JP2011114792A - Frequency conversion circuit and receiving apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To remove image signals without fail, in a frequency conversion circuit that uses a sub-harmonic mixer. <P>SOLUTION: In a frequency conversion part 30, a phase shift circuit 31 converts a received signals, RF signals into signals RF1, RF2 having 90 degrees for the phase differences; mixers 34a, 34b multiplies (or combines) each of the signals RF1, RF2 by a locally-generated signal Lo generated by a locally-generated signal-generating section 32; LPF35a, 35b extract a signal for a difference frequency ¾F<SB>RF</SB>-F<SB>Lo</SB>×N¾ with Nth harmonic for the RF signal and locally-generated signal Lo; and a PPF36 removes the image signal to output as an IF signal of a desired frequency. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a frequency conversion circuit and a receiving apparatus including the frequency conversion circuit.

  In a superheterodyne wireless communication device, a mixer is used to multiply (synthesize) a received signal by a local oscillation signal (local signal) generated by a local oscillator inside the device, thereby generating an intermediate frequency signal (IF signal). Frequency conversion for conversion (down-conversion) is performed.

  For example, a polyphase filter as disclosed in Patent Document 1 is used to remove an image signal (a signal having a frequency symmetrical to the frequency of a reception desired signal centered on the frequency of the local oscillation signal) generated by performing this frequency conversion. . Specifically, by multiplying the received signal by two local oscillation signals Lo1 and Lo2 whose phases are different from each other by 90 degrees, it is converted into an intermediate frequency signal (IF signal) and has a phase difference of 90 degrees. The image signal is removed (cancelled) by converting to an orthogonal signal (I, Q signal) and passing the converted orthogonal signal through a polyphase filter.

  In order to remove the image signal satisfactorily, the phases of the local oscillation signals Lo1 and Lo2 need to be accurately shifted by 90 degrees. Actually, the phase difference between the local oscillation signals Lo1 and Lo2 may not be exactly 90 degrees due to variations in circuit elements of the phase shift circuit, but there is an allowable error θ of about ± 3 degrees, for example. The phase shift circuit may be configured to satisfy θ.

JP 2008-193363 A

  By the way, when performing frequency conversion to an IF signal, it is common to use a local oscillation signal Lo having a frequency separated from the frequency of the received signal by an intermediate frequency. However, as the received signal becomes higher in frequency, the local oscillation signal Lo becomes higher in frequency, which causes problems that the local oscillator becomes more expensive and consumes more power. Therefore, using a local oscillator having an oscillation frequency 1 / N of the required local oscillation signal Lo, a sub-harmonic that obtains a signal having a difference frequency between the frequency of the received signal and the Nth harmonic (Nth harmonic) of the local oscillation signal. A method using a mixer can be considered.

  However, in the subharmonic mixer, in order to extract a signal having a frequency difference between the frequency of the received signal and the harmonic component (N harmonic) of the local oscillation signal Lo, when performing image removal using the above-described polyphase filter, If N harmonics are used, the allowable phase error of the local oscillation signals Lo1 and Lo2 is θ / N. That is, the allowable error θ / N decreases as the harmonic order N increases. For example, in the case of a subharmonic mixer using the eleventh harmonic, an allowable phase error is θ / 11. If θ = ± 3 degrees, only a phase error within ± 0.3 degrees is allowed. . Generation of a phase shift circuit satisfying such an allowable error θ / N is very difficult and is not practical. Therefore, there is a need for a method for reliably removing an image signal in a frequency conversion circuit using a subharmonic mixer.

  The present invention has been made in view of the above circumstances, and an object thereof is to reliably remove an image signal in a frequency conversion circuit using a subharmonic mixer.

  A first form for solving the above-described problem is that a received signal of a high-frequency signal whose frequency is defined in advance is multiplied by a local oscillation signal, and the difference frequency between the harmonic of the local oscillation signal and the frequency of the high-frequency signal is calculated. A frequency conversion circuit for extracting a signal, the phase conversion unit for converting a phase of the reception signal to generate a pair of phase difference reception signals having a phase difference of 90 degrees, and a pair of the phase difference reception signals A mixer that multiplies a local oscillation signal, a filter that extracts a signal component of the difference frequency from the output signal of the mixer, and an image signal removal circuit that removes an image signal from the signal extracted by the filter Is a frequency conversion circuit.

  According to the first aspect, in the frequency conversion circuit that multiplies the reception signal of the high-frequency signal by the local oscillation signal and extracts a signal having a difference frequency between the harmonic of the local oscillation signal and the frequency of the high-frequency signal, the phase of the reception signal Each of the pair of phase difference reception signals having a phase difference of 90 degrees generated by converting the signal is multiplied by the local oscillation signal, the signal component of the difference frequency is extracted, and the image signal is removed.

