JP3067835B2 - Quadrature demodulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a demodulation circuit, and more particularly to a quadrature demodulation circuit for mixing two orthogonal frequencies.

[0002]

2. Description of the Related Art Conventionally, various detection circuits have been proposed. A quadrature demodulation circuit for selecting an oscillation frequency of a local oscillation circuit equal to a carrier frequency, setting the beat frequency to zero, and directly converting the signal to an audible signal has attracted attention. ing. This quadrature demodulation circuit has the advantage that the circuit configuration is simple and the number of adjustment points is small, so that reliability can be improved.

FIG. 6 is a principle diagram of a quadrature demodulation circuit. A carrier RF modulated by an audible signal input from an input terminal is input to mixers 3021 and 3022. The local oscillation wave LF having the same frequency as the carrier frequency oscillated by the local oscillator 301 is split into two, one of which is input to the 90 ° phase shifter 303 and is shifted by 90 ° in phase.
F '.

[0004] These two local oscillation waves are LF
021, LF ′ is directed to mixer 3022 and the carrier R
Mixed with F. Two mixers 3021 and 3022
Are output through a low-pass filter or band-pass filters 3041 and 3042, respectively.
50 is input.

After that, in the demodulation processing unit 350, for example, in the case of AM detection, two filters 3041 and 3042 are used.
An average value is calculated and an audible signal is output.

[0006]

However, in this quadrature demodulation circuit, if the two local oscillation waves do not have orthogonality, or if the characteristics of a filter installed at the subsequent stage of the mixer are misaligned, the audible signal is reduced. This results in distortion. Now, the signal of the carrier wave RF is represented by the following equation.

[0007]

(Equation 1)

The oscillation wave LF oscillated by the local oscillator 301 is represented by the following equation.

[0009]

(Equation 2)

Then, the output LF ′ of the 90 ° phase shifter 303 is expressed by the following equation.

[0011]

(Equation 3)

Here, Δθ ₀ indicates a phase error generated in the 90 ° phase shifter 303. Then, the output of the mixer 3021 and the output of 3022 are expressed by the following equations.

[0013]

(Equation 4)

[0014]

(Equation 5)

The mean square of the equations (4) and (5) calculated by the demodulation processing unit 350 is as follows.

[0016]

(Equation 6)

Therefore,

[0018]

(Equation 7)

If so, the audible signal S contains distortion. SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and has as its object to provide a quadrature demodulation circuit capable of removing distortion by correcting a phase difference between outputs of two filters.

[0020]

FIG. 1 is a diagram showing the basic configuration of the present invention, in which a local oscillator for transmitting a local oscillation wave having the same frequency as a carrier wave or the same frequency as a converted signal obtained by frequency-converting a carrier wave. Circuit 101 and a first mixer 1021 for mixing a first output and an input signal of local oscillation circuit 101
A phase shifter 103 for shifting the phase of the second output of the local oscillation circuit 101 by 90 °, a second mixer 1022 for mixing the output of the phase shifter 103 and an input signal, and a first mixer 1021 And a second filter 101 that performs a low-pass or band-pass filtering process on the output of the second mixer 1022.
42, a demodulation processing unit 105 that inputs the output of the first filter 1041 and the output of the second filter 1042 and executes demodulation processing, the output of the first filter 1041 and the output of the second filter 1042, Phase difference detecting means 106 for detecting the phase difference between the first filter 1041 and the output of the second filter 1042 based on the phase difference detected by the phase difference detecting means 106. A first phase difference correction unit 107 for adding the phase difference correction signal, and a correction output of the first phase difference correction unit 107 based on the phase difference detected by the phase difference detection unit 106. 2 phase difference correction means 108;
Consists of

According to the second aspect of the invention, the local oscillation circuit is controlled based on the outputs of the first filter 1041 and the second filter 1042 in order to set the oscillation frequency of the local oscillation circuit to a frequency shifted from the frequency of the carrier by a predetermined amount. An offset frequency control means 109 for controlling the oscillation frequency of the oscillation circuit is included.

