JPH0511521U - Quadrature demodulation circuit - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to a quadrature demodulation circuit, in which an appropriate AGC is applied even when correcting the absolute value levels of two signals which are orthogonal to each other before deriving the root mean square. The purpose is to realize a demodulation circuit. In a quadrature demodulation circuit that obtains a demodulated signal by deriving the root mean square of two orthogonal signals, the quadrature demodulation circuit is provided in a stage before deriving the root mean square, and is corrected so that the absolute value levels of the two quadrature signals match. Level matching correction means, and automatic gain control means for controlling so that the intensities of the two signals orthogonal to each other at a stage before the level matching correction means fall within a predetermined range.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a quadrature demodulation circuit used in a direct detection type audio wireless receiver such as an AM radio.

[0002]

[Prior Art]

 In voice radio receivers such as AM radio, a direct conversion reception system using a quadrature demodulation circuit has been used instead of the conventional super-heterodyne system. The direct conversion reception method has the advantage that the circuit configuration can be simplified and high reliability can be obtained. In the quadrature demodulation circuit, a demodulated signal is obtained by deriving the root mean square of two signals that are orthogonal to each other. The applicant of the present invention has proposed a quadrature demodulation circuit having a simple structure and high reliability in JP-A-61-273005.

In a voice radio receiver such as an AM radio, the intensity of the received radio wave is not constant depending on the receiving position and the like, so that if the demodulated signal is output as it is, a stable volume cannot be obtained. Therefore, a method called automatic gain control (hereinafter referred to as AGC) is used to automatically keep the signal strength constant so that a constant volume can be obtained regardless of the strength of the received radio wave. FIG. 3 shows an example of the configuration of a quadrature demodulation circuit having an AGC function. In the drawings, the same functional parts are designated by the same reference numerals, which are sequentially denoted by lowercase letters.

As shown in FIG. 3, in the received radio wave from the antenna, only the frequency component corresponding to the carrier wave is extracted by the band filter 13c and amplified by the RF amplifier 81c. The RF amplifier 81c is a variable gain amplifier and adjusts the signal strength for AGC. The oscillator 11c generates a reference wave, and the phase shifter 12c becomes another reference wave having a 90 ° phase difference. The two reference waves are mixed with the received signal by mixers 2c and 3c to generate two orthogonal signals. From these quadrature signals, low-frequency components containing no DC component are extracted by the BPFs 4c and 5c and input to the arithmetic circuit 6c. In the arithmetic circuit 6c, the root mean square is derived from the arithmetic units 61c to 64c and becomes a demodulated signal.

Normally, when applying AGC, the demodulated signal is processed by the AGC controller 8c as shown in FIG. 3 and then fed back to the RF amplifier 81c to obtain a constant demodulated output. In the circuit of FIG. 3, the dynamic range of each component is set to be optimum when a signal of a predetermined intensity is output from the RF amplifier 81c, and when AGC is not applied, a constant volume is maintained. Not only can it not be obtained, but the problem arises that the signal is distorted.

When a quadrature demodulation circuit obtains a demodulated signal by square-rooting two orthogonal signals, it is necessary that the two orthogonal signals be exactly orthogonal and have equal absolute value levels. If these conditions are not met, the demodulated signal will be distorted. There are some factors such as the error of the 90 ° phase shifter 12c and the difference in the characteristics of the BPFs 4c and 5c that cause the two signals that are orthogonal to each other not to be exactly orthogonal and have the same absolute value level. . Therefore, before deriving the root mean square, two orthogonal signals are accurately orthogonalized and corrected so that they have equal absolute value levels. FIG. 4 is a diagram showing a configuration of a conventional example in which the absolute value levels of two orthogonal signals are matched before deriving the root mean square.

FIG. 4 shows an example in which the root mean square derivation and the above correction are performed by digital processing using a digital signal processor (hereinafter referred to as DSP). As shown in the figure, the absolute values of two orthogonal signals are calculated, and only the low frequency component is extracted. Since this is an absolute value level, the ratio is calculated, and amplification is performed by that ratio to make them match. The same applies to the phase correction, and the applicant of the present application discloses a quadrature demodulation circuit that performs the phase correction by digital processing in Japanese Patent Application No. 3-113118. Since absolute value level changes when phase correction is performed, it is desirable to correct absolute value level when performing phase correction.

[0008]

[Problems to be solved by the device]

 As described above, the quadrature demodulation circuit provided with absolute value level correction means for performing correction so as to match the absolute value levels of two signals orthogonal to each other prior to the step of calculating the root mean square, demodulation as shown in FIG. Consider the case where a signal is detected and AGC is applied. In this case, a signal having a constant intensity is obtained as the demodulated signal, but the demodulated signal includes the result of the correction of the absolute value level. Therefore, it cannot be said that the levels of the demodulated signal and the input signal completely correspond, and when AGC is applied based on the demodulated signal, the correction amount of the absolute value level has an influence.

As described above, the AGC not only serves to obtain a demodulated signal of constant intensity, but also serves to ensure that the dynamic range of each constituent element of the circuit is within an appropriate range with respect to the input signal. Especially when the input signal exceeds the dynamic range, the signal is immediately distorted. If AGC is applied by a demodulated signal whose absolute value level has been corrected in the middle, there is a problem that an appropriate AGC does not occur and an appropriate input signal level cannot be secured for the dynamic range of the constituent elements. Occur.

