass filter (band-pass filter) that suppresses the amplified signal amplified by the frequency amplifier 4, and is branched to a received detection signal generation line and a control signal line, which are power transmission paths, on the output side. Reference numeral 6 denotes an IF buffer provided on the reception detection signal generation line side, and branches the IF signal on the output side.

  7 is a phase shifter that changes the phase of one of the branched IF signals, 8 is a first amplitude limiting amplifier that amplifies the amplitude of the IF signal whose phase is changed by the phase shifter 7, and 9 is the other of the branched IF signals. A second amplitude limiting amplifier 10 that amplifies the amplitude of the IF signal is a phase comparator that compares the phases of the amplified signals of the first amplitude limiting amplifier 8 and the second amplitude limiting amplifier 9. Output as a signal.

  Reference numeral 11 denotes an RSSI (Receiving Signal Strength Indicator) circuit that amplifies the IF signal that has been amplified by the intermediate frequency amplifier 4 and that amplifies the IF signal that has passed through a predetermined frequency, and converts the power amplified signal to a voltage to generate a received signal strength index signal. The output line is divided into a feedback line and an output control line. An AGC (Auto Gain Control) circuit 12 is provided in the feedback line on the output side of the RSSI circuit 11 and controls the variable attenuator 2. Reference numeral 13 denotes a determination unit (detection signal ON / OFF determination unit) that is provided in the output control line on the output side of the RSSI circuit 11, detects the input power level, and sends a switching signal according to the output value of the RSSI circuit 11. . Reference numeral 14 denotes switching means constituted by a switch for turning on / off the detection signal output from the comparator 10, and is controlled by the determination unit 13.

  Next, the operation will be described. In FIG. 1, a received RF signal (FM modulated wave) is amplified by an amplifier 1, level-adjusted by a variable attenuator 2, and frequency converted to an IF signal by a down mixer 3.

  The IF signal is amplified by the intermediate frequency amplifier 4, becomes a signal having a desired IF bandwidth by a band pass filter (BPF) 5, and is band-limited.

  The IF signal of the band-limited reception detection signal generation line is input to the IF buffer 6 and the output is branched. One branch output changes the phase difference by 90 ° within the desired carrier frequency band. , The first amplitude limiting amplifier 8 amplifies the signal by a predetermined amount, and the other branched branch output is amplified by the second amplitude limiting amplifier 9 by a predetermined amount. The amplitude signal amplified by a predetermined amount with the amplitude limiting amplifier 9 is input to the phase comparator 10 and phase-compared to perform FM detection, and becomes an FM detection output.

  Further, the IF signal of the band-limited control signal line is input to the RSSI circuit 11, and the RSSI circuit 11 outputs an RSSI output signal (received signal strength indicator signal) proportional to the power level of the input IF signal. To do. The RSSI output signal is divided and input to the AGC circuit 12 provided in the feedback line and the determiner 13 provided in the output control line.

  The AGC circuit 12 controls the variable attenuator 2 to increase / decrease the attenuation amount of the variable attenuator 2 within a range where the received signal is larger than a certain value, and keep the power input to the RSSI circuit 11 constant. The level set to this constant value is set to a level at which the occurrence of nonlinear distortion generated in the amplifier 1 or IF amplifier 4 can be suppressed to a desired value or less.

  The output of the determiner 13 becomes a control signal for the switch 14, the RSSI output signal is compared with the voltage set by the reference potential of the determiner, and the output of the detection signal is opened and closed (ON / OFF) in the transmission path.

  Next, a method for opening and closing (ON / OFF) the output of the detection signal will be described. Normally, in the AGC circuit, even if the RF input power changes, for example, the IF signal power input to the RSSI circuit 11 is constant, so the RSSI output voltage also becomes a constant value as shown in FIG. Therefore, since the signal proportional to the power level of the signal input to the receiver or the like is not output, it is difficult to determine whether the detection output signal is ON / OFF with respect to the weak RF input power.

  On the other hand, in the first embodiment, when the RF input power is weak, the variable voltage of the variable attenuator 2 is stopped by setting the control voltage from the AGC circuit 12 to the minimum control voltage, and the RF input power Pass through without attenuation. That is, as shown in FIG. 3, the variable attenuator 2 has a minimum attenuation when the RF input power is weak, and has a variable function as the RF input power gradually increases.

  FIG. 4 is a diagram for explaining the relationship with the RSSI output voltage according to each level of the RF input power. In FIG. 4, the RF input power in a region where the RF input power is weak is RFin1, and the RF input power for functioning the AGC is RFin2.

  When the input power is equal to or less than RFin1, the switch 14 is turned off (opened) and the detection output signal is not output. When the input power is between RFin1 and RFin2, the nonlinear distortion generated in the amplifier 1 and the IF amplifier 4 can be ignored. Therefore, the AGC function is stopped and a signal proportional to the RF input power is obtained at the RSSI signal output. When the input power is greater than or equal to RFin2, the AGC is functioned to prevent the occurrence of nonlinear distortion, and the RSSI signal output is set to a constant value with respect to the RF input power.