In the third invention, the signal processing means 10
5 is characterized in that an AM demodulated output is taken out. 4th
The present invention is characterized in that the FM demodulated output is extracted from the offset frequency control means 109. FIG. 2 is a block diagram of the fifth aspect of the present invention.
Squaring means 1 for squaring the output of filter 1041
0511 and the second filter 1042 with the phase difference corrected
A second squaring means 10512 for squaring the output of the first squaring means, and a first adding means 1052 for adding the outputs of the first squaring means 10511 and the second squaring means 10512. The detecting means 106 determines whether the first filter 1041
A multiplication unit 1061 for multiplying the output of the second filter 1042 by the output of the second filter 1042, a first filtering unit 1062 for filtering the output of the multiplication unit 1061,
And a coefficient multiplying unit 1063 for multiplying the output of the first filtering unit 1062 by a predetermined factor. The first phase correction unit 107 filters the output of the first adding unit 1052 in the demodulation processing unit 105. Second filtering means 1071 and phase difference detecting means 106
The output of the coefficient multiplying means 1063 is used as the dividend and the second
A first division unit 1072 for performing division using the output of the filtering unit 1071 as a divisor, a second multiplication unit 1073 for multiplying the output of the first filter 1041 by the output of the first division unit 1074, Multiplying means of 2 10
A second adder 1074 for adding the output of the first filter 73 to the output of the second filter 1042, and a second phase corrector 108 is provided for the first divider 1072 of the first phase corrector 107. Third squaring means 10 for squaring the output
81, a subtracting means 1082 for subtracting the output of the third squaring means 1081 from the reference value as a subtracted number, and a subtracting means 1
The square root extraction means 1083 for square rooting the output of the signal generator 822 and the second output of the first phase compensator 107 for the divisor
And a second divider 1084 that uses the output of the adder 1075 as the dividend.

[0023]

In the first invention, the phase difference between the output of the first filter and the output of the second filter is detected, and the first phase difference is compensated so that the phase difference becomes substantially zero. In addition, a second phase difference compensation is performed to make the phase difference closer to zero more accurately.

In the second invention, feedback control is performed so that the oscillation frequency of the local oscillation circuit has a constant offset.

[0025]

FIG. 3 is a block diagram of an embodiment of the present invention, in which a local oscillation wave LF oscillated by a local oscillation circuit 301 is branched into two, one of which has a phase of 90 ° by a 90 ° phase shifter 303.
Is shifted and becomes the second local oscillation wave LF '. The first local oscillation wave LF is applied to a carrier RF
And the second local oscillation wave LF ′ is mixed with the second mixer 3
At 022, it is mixed with the carrier RF.

The outputs of the first mixer 3021 and the second mixer 3022 are filtered by a first filter 3041 and a second filter 3042, respectively, and then, for example, a digital signal processor (DS)
P) is input to the demodulation processing unit 310. First
In the present invention, the demodulation processing means (105), the phase difference detection means (106), and the first phase difference compensation means (107) shown in FIG.

That is, the output of the first filter 3041 and the second
The output of the filter 4042 and the first multiplication means 1061
, The output is:

[0028]

(Equation 8)

The cutoff frequency fc is determined by the phase angle (2
( ₁ ) When the first filtering means 1062 having a low-pass characteristic and having a frequency sufficiently lower than the frequency corresponding to (θ _i -Δθ ₀ ) performs filtering, the output becomes the following equation.

[0030]

(Equation 9)

The output of the second filter 3042 is subjected to phase difference correction to remove the term Δθ ₀ , and this signal is represented by the following equation.

[0032]

(Equation 10)

The first filter 3041 and the corrected second
And the output of the filter 3042 of the
11 and 10512, and the first addition means 105
Adding by 2 gives the following equation.

[0034]

[Equation 11]

When this is filtered by the second filtering means 1071 having a sufficiently low cut-off frequency (for example, the same fc as the first filtering means 1062), the following equation is obtained.

