JPH0964649A - Frequency converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a frequency converter for removing an image signal by the use of a phase shifter constituted of an analog circuit with small and simple constitution. SOLUTION: A required signal and an image signal inputted to an input terminal are respectively frequency-converted in a mixer 22 by the use of a signal obtained by phase-shifting an output signal from a local oscillator 24 by +45 deg. through a +45 deg. phase shifter 25 and in an mixer 23 by the use of a signal obtained by phase-shifting the output signal of the oscillator 24 by -45 deg. through a -45 deg. phase shifter 26. An output signal from the mixer 22 is frequency-converted in an mixer 27 by the use of a signal obtained by phase- shifting an output signal from a local oscillator 29 by +45 deg. through a +45 deg. phase shifter 30, and an output signal from the mixer 23 is frequency-converted in a mixer 28 by the use of a signal obtained by phase-shifting the output signal of the oscillator 29 by -45 deg. through a -45 deg. phase shifter 31. Output signals from the mixers 27, 28 are mutually subtracted by a subtractor 32 to remove an image signal included in a required signal band. An unnecessary band signal is removed by a filter 33 and a required signal is outputted from an output terminal 34.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency converter used in a superheterodyne system or double superheterodyne system receiver.

[0002]

2. Description of the Related Art Conventionally, as an image rejection type frequency converter used in a super-heterodyne receiver, there is a circuit of Japanese Patent Laid-Open No. 6-343088 shown in FIG. 5, which will be described below.

In FIG. 5, the reception R modulated by BPSK is shown.
The F signal is down-converted into IF signals that are orthogonal to each other. This directly supplies the first local oscillation signal generated by the local oscillator 2 to the mixer 101,
This is achieved by inputting a signal shifted by 90 degrees to 02 through the 90-degree phase shifter 103. The IF signal output from the mixer 101 is input to the mixer 3 and mixed with a signal from the synchronous oscillator 8 that generates a second local oscillation signal having the same frequency and phase as the IF signal to form a baseband signal. On the other hand, the IF signal, which is the output of the mixer 102 and is shifted by 90 degrees, is input to the mixer 4, and the synchronous oscillator 8
The phase of the second local oscillation signal generated in
And is mixed with the signal shifted by 90 degrees to form a baseband signal. By adding the output signals of the mixer 3 and the mixer 4 by the adder 9 and passing them through the low-pass filter 11, the image component and unnecessary harmonic components can be removed, and the baseband signal I can be obtained. Further, the output signals of the mixer 5 and the mixer 6 are subtracted by the subtractor 10 and passed through the low-pass filter 12, whereby the image component and unnecessary harmonic components can be removed, and the baseband signal Q can be obtained.

However, in order to remove the image signal, the output signal of the local oscillator 2 and the 90-degree phase shifter 103 are used.
The output signal of the local oscillator 8 and the output signal of the 90-degree phase shifter 7 must have the same amplitude. If there is a difference in amplitude, the image signal may not be removed sufficiently. If the 90-degree phase shifter 103 and the 90-degree phase shifter 7 satisfying such conditions are to be realized by an analog circuit, a function such as amplitude adjustment is usually required, which causes the circuit to become large-scale and complicated. Was becoming.

[0005]

According to the above-mentioned conventional circuit, in order to remove the image signal, the amplitudes of the output signals of the respective local oscillators and the output signals of the 90-degree phase shifter are equal to each other. However, if there is a difference in amplitude, the image signal cannot be sufficiently removed. If an attempt is made to realize each 90-degree phase shifter satisfying such conditions with an analog circuit, amplitude adjustment or the like is usually required. Function is required, which causes the circuit to become large-scale and complicated.
It is an object of the present invention to provide a frequency converter that removes the image signal by using a phase shifter having an analog circuit of a small scale and a simple structure.

[0006]

In order to solve the above-mentioned problems, according to the present invention, a first local oscillator for generating a first local oscillation signal and the first local oscillation signal are phase-shifted by α degrees. A first α degree phase shifter, a first (α-90) degree phase shifter that shifts the first local oscillation signal by (α-90) degrees, a received signal and the first α degree. A first mixer for mixing the output signal of the phase shifter, the received signal and the first (α-
90) a second mixer for mixing the output signal of the phase shifter, a second local oscillator for generating a second local oscillation signal, and a second for phase shifting the second local oscillation signal by α degrees. [Alpha] degree phase shifter, a second ([alpha] -90) degree phase shifter that shifts the second local oscillation signal by ([alpha] -90) degrees, an output signal of the first mixer and the first [alpha] degree phase shifter. A third mixer for mixing the output signal of the second α-degree phase shifter, and a third mixer for mixing the output signal of the second mixer and the output signal of the second (α-90) -degree phase shifter. 4 mixer and a subtractor for subtracting the output signal of the third mixer and the output signal of the fourth mixer. As a result, even if a small-scale and simple analog circuit is used for the α-degree phase shifter or the (α-90) -degree phase shifter, sufficient performance of removing the image signal can be obtained.