  A frequency converter circuit (so-called sub-harmonic mixer) that extracts the signal of the difference frequency between the received signal and the harmonics of the local oscillation signal converts the phase of the local oscillation signal to generate two signals with a phase difference of 90 degrees. When the conventional method is adopted, the allowable error of the phase difference of the local oscillation signal is θ / N, which is influenced by the harmonic order N of the local oscillation signal. On the other hand, as in the first embodiment, the phase of the received signal is converted to generate a pair of phase difference received signals having a phase difference of 90 degrees, and a local oscillation signal is applied to each of the phase difference received signals. By multiplying, the allowable error θ of the phase difference of the phase difference reception signal generated by the phase conversion unit is not affected by the order N of the harmonics (Nth harmonic) of the local oscillation signal, so that reliable image signal removal is possible. Is realized.

  Further, as a second mode, phase conversion for generating a first reception signal and a second reception signal having a phase difference of 90 degrees is performed by converting the phase of a reception signal of a high-frequency signal having a predetermined frequency. A first mixer unit that multiplies the first reception signal by a local oscillation signal, a second mixer unit that multiplies the second reception signal by a local oscillation signal, the first mixer unit, A filter unit that extracts a signal component of a difference frequency between the harmonic frequency of the local oscillation signal and the frequency of the high-frequency signal from the output signal of the second mixer unit, and an image signal from the signal extracted by the filter unit You may comprise the frequency converter circuit provided with the image signal removal circuit part which removes.

  According to the second embodiment, the first received signal and the second received signal having a phase difference of 90 degrees are multiplied by the local oscillation signal, the signal component of the difference frequency is extracted, and the image signal is removed. The Thus, similarly to the frequency conversion circuit of the first embodiment, the allowable error θ of the phase difference of the phase difference reception signal generated by the phase conversion unit is set to the order N of the harmonic (Nth harmonic) of the local oscillation signal. Since it is not affected, reliable image signal removal is realized.

  As a third mode, there is provided a frequency conversion circuit according to the first or second mode, wherein the harmonic frequency is N times (N ≧ 3) the local oscillation signal. It may be configured.

  According to the third mode, in the frequency conversion circuit, the difference frequency between the harmonic (Nth harmonic) whose frequency is N times (N ≧ 3) the local oscillation signal and the frequency of the high-frequency signal that is the reception signal. Are extracted.

  Further, as a fourth mode, the frequency conversion circuit according to any one of the first to third modes, wherein the image signal removal circuit unit may constitute a frequency conversion circuit formed of a polyphase filter.

  Further, as a fifth mode, the frequency conversion circuit according to any one of the first to fourth modes, wherein the high-frequency signal may constitute a frequency conversion circuit that is a satellite signal transmitted from a positioning satellite. .

  Furthermore, as a sixth embodiment, a receiving device including the frequency conversion circuit of the fifth embodiment may be configured.

The block block diagram of a GPS receiver. The block block diagram of a frequency conversion part.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following, the case where the present invention is applied to a GPS receiver that receives a GPS signal (high frequency signal) transmitted from a GPS (Global Positioning System) satellite which is a kind of positioning satellite will be described. The applicable embodiment of the invention is not limited to this.

[Constitution]
FIG. 1 is a block configuration diagram of a GPS receiver 1 according to the present embodiment. As shown in FIG. 1, the GPS receiver 1 includes a GPS antenna 10, an RF (Radio Frequency) receiving circuit unit 20, and a baseband unit 40.

  The GPS antenna 10 receives an RF signal including a GPS satellite signal transmitted from a GPS satellite that is a kind of positioning satellite. The GPS satellite signal is a 1.57542 [GHz] communication signal directly modulated by a spread spectrum system with a PRN (Pseudo Random Noise) code, which is a kind of spreading code different for each GPS satellite. The PRN code is a pseudo random noise code having a repetition period of 1 ms with a code length of 1023 chips as one frame.

  The RF receiving circuit unit 20 includes a SAW (Surface Acoustic Wave) filter 21, an LNA (Low Noise Amplifier) 22, a frequency conversion unit (frequency conversion circuit) 30, an amplification unit 23, and an ADC (Analog to Digital Converter) 24. And have.

  The SAW filter 21 is a band-pass filter, and allows a signal in a predetermined band to pass through the RF signal received by the GPS antenna 10 and blocks out-of-band frequency components. The LNA 22 is a low noise amplifier and amplifies the RF signal output from the SAW filter 21.

Frequency converting unit 30, the RF signal outputted from the LNA 22, is multiplied by a local oscillation signal Lo frequency _{F Lo} of approximately 1 / N of the frequency _{F RF,} frequency the RF signal _| F RF _{-F Lo} × Sub-harmonic frequency conversion is performed for conversion to an N | intermediate frequency signal (IF signal). In this embodiment, N ≧ 3. However, by setting N ≧ 10, a local signal oscillator with lower power consumption can be obtained.