[0036]

(Equation 12)

Therefore, the first filtering means 1062
Is multiplied by (−2) by the coefficient multiplying means 1063 and the output of the second filtering means 1071 is divided by the first dividing means 1072 as the divisor, whereby the first and second filters 3041 and 3042 The following equation is obtained as information on the phase difference between outputs.

[0038]

(Equation 13)

The first filter 304 is provided by the multiplication means 1073.
1 and multiplying the result by the second adding means 107
In step 4, the output of the second filter 3042 is added. Therefore, the output of the second adding means 1074 is as follows.

[0040]

[Equation 14]

Therefore,

[0042]

(Equation 15)

If it can be regarded as 90 °
If the phase shift error of the phase shifter 301 is small,

[0044]

(Equation 16)

The distortion can be improved. The second invention aims at compensating for the error existing in the approximation of the equation (15). That is, the third square means 10
At 81, the output of the first division means 1072 is squared, and the subtraction means 1082 subtracts it from the reference value "1".
The square root process is represented by the following equation.

[0046]

[Equation 17]

Therefore, the second addition means 1074 is calculated using this value.
Is divided, the following equation is established, and the phase difference is completely compensated.

[0048]

(Equation 18)

FIG. 4 is a vector diagram of the above description.
Assuming that the phase difference between the output of the first filter and the output of the second filter, which should be orthogonal to each other, is (90 ° + Δθ ₀ ), they are corrected so as to be orthogonal to each other by the first phase difference correction. This shows that the gain is corrected by the second phase difference correction. In the quadrature demodulation circuit according to the present invention, if the carrier frequency and the local oscillation frequency are completely matched, a DC component is included in the demodulated signal, and it is necessary to convert the circuit to DC.

However, the use of a DC circuit not only complicates the circuit configuration but also raises a problem of stability.
On the other hand, if a capacitor is inserted into the circuit for removing the DC component, a part of the information is also removed, and it becomes impossible to correctly demodulate. In order to solve this problem, the present applicant has proposed a demodulation circuit that controls the oscillation frequency of the local oscillation circuit to have a predetermined offset with respect to the carrier wave frequency and obtains a demodulated signal that does not include a DC component. (Japanese Patent Application No. 2-
271169).

FIG. 5 is a functional block diagram of the offset frequency control means 109 proposed by the present applicant, which comprises frequency discrimination means 1091 and local oscillation frequency control means 1092. That is, the addition of the offset frequency control means 109 to the first or second invention not only eliminates the necessity of converting the circuit into a direct current, but also allows the first and second inventions to track the variation of the carrier frequency.
Even if the cutoff frequency of the filtering means 1061 and 1071 is fixed, the average value is accurately calculated, and the distortion can be further improved.

As apparent from the equation (11), the first
, An audible signal for AM reception can be extracted. The output of the frequency discriminating means 1091 is F
It can be used as an audible signal for M reception. Further, the present invention has the same effect in a quadrature demodulation circuit in which a signal after frequency conversion of a carrier frequency is used as an input signal.
In this case, a frequency converter is provided at the input end of the carrier RF in the embodiment of FIG. 3 so that the frequency of the local oscillation wave oscillated from the local oscillation circuit 301 becomes the same value as the frequency converted by the frequency converter. Is set.

[0053]

According to the present invention, in the quadrature demodulation circuit, 9
By correcting the difference between the characteristics of the 0 ° phase shifter and the two filters, an audible signal without distortion can be obtained. Further, by controlling the offset frequency, the calculation accuracy of the average value by the filtering means is improved, and the distortion can be further reduced.

[Brief description of the drawings]

FIG. 1 is a basic configuration diagram of a quadrature demodulation circuit according to the present invention.

FIG. 2 is a functional diagram of a quadrature demodulation circuit according to the present invention.

FIG. 3 is a hardware configuration diagram of a quadrature demodulation circuit according to the present invention.

FIG. 4 is a vector diagram for explaining an effect of the present invention.

FIG. 5 is a functional diagram of an offset frequency control circuit.

FIG. 6 is a configuration diagram of a conventional quadrature demodulation circuit.