[0007]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit configuration diagram for explaining one embodiment of the present invention. The desired signal and the image signal input to the input terminal 21 are output from the local oscillator 24 by +4 in the mixer 22.
The 5 ° phase shifter 25 performs frequency conversion using the signal whose phase is shifted by + 45 °, and the mixer 23 uses the local oscillator 2
The output signal of No. 4 is frequency-converted by the -45 degree phase shifter 26 using a signal whose phase is shifted by -45 degrees. The output signal of the mixer 22 is frequency-converted by using the signal output from the local oscillator 29 in the mixer 27 and the signal whose phase is shifted by +45 degrees in the +45 degree phase shifter 30, and the output signal of the mixer 23 is converted into the mixer 28. Then, the output signal of the local oscillator 29 is frequency-converted by using the signal whose phase is shifted by -45 degrees by the -45 degree phase shifter 31. The output signals of the mixers 27 and 28 are subtracted by using a subtractor 32, and the image signal within the desired signal band is removed at this time. Further, the filter 33 removes unnecessary band signals, and the output terminal 3
Output from 4.

In the +45 degree phase shifter 25 and the -45 degree phase shifter 26, since the signals to be handled are only local oscillation signals, the frequency of the local oscillation signal as shown in FIG. 2, for example, is cut off. A first-order high-pass filter or low-pass filter having a frequency may be used. In addition, Vcc is a power source, GND is a ground point, Q1 to Q4 are transistors, and R1 is
-R6 are resistors, C1-C4 are capacitors, V1-V2 are bias voltage sources, and I1-I4 are current sources.

Here, the frequency of the input local signal is f
If l, then R = R3 = R6, C = C2 = C4 and f
By setting a resistor R and a capacitor C such that l = 1 / (2πRC), a +45 degree phase shifter 25 and − at the frequency fl.
The output signal amplitudes of the 45-degree phase shifter 26 match.

FIG. 3 shows the spectrum of the signal at each point (a) to (f) in FIG. However, the signal in the unnecessary band removed by the filter 33 in the subsequent stage is not shown. The horizontal axis in FIG. 3 represents frequency, the vertical axis represents level, and + and − represent symbols in the calculation formula described later.

In FIG. 1, the desired signal and the image signal input to the input terminal 21, and the local oscillators 24 and 29.
Letting the respective output signals fif, fim, fl1, and fl2 of fif = sinωift fim = sinωimt fl1 = sinωl1t fl2 = sinωl2t, the output signal of the mixer 22 is fif2 = −1 / 2 · cos (ωift + ωl1t + 45 °). +1/2 ・ cos (ωift -ωl1t-45 °) (1) fim2 = -1 / 2 ・ cos (ωl1t + ωimt + 45 °) +1/2 ・ cos (ωl1t -ωimt + 45 °) (2) . In equations (1) and (2), the second term is the desired signal and the image signal within the desired signal band, respectively, and is shown in FIG. 3 (b). The output signal of the mixer 23 is fif3 = -1 / 2.cos (ωift + ωl1t-45 °) +1/2 ・ cos (ωift-ωl1t + 45 °) (3) fim3 = -1 / 2 ・ cos ( ωl1t + ωimt−45 °) + 1/2 · cos (ωl1t−ωimt−45 °) (4) The second term in the equations (3) and (4) is the desired signal and the image signal in the desired signal band, respectively, and is shown in FIG. 3 (d). Further, the output signal of the mixer 27 is fif7 = -1 / 4 · cos (ωift + ωl1t + ωl2t) + 1/4 · sin (ωift + ωl1t −ωl2t) + 1/4 · sin (ωift −ωl1t + ωl2t) +1/4 ・ cos (ωift -ωl1t -ωl2t) (5) fim7 = -1 / 4 ・ cos (ωl1t + ωimt + ωl2t) +1/4 ・ sin (ωl1t + ωimt -ωl2t) +1/4 ・cos (ωl1t −ωimt + ωl2t) −1 / 4 · sin (ωl1t −ωimt −ωl2t) (6) Of the expressions (5) and (6), the fourth term is the desired signal and the image signal in the desired signal band, respectively, and is shown in FIG. 3 (c). The output signal of the mixer 28 is fif8 = 1/4 · cos (ωift + ωl1t + ωl2t) + 1/4 · sin (ωift + ωl1t−ωl2t) + 1/4 · sin (ωift−ωl1t + ωl2t) -1 / 4 ・ cos (ωift -ωl1t -ωl2t) (7) fim8 = 1/4 ・ cos (ωl1t + ωimt + ωl2t) +1/4 ・ sin (ωl1t + ωimt -ωl2t) -1/4 ・ cos (ωl1t) -ωimt + ωl2t) -1 / 4 · sin (ωl1t-ωimt-ωl2t) (8) In equations (7) and (8), the fourth term is the desired signal and the image signal within the desired signal band, respectively, and is shown in FIG. 3 (e). Then, the output signal of the subtractor 32 is fif12 = fif7-fif8 = -1 / 2 * cos (ωift + ωl1t + ωl2t) + 1/2 * cos (ωift -ωl1t -ωl2t) (9) fim12 = fim7- fim8 = -1 / 2 * cos (ωl1t + ωimt + ωl2t) + 1/2 * cos (ωl1t-ωimt-ωl2t) (10) In the expressions (9) and (10), the desired signal is the second term of the expression (9), and the component including the image signal within the desired signal band is canceled. Although the first term and the equation (10) of the equation (9) are unnecessary signals, they can be removed by using the filter 33. For this filter 33,
The pass band is 0 to several MHz, and the cutoff band is tens of MHz
The above is sufficient, and for example, a low-order low-pass filter or the like may be used.