  The amplification unit 23 amplifies the IF signal output from the frequency conversion unit 30. The ADC 24 converts the IF signal that is an analog signal output from the amplifying unit 23 into a digital signal.

  The baseband unit 40 performs correlation processing on the IF signal output from the RF receiving circuit unit 20 to capture and extract GPS satellite signals, decodes the data to extract navigation messages and time information, and calculates pseudo distances. And positioning calculation.

  FIG. 2 is a block configuration diagram of the frequency conversion unit 30. As shown in FIG. 2, the frequency conversion unit 30 includes a phase shift circuit (phase conversion unit) 31, a local oscillation signal generation unit 32, mixers 34 a and 34 b (mixer unit), LPFs 35 a and 35 b, and a PPF (polypole). Phase filter) (image signal removal circuit) 36.

  The phase shift circuit 31 converts the phase of the input RF signal and generates a pair of phase difference reception signals (orthogonal signals) having a phase difference of 90 degrees. The phase shift circuit 31 is realized by, for example, an RC filter (HPF and LPF), and generates a signal RF1 in which the phase of the RF signal is advanced by 45 degrees and a signal RF2 in which the phase is delayed by 45 degrees, so that the phase difference is generated. Two signals RF1 and RF2 of 90 degrees are generated.

The local oscillation signal generation unit 32 includes an oscillator such as a VCO (Voltage Controlled Oscillator), and generates a local oscillation signal (local signal) Lo having a frequency F _{Lo that} is approximately 1 / N of the frequency F _RF of the received RF signal. To do. More specifically, a local oscillation signal Lo having a frequency F _Lo that satisfies | F _RF −F _Lo × N | = intermediate frequency is generated. Since the GPS receiver 1 receives only one wave called a GPS satellite signal, the local oscillation signal generator 32 may generate only one wave. For this reason, a highly accurate frequency can be generated with low power consumption.

The mixers 34a and 34b multiply (synthesize) the local oscillation signal Lo generated by the local oscillation signal generation unit 32 with respect to the signals RF1 and RF2 output from the phase shift circuit 31, respectively. The mixers 34a and 34b are realized by, for example, a Gilbert cell mixer, and the signals RF1 and RF2 output from the phase shift circuit 31 are converted into differential signals by the differential amplifier circuits 33a and 33b in the previous stage. Is input. That is, the mixer 34a is a differential signal of the signal RF1 output from the phase shift circuit 31 (original signal RF1 ₊ and an inverted signal RF1 _-), respectively, multiplied by the local oscillation signal Lo the (synthesis) signal (I signal) Generate. Further, the mixer 34b is a differential signal of the signal RF2 outputted from the phase shift circuit 31 (original signal RF2 ₊ and the inverted signal RF2 _-), respectively, multiplied by the local oscillation signal Lo the (synthesis) signal (Q signal) Generate.

Further, since the mixers 34a and 34b are used as sub-harmonic mixers, the signal output from each of the mixers 34a and 34b includes a difference frequency | F _RF −F _Lo between the RF signal and the Nth harmonic of the local oscillation signal _Lo. XN | signals are included.

The LPFs 35a and 35b each include a low frequency component including a difference frequency | F _RF −F _Lo × N | between the RF signal and the Nth harmonic of the local oscillation signal Lo with respect to the signals output from the mixers 34a and 34b. The band signal is passed and the frequency component outside the band is cut off. The LPFs 35a and 35b extract desired IF signals from the output signals of the mixers 34a and 34b. That is, the in-phase component signal (I signal) of the IF signal is extracted by the LPF 35a, and the quadrature component signal (Q signal) of the IF signal is extracted by the LPF 35b.

The PPF (polyphase filter) 36 removes the image signal from the IF signal output from the LPFs 35a and 35b. The PPF 36 is a four-phase RC filter, and four signals whose phases are shifted by 90 degrees are input. That is, the differential signals I ₊ and I ₋ of the I signal from the LPF 35a and the differential signals Q ₊ and Q ₋ of the Q signal from the LPF 35b are input.

  In the present embodiment, the harmonic of the local oscillation signal Lo used in the frequency converter 30 is preferably the third harmonic or higher (N ≧ 3), and the tenth harmonic or higher (N ≧ 10). More preferred. That is, the oscillation frequency of the oscillator (local oscillator) included in the local oscillation signal generation unit 32 becomes a lower frequency, and power consumption can be reduced.