The present invention has been made in view of the above problems, and is suitable for AGC even when performing absolute value level correction for correcting the absolute value levels of two signals that directly intersect each other before deriving the root mean square. It is an object of the present invention to realize a quadrature demodulation circuit that can be multiplied.

[0011]

[Means for Solving the Problems]

 In order to solve the above problems, the quadrature demodulation circuit of the present invention applies AGC so that the signal strength falls within a predetermined range before the absolute value level correction. That is, the quadrature demodulation circuit of the present invention mixes reference waves having different 90 ° phases with the received wave, respectively, extracts only the frequency components necessary for demodulation of the obtained two orthogonal signals, and then calculates the root mean square. In a quadrature demodulation circuit for deriving and obtaining a demodulation signal, a level coincidence correction unit and a level coincidence correction unit, which are provided at a stage before deriving a root mean square and correct the absolute value levels of two orthogonal signals In a step before the step of (1), an automatic gain control means is provided for controlling the strengths of the two signals which are orthogonal to each other so as to fall within a predetermined range.

[0012]

[Action]

 Since the AGC is applied at the stage before the correction so that the absolute value levels of the two orthogonal signals are made to coincide with each other, an appropriate AGC can be applied without being affected by the correction of making the absolute value levels coincide.

[0013]

【Example】

 In the quadrature demodulation circuit that derives the root mean square, the process of deriving the root mean square, the above-described correction of the absolute value level and the correction of the phase difference require complicated correction, and therefore the circuit is not realized by the analog circuit. There is a problem of getting bigger. Therefore, it is desirable from the standpoint of reliability and cost to perform the above-mentioned processing by digital processing using a DSP. Therefore, an embodiment will be described in which AGC is applied to a circuit for performing root mean square and correction by digital processing.

FIG. 1 is a diagram showing the configuration of this embodiment. Although description of the portions common to those shown in FIGS. 3 and 4 is omitted, variable gain amplifiers 81a, 82a and 83a are provided to change the signal strength by the AGC. The level correction section 71a is a section for performing correction so as to match the absolute value levels of the two orthogonal signals shown in FIG. 4, and the phase correction section 72a is a circuit as disclosed in Japanese Patent Application No. 3-113118. Is an example.

In FIG. 1, reference numeral 8a denotes an AGC control unit, which detects the intensities of two digitally-converted orthogonal signals and feeds them back to the amplifiers 81a, 82a, and 83a so as to apply AGC. The processing of the AGC controller 8a is performed by digital processing, and after being converted into an analog signal, it is fed back to each amplifier. FIG. 2 is a diagram showing a circuit configuration of the AGC control section 8a, which is a widely known circuit. Two orthogonal signals are independently subjected to AGC, and feedback is applied to the first amplifier 81a based on the combined signal of the two orthogonal signals.

In this embodiment, as shown in FIGS. 1 and 2, the AGC includes not only the first RF amplifier 81a but also the amplifier 82a in front of the two orthogonal A / D converters 91a and 92a. You can also call 83a. In the A / D converters 91a and 92a, the relationship between the input signal and the dynamic range is particularly strict. When the input signal exceeds the reference voltage of the A / D converter, it becomes a saturated digital signal, and the input signal becomes This is because if it is lower than the reference voltage, the digital signal will be lost. However, if the characteristics of the LPF or the like have sufficient accuracy, it is not necessary to apply AGC in the amplifiers 82a and 83a, and this portion may be omitted.

As described above, when the root mean square is derived or corrected by digital processing, there is a problem that precision is lowered due to digit cancellation during conversion from an analog signal to a digital signal and during other calculations. , AGC, it is particularly important to make the signal strength an appropriate value for the dynamic range.

[0018]

[Effect of the device]

 Even when the present invention corrects the absolute value levels of two signals that are orthogonal to each other to obtain a demodulated signal without distortion, it is possible to apply an appropriate AGC, and a better demodulated signal can be obtained. A quadrature demodulation circuit can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a diagram showing an example of the AGC circuit of FIG.

FIG. 3 is a diagram showing a configuration of a conventional quadrature demodulation circuit having an AGC function.

FIG. 4 is a diagram showing a conventional example in which absolute values of two orthogonal signals are corrected by digital processing so as to match each other.

[Explanation of symbols]

 2a, 3a ... Mixer 4a, 5a ... Bandpass filter 8a ... AGC control section 11a ... Oscillator 12a ... 90 ° phase shifter 71a ... Level correction section 72a ... Phase correction section 81a ... RF variable amplifier 82a, 83a ... Variable amplifier 84a , 85a, 86a ... D / A converter 91a, 92a ... A / D converter

Claims (1)

[Claims for utility model registration] [Claim 1] Mixing reference waves with different 90 ° phases into the received waves, extracting only the frequency components necessary for demodulation of the two orthogonal signals obtained, and then taking the root mean square In a quadrature demodulation circuit for deriving a demodulated signal by deriving the quadrature signal and a level coincidence correction means for correcting the absolute value levels of the two orthogonal signals so as to coincide with each other, and the level. A quadrature demodulation circuit comprising automatic gain control means for controlling the intensities of the two orthogonal signals to fall within a predetermined range at a stage before the coincidence correction means.