[Explanation of symbols]

 101: Local oscillation circuit 1021, 1022: Mixer 103: 90 ° phase shifter 1041, 1042: Filter 105: Demodulation processing means 106: Phase difference detection means 107: First phase difference correction means 108: Second phase difference correction Means 109: Offset frequency control means

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H03D 1/00 H03D 1/22 H03D 3/06 H04L 27/14

Claims (5)

(57) [Claims] 1. A local oscillation circuit (101) for transmitting a local oscillation wave having the same frequency as a carrier wave or the same frequency as a signal obtained by frequency-converting a carrier wave, and a first oscillator of the local oscillation circuit (101). A first mixer (1021) for mixing an output and an input signal, and a second output of the local oscillator circuit (101) having a phase of 90 °
A phase shifter (103) for deviating, and a second mixing unit for mixing an output of the phase shifter (103) and an input signal.
, A first filter (1041) that performs low-pass or band-pass filtering on the output of the first mixer (1021), and an output of the second mixer (1022) A second filter (1042) that performs a low-pass or band-pass filtering process on the data, and outputs the output of the first filter (1041) and the output of the second filter (1042) to execute demodulation processing A quadrature demodulation circuit comprising: a demodulation processing means (105) for detecting a phase difference between an output of the first filter (1041) and an output of the second filter (1042). ) And the output of the first filter (1041) or the output of the second filter (1042) based on the phase difference detected by the phase difference detection means (106). While in any of the output,
A first phase difference correction unit (107) for adding a phase correction signal based on the other, and a phase difference detection unit based on the phase difference detected by the phase difference detection unit (106).
The correction output of the first phase difference correcting means (107)
Second phase difference correcting means for further correcting the force (108)
And a quadrature demodulation circuit characterized by including: 2. The output of the first filter (1041) and the output of the second filter (1042) in order to set the oscillation frequency of the local oscillation circuit (101) to a frequency shifted from the frequency of a carrier by a predetermined amount. The quadrature demodulation circuit according to claim 1 , further comprising an offset frequency control means (109) for controlling an oscillation frequency of the local oscillation circuit (101) based on the offset frequency control means (109). 3. A quadrature demodulation circuit according to claim 1 or 2 to eject the AM demodulated output from said signal processing means (105). 4. The offset frequency control means (10)
Quadrature demodulation circuit described from 9) to claim 1 or 2 to eject the FM demodulation output. 5. The demodulation processing means (105): a first squaring means (10511) for squaring the output of the first filter (1041); and a phase difference corrected second filter (105). 1042) a second squaring means (10512) for squaring the output, and a first addition for adding outputs of the first squaring means (10511) and the second squaring means (10512). Means (1
052), wherein the phase difference detecting means (106) outputs the output of the first filter (1041) and the second filter (1042).
Multiplication means (1061) for multiplying the output of the first filter means, first filtering means (1062) for filtering the output of the multiplication means (1061), and multiplying the output of the first filtering means (1062) by a predetermined factor The first phase correction means (107) filters the output of the first addition means (1052) in the demodulation processing means (105). Filtering means (1
071), and first division means for executing division with the output of the coefficient multiplication means (1063) in the phase difference detection means (106) as a dividend and the output of the second filtering means (1071) as a divisor. (1072); second multiplication means (1073) for multiplying the output of the first filter (1041) by the output of the first division means (1074); and the second multiplication means (1073) The second adding means (1) adds the output of the second filter (1042) to the output of the second filter (1042).
074), wherein the second phase correction means (108) squares the output of the first division means (1072) of the first phase correction means (107). Means (1081); subtraction means (1082) for subtracting an output of the third squaring means (1081) from a reference value as a subtracted number; and square rooting means (1) for square rooting the output of the subtraction means (1082).
083), a second dividing means (1084) using the output of the square root means (1083) as a divisor and the output of the second adding means (1075) of the first phase compensating means (107) as a dividend. straight <br/>交復regulating circuit according to claim 1 or 2 shall be the characterized in that they are composed of.