  Moreover, this embodiment is a GPS receiver, and the frequency to receive is one type (1.57542 [GHz]). For this reason, one type of oscillation frequency is required for the local oscillator. In a receiving device that uses a subharmonic mixer, the harmonics of the local oscillation signal Lo are used, so the accuracy of the local oscillator is important. In a receiving apparatus that is premised on reception of a plurality of types of frequencies, a highly accurate oscillation frequency corresponding to each reception frequency is required. However, in the present embodiment, since it is sufficient to have one type of local oscillation frequency determined in advance, the configuration is simpler than that of a receiving device that receives a plurality of types of frequencies.

[Action / Effect]
Thus, according to the present embodiment, in the frequency converter 30 of the GPS receiver 1, the RF signal that is the received signal is converted into the signals RF1 and RF2 having a phase difference of 90 degrees by the phase shift circuit 31. . The mixers 34a and 34b multiply (synthesize) the signals RF1 and RF2 with the local oscillation signal Lo generated by the local oscillation signal generation unit 32, respectively. Next, a signal having a difference frequency | F _RF −F _Lo × N | between the RF signal and the Nth harmonic of the local oscillation signal Lo is extracted by the LPFs 35a and 35b. Further, the image signal is removed by the PPF 36 and output as an IF signal having a desired frequency.

  That is, as in the prior art, instead of the local oscillation signal Lo, the phase of the received RF signal is converted to generate signals RF1 and RF2 having a phase difference of 90 degrees, and the local oscillation signal is generated for each of the signals RF1 and RF2. Since the I and Q signals are separated by multiplication by Lo, the allowable error θ required for the phase shift circuit 31 is not affected by the harmonic order N of the local oscillation signal Lo, and is reliably obtained in the sub-harmonic mixer. Image removal is realized.

[Modification]
It should be noted that embodiments to which the present invention can be applied are not limited to the above-described embodiments, and can be appropriately changed without departing from the spirit of the present invention.

(A) Phase shift circuit 31
For example, in the above-described embodiment, the phase shift circuit 31 generates the signal RF1 obtained by advancing the phase of the RF signal by 45 degrees and the signal RF2 delayed by 45 degrees, whereby the signals RF1 and RF2 having a phase difference of 90 degrees. However, it is only necessary to generate two signals whose phases are different from each other by 90 degrees, for example, by generating a signal delayed by 90 degrees in phase.

(B) Mixers 34a and 34b
In the above-described embodiment, the mixers 34a and 34b are Gilbird cell mixers, but may be any sub-harmonic mixer such as a double balance type mixer.

(C) Receiving Device In the above-described embodiment, a GPS receiving device that receives GPS signals is used. However, the same applies to receiving devices that receive satellite signals in other satellite positioning systems such as GLONASS (GLObal Navigation Satellite System). It is applicable to.

  DESCRIPTION OF SYMBOLS 1 GPS receiver, 10 GPS antenna, 20 RF receiving circuit part, 21 SAW filter, 22 LNA, 23 Amplifying part, 24 ADC, 30 Frequency conversion part, 31 Phase shift circuit, 32 Local oscillation signal generation part, 33a, 33b Differential amplifier circuit, 34a, 34b mixer, 35a, 35b LPF, 36 PPF, 40 baseband section

Claims (6)

A frequency conversion circuit for extracting a signal having a frequency difference between a frequency of a harmonic of the local oscillation signal and a frequency of the high frequency signal by multiplying a reception signal of a high frequency signal having a predetermined frequency by a local oscillation signal,
A phase converter that converts a phase of the received signal to generate a pair of phase difference received signals having a phase difference of 90 degrees;
A mixer for multiplying the local oscillation signal by each of the pair of phase difference reception signals;
A filter unit for extracting the signal component of the difference frequency from the output signal of the mixer unit;
An image signal removing circuit unit for removing an image signal from the signal extracted by the filter unit;
A frequency conversion circuit comprising: A phase converter that generates a first reception signal and a second reception signal having a phase difference of 90 degrees by converting the phase of the reception signal of a high-frequency signal having a predetermined frequency;
A first mixer for multiplying the first received signal by a local oscillation signal;
A second mixer for multiplying the second received signal by a local oscillation signal;
A filter unit for extracting a signal component of a difference frequency between a frequency of a harmonic of the local oscillation signal and a frequency of the high-frequency signal from output signals of the first mixer unit and the second mixer unit;
An image signal removing circuit unit for removing an image signal from the signal extracted by the filter unit;
A frequency conversion circuit comprising:   The frequency conversion circuit according to claim 1, wherein a frequency of the harmonic is a frequency N times (N ≧ 3) the local oscillation signal.   The frequency conversion circuit according to claim 1, wherein the image signal removal circuit unit is a polyphase filter.   The frequency conversion circuit according to any one of claims 1 to 4, wherein the high-frequency signal is a satellite signal transmitted from a positioning satellite.   A receiving device comprising the frequency conversion circuit according to claim 5.

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