  RFin1 is set for a weak reception input, and the variable attenuator 2 is determined by the output of the RSSI circuit 11 at the time of the minimum attenuation, and the level of the reception signal strength indicator signal (RSSI signal output) is low. Since the transmission of the detection signal is interrupted by opening (OFF) the switch (switching means) 14, a reference potential is provided in the determiner, this reference potential is set as a threshold voltage, and the received signal strength indicator signal is lower than the threshold voltage. On the other hand, the transmission of the detection signal is interrupted. On the contrary, when the level of the received signal strength index signal is higher than the threshold voltage, the transmission of the detection signal is continued by closing (turning on) the switch 14.

  RFin2 is set to be equal to or less than a reception input at which nonlinear distortion generated in the amplifier 1 and the IF amplifier 4 occurs, and the level of the received signal strength index signal of the RSSI circuit 11 corresponding to the input is set. When the level of the received signal strength index signal is higher than the threshold voltage provided in the AGC circuit 12, the AGC circuit 12 applies a variable control voltage higher than the minimum control voltage to the variable attenuator 2 to increase or decrease the attenuation amount of the variable attenuator. The output of the amplifier 1 is adjusted so that the received signal strength index signal is constant.

  By setting the power levels of RF1n1 and RFin2 as described above, and comparing and controlling the output voltage of the RSSI circuit 11 with the threshold voltage with respect to the reference voltage provided in the AGC circuit 12 and the determiner 13 correspondingly, RF In a region where the input power level is low and the nonlinear distortion generated in the amplifier 1 or IF amplifier 4 can be ignored, if the required C / N ratio is not ensured depending on the reception situation, the IF signal is output from the RSSI signal output proportional to the RF input power. Further, by turning off the output of the detection signal, it is possible to prevent unnecessary transmission of the output for the weak signal.

  Further, in the region where the RF input power level is high and the nonlinear distortion generated in the amplifier 1 or the IF amplifier 4 is generated, the C / N ratio is secured, so that the AGC circuit 12 is operated to output the amplifier 1 or the IF amplifier 4. It is possible to cope with a wide range of received power levels by preventing nonlinear distortion waves due to power saturation.

  In the first embodiment, the AGC circuit 12 sets the minimum attenuation by giving the minimum control voltage to the variable attenuator 2. However, the AGC circuit 12 gives the maximum control voltage to the variable attenuator 2 and reduces this to the minimum attenuation. A variable attenuator may be used.

  In the first embodiment, the switching means 14 is provided on the output side of the phase comparator 10. However, since the switching means 14 is intended to turn off the detection output, the first signal line for phase comparison and the You may provide in the input stage of 2 signal lines.

It is a block block diagram of the FM detection circuit by Embodiment 1 of this invention. It is a characteristic view of input electric power and RSSI output voltage using an AGC circuit. It is a characteristic diagram of the control voltage and attenuation amount of the variable attenuator used for the FM detection circuit by Embodiment 1 of this invention. It is a characteristic diagram of the input input power and RSSI output voltage of the FM detection circuit by Embodiment 1 of this invention.

Explanation of symbols

1 .. RF amplifier (amplifier) 2. Variable amplifier 3 Down mixer (mixer)
4. IF amplifier 5. BPF (band pass filter) 6. IF buffer 7. Phase shifter 8. First amplitude limiting amplifier 9. Second amplitude limiting amplifier 10. Phase comparator 11.・ ・ RSSI circuit 12 ・ ・ AGC circuit 13 ・ ・ Determinator (Detection signal output ON / OFF determiner)
14..Switch (switching means)

Claims (1)

An amplifier that amplifies a high-frequency signal, a variable attenuator that varies the output of the amplifier, a mixer that down-converts the high-frequency signal that has passed through the variable attenuator to an intermediate frequency, the intermediate frequency is branched, and one branch output Through the phase shifter, the output of the first signal line passing through the first amplitude limiting amplifier and the second signal line passing through the second amplitude limiting amplifier are compared, and the first signal line And a phase comparator that outputs a detection signal proportional to the phase difference between the first signal line and the second signal line, and the one branch output transmission line excluding the first signal line and the second signal line. A switching means, an RSSI circuit for generating a received signal strength index signal from the other branch output of the intermediate frequency, an AGC circuit having a gain adjustment comparison function to which one output of the RSSI circuit is input, and the RSS A determination unit to which the other output of the circuit is input, and when the level of the received signal strength indicator signal is lower than a threshold voltage provided in the AGC circuit, the AGC circuit minimizes the attenuation of the variable attenuator. The detection signal by opening the switching means when the level of the received signal strength indicator signal is lower than the threshold voltage provided in the determination unit within the minimum value maintenance range of the variable attenuator. When the level of the received signal strength index signal is higher than the threshold voltage provided in the determiner, the transmission of the detection signal is continued by closing the switching means, and provided in the AGC circuit. When the level of the received signal strength indicator signal is higher than the threshold voltage, the AGC circuit increases or decreases the attenuation amount of the variable attenuator to keep the output level of the intermediate frequency constant. M detection circuit.

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