Frequency converting device, television device and frequency converting method

According to an embodiment, a frequency converting device is provided with a duty adjusting unit that generates a 1/N local signal, which is a local signal with a duty ratio of 1/N, when N is an integral number not smaller than 3 and an N-th high-frequency component included in the local signal is a target of inhibition. Further, this is provided with a mixer that outputs difference or sum between/of the 1/N local signal and an input signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2010-191153, filed on Aug. 27, 2010; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments generally relate to a frequency converting device, a television device, and a frequency converting method.

BACKGROUND

A mixer circuit used for a tuner integrated circuit (IC) for television (TV) and the like that frequency-converts an input signal is generally provided with an input signal port, an output signal port, and a local signal port. The mixer circuit outputs a signal having a frequency, which is difference or sum between/of a frequency (input frequency) of a signal (for example, a received TV broadcast signal) input from the input signal port and a frequency (local frequency) of a local signal input from the local signal port from the output port.

In general, the local signal includes not only a desired frequency (local frequency) but also a harmonic component. In the above-described mixer circuit, the input signal also couples to the harmonic. Therefore, the output signal also includes a signal in which the input signal couples to the harmonic to be frequency-converted.

When it is required to equally receive a frequency signal of a wideband as when receiving the TV broadcast, in the mixer circuit, it is required that the mixer does not respond to the harmonic component of the local signal.

A reason for this is hereinafter described by a specific example. For example, when a received frequency range is from 45 MHz to 1000 MHz, an electric wave of this wide frequency range is equally input to the mixer circuit. At that time, when a direct conversion system in which the input frequency and the local frequency are equal to each other and the output signal is a base band signal is used, the mixer circuit frequency-converts the input signal of which input frequency is 100 MHz using the local signal of 100 MHZ to output as the base band signal centered around 0 Hz. However, since the frequency range of the input signal is wide, an interfering wave of 300 MHz three times as high as the local frequency of 100 MHz is also input to the mixer circuit as the input signal, for example, to be coupled to a third harmonic (300 MHz) of the local frequency and is frequency-converted to a frequency band around 0 Hz as a desired base band signal. As a result, poor reception occurs.

DETAILED DESCRIPTION

According to an embodiment, a frequency converting device is provided with a duty adjusting unit that generates a 1/N local signal, which is the local signal with a duty ratio of 1/N, when N is an integral number not smaller than 3 and an N-th high-frequency component included in the local signal is a target of inhibition. Further, this is provided with a mixer that outputs difference or sum between/of the 1/N local signal and an input signal.

The frequency converting device, a television device, and a frequency converting method according to the embodiment are hereinafter described in detail with reference to the attached drawings. Meanwhile, the present invention is not limited by the embodiments.

First Embodiment

FIG. 1is a view of a functional configuration example of the frequency converting device according to a first embodiment. As illustrated inFIG. 1, the frequency converting device of this embodiment is composed of an oscillator31that generates the local signal having a predetermined local frequency, a duty adjusting circuit32that adjust a duty of the local signal, and a mixer33that frequency-converts the input signal to a base band signal using the local signal.

In a conventional frequency converting device, the local signal is generated as the signal with the duty of ½, in general. That is to say, when one period of the local signal is set to T, a pulse width of the local signal is T/2. In this case, a local signal f(t) may be expanded using Fourier series expansion as expressed by a following equation (1). Meanwhile, ω0represents an angular speed (ω0=2π/T) corresponding to the local frequency and t represents a time.

Second and third terms of the equation (1) represent third and fifth harmonics (waves with frequencies three times and five times as high as the local frequency, respectively). In the conventional frequency converting device, when there is a harmonic component of the local frequency as represented by the second and third terms of the equation (1), a signal generated by coupling of the harmonic component and the input signal (a signal obtained by frequency-converting the input signal by the harmonic component) is included in an output signal after frequency conversion. The signal generated by the coupling of the harmonic component and the input signal (hereinafter, referred to as a high-order output signal) is present in a region of a desired base band signal when the input signal has a frequency component about the same as the harmonic component, and this causes deterioration in receiving sensitivity.

In this embodiment, the duty adjusting circuit32adjusts the duty of the local signal to 1/N (N is an integral number not smaller than 3), thereby decreasing the high-order output signal generated by the harmonic component with a frequency N times as high as the local frequency included in the local signal (hereinafter, referred to as an N-th local signal). In other words, in this embodiment, sensitivity of the frequency converting device to an N-th local signal is suppressed.

For example, when an effect by a third local signal is suppressed, the duty adjusting circuit32sets the duty of the local signal to ⅓. At that time, the local signal f(t) may be expressed by a following equation (2).

As is clear from the equation (2), there is not a frequency component three times as high as the local frequency (third harmonic component: a term including 3ω0t) in the local signal. Therefore, when using the local signal with the duty of ⅓, the effect by the third local signal (the high-order output signal by the coupling of the third local signal and the input signal) may be suppressed.

Next, by generalizing this, it is described that the local signal with the duty of 1/N does not include a component corresponding to the N-th harmonic. A periodic function f(t) of the period T may be expanded using the Fourier series expansion as expressed by a following equation (3).

Coefficients anand bnin the above-described equation (3) represent the coefficients of the n-th harmonic. Herein, when f(t) is a waveform with the duty of 1/N, f(t) may be expressed by a following equation (4) in a section from −T/2 to T/2.

A following equation (5) is obtained by the equations (3) and (4).

Therefore, when n=N in the equation (5), both of anand bnare 0. As described above, the local signal with the duty of 1/N does not include the component corresponding to the N-th harmonic. A result obtained by confirming this effect by simulation is hereinafter described.