FIG. 4 is a circuit configuration diagram for explaining another embodiment of the present invention. This embodiment is different from the embodiment of FIG. 1 in that the +45 degree phase shifter 30 and the −45 degree phase shifter 31 are simply replaced with each other, and the subtractor 32 is an adder 35. This embodiment and the embodiment of FIG. 1 are equivalently the same circuit configuration, and the operation and effect are also the same, and the description thereof is omitted here.

In each of the above-described embodiments, the α-degree phase shifter and the (α-90) -degree phase shifter are described as + 45-degree phase shifter and -45-degree phase shifter, respectively. To (α
The +90) degree phase shifter may be used, and the (α-90) degree phase shifter may be used as the α degree phase shifter.

[0014]

As described above, according to the frequency converter of the present invention, the frequency conversion of the input signal is performed while removing the image signal by using the phase shifter having the analog circuit of the small scale and the simple structure. It can be performed.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram for explaining an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram for explaining an example of a +45 degree phase shifter and a −45 degree phase shifter of FIG.

FIG. 3 is an explanatory diagram for explaining a spectrum of a signal at each of points (a) to (f) in FIG.

FIG. 4 is a circuit configuration diagram for explaining another embodiment of the present invention.

FIG. 5 is an explanatory diagram for explaining a conventional image rejection type frequency converter.

[Explanation of symbols]

21 ... Input terminal, 22, 23, 27, 28 ... Mixer,
24, 29 ... Local oscillator, 25, 30 ... +45 degree phase shifter, 26, 31 ... -45 degree phase shifter, 32 ... Subtractor, 33
... filter, 34 ... output terminal, 35 ... adder.

Claims (5)

[Claims] 1. A first local oscillator that generates a first local oscillation signal, a first α-degree phase shifter that shifts the first local oscillation signal by α degrees, and the first local oscillation. First phase shift of signal by (α-90) degrees
(Α-90) degree phase shifter, a first mixer that mixes the received signal with the output signal of the first α degree phase shifter, the received signal and the first (α-90) degree A second mixer for mixing with the output signal of the phase shifter; a second local oscillator for generating a second local oscillation signal; and a second α degree for phase-shifting the second local oscillation signal by α degrees. A phase shifter and a second phase shifter for shifting the second local oscillation signal by (α-90) degrees.
(Α-90) degree phase shifter, a third mixer for mixing the output signal of the first mixer with the output signal of the second α degree phase shifter, and the output of the second mixer Signal and the second (α-9
A fourth mixer for mixing the output signal of the 0) degree phase shifter, and a subtractor for subtracting the output signal of the third mixer and the output signal of the fourth mixer. And a frequency converter. 2. A first local oscillator that generates a first local oscillation signal, a first α-degree phase shifter that shifts the first local oscillation signal by α degrees, and the first local oscillation. First phase shift of signal by (α-90) degrees
(Α-90) degree phase shifter, a first mixer that mixes the received signal with the output signal of the first α degree phase shifter, the received signal and the first (α-90) A second mixer that mixes with the output signal of the phase shifter, a second local oscillator that generates a second local oscillator signal, and a phase shift of the second local oscillator signal by (α-90) degrees. Second
(Α-90) degree phase shifter, a second α degree phase shifter that shifts the second local oscillation signal by α degrees, an output signal of the first mixer and the second (α -9
A third mixer that mixes the output signal of the 0) degree phase shifter, and a fourth mixer that mixes the output signal of the second mixer and the output signal of the second α degree phase shifter, A frequency converter comprising an adder for adding the output signal of the third mixer and the output signal of the fourth mixer. 3. The first and second α degree phase shifters are +4.
A phase shifter of 5 degrees is used, and the first and second (α-90)
3. The frequency converter according to claim 1, wherein the phase shifter is a -45 degree phase shifter. 4. The first and second + 45-degree phase shifters are high-pass filters, and the first and second −45-degree phase shifters are low-pass filters. The described frequency converter. 5. The first +45 degree phase shifter and the first −45 degree phase shifter are a first-order high-pass filter and a first-order low-pass filter having a common cut-off frequency, respectively. 2. The second +45 degree phase shifter and the second −45 degree phase shifter are a first-order high-pass filter and a first-order low-pass filter having a common cutoff frequency, respectively. 2. The frequency converter described in 2.