First, in a model in which a conventional switching mixer is simplified, the input signal including an interfering wave (third-order harmonic wave) having a frequency component three times as high as that of a desired wave (D wave) is input and the duty of the local signal is set to ½ in a conventional manner. In a specific simulation circuit example, third-order interference outputs −28.13 dBm.

On the other hand, when the duty of the local signal is set to ⅓ and another condition is made similar to that of the above-described conventional switching mixer, the interfering wave of −82.29 dBm is generated in the output signal by the simulation (the signal after the frequency conversion). Therefore, when there is the third-order harmonic wave, improvement of 54.16 dB is expected by setting the duty of ½ to the duty of ⅓.

The frequency converting device of this embodiment may be applied also when a differential signal is input.FIG. 2is a view of a functional configuration example of the frequency converting device of this embodiment when the differential signal is input. The frequency converting device illustrated inFIG. 2is similar to the frequency converting device inFIG. 1except that the input signal is the differential signal, the local signal is differentiated, and the output signal is the differential signal. In this case, the duty adjusting circuit32adjusts to set the duty of the differentiated local signal to 1/N.

Although the frequency converting device of this embodiment may be used as an LSI for TV tuner for receiving TV broadcast, for example, there is not a limitation, and this may also be used as the frequency converting device of another application as far as this is a device that frequency-converts the input signal using the local signal such as the frequency converting device in a communication device that receives a wideband signal. Although a case in which the input signal is converted to the base band frequency is described in this embodiment, in a case in which the input signal is converted to an intermediate frequency and the like also, an N-th frequency of the local signal may be suppressed by similarly setting the duty of the local signal to 1/N.

Next, a configuration example of the duty adjusting circuit32of the frequency converting device of this embodiment is described.FIG. 3is a view of the configuration example of the duty adjusting circuit32when a divide-by-N divider321is used as the duty adjusting circuit32. Although an example in which the differential signal is not used is illustrated inFIG. 3, when using the differential signal, an output from the oscillator31and an output from the divide-by-N divider321are made the differential signals. Meanwhile, the configuration of the duty adjusting circuit32is not limited to this and any configuration is possible.

It is also possible that the oscillator31is provided with a function as the duty adjusting circuit32. In other words, the duty adjusting circuit32has a function of the oscillator31.FIG. 4is a view of a configuration example of the oscillator provided with the function as the duty adjusting circuit32. The oscillator inFIG. 4is provided with a ring oscillator311obtained by connecting N differential amplifiers in series in a ring manner, and a signal extracting unit312that selects a signal to be output as a clock signal out of outputs from the differential amplifiers, which compose the ring oscillator. The signal extracting unit312appropriately selects the output of the differential amplifier, thereby generating the local signal with the duty of 1/N. Meanwhile, the oscillator provided with the function as the duty adjusting circuit32is not limited to the example inFIG. 4and may be realized by any configuration. When using the oscillator provided with the function as the duty adjusting circuit32, it is not required that the duty adjusting circuit32is provided.

Meanwhile, although the frequency converting device is provided with the oscillator31in this embodiment, it is also possible to use a frequency signal input from outside as the local signal without the oscillator31being provided. In this case, the duty adjusting circuit32may generate the local signal with the duty of 1/N using the frequency signal input from outside.

FIG. 5is a view of a configuration example of the television device provided with the LSI for TV tuner, which is the frequency converting device of this embodiment. As illustrated inFIG. 5, the television device of this embodiment is composed of an antenna1, a differentiating unit9, a filter2, an LSI for TV tuner3, a filter circuit4, an analog/digital (A/D) converter5, a digital filter6, a digital signal processor (DSP)7, and a display8. Meanwhile, although an example in which the frequency converting device of this embodiment is used in the television device including also the display8is herein illustrated, the frequency converting device of this embodiment may be used as a receiving device that receives the TV broadcast. In this case, the receiving device is composed of the antenna1, the differentiating unit2, the LSI for TV tuner3, the filter circuit4, the A/D converter5, the digital filter6, and the DSP7, for example.

Meanwhile,FIG. 5is merely an example and the frequency converting device of this embodiment may be used as the LSI for TV tuner of the television device or the receiving device other than the configuration example illustrated inFIG. 5.

Operation of the television device illustrated inFIG. 5is described. First, the antenna1receives a TV broadcast signal and the differentiating unit9differentiates the TV broadcast signal. The filter2removes an electric wave out of a desired frequency band (TV frequency band) from the differentiated TV broadcast signal. Then, the LSI for TV tuner3frequency-converts the TV broadcast signal to the base band signal using the local signal with the duty of 1/N as described above. Thereafter, the filter circuit4performs a filtering process from the base band signal to the base band signal and the A/D converter5converts the base band signal after the filtering process from an analog signal to a digital signal. Next, the digital filter6performs the filtering process to the digital signal and the DSP7performs a predetermined process to the digital signal after the filtering process to output to the display8as a TV image signal. That is to say, the DSP7has a function as an image processor. Then, the display8displays the TV image signal as a TV image.

Meanwhile, although an example in which the input signal (TV broadcast signal) is differentiated is illustrated inFIG. 5, it is not required that the input signal is differentiated. When this is not differentiated, it is not required that the differentiating unit9is provided.

As described above, in this embodiment, it is configured that the duty adjusting circuit32generates the local signal with the duty of 1/N and the mixer33frequency-converts the input signal using the local signal. Therefore, the N-th high-frequency component of the local signal may be suppressed. Therefore, generation of the interfering wave generated by the N-th high-frequency component may be decreased and deterioration in the receiving sensitivity is prevented.

Second Embodiment

FIG. 6is a view of a functional configuration example of the frequency converting device according to a second embodiment. The frequency converting device of this embodiment is composed of the oscillator31, a duty adjusting circuit34, mixers33-1to33-N (N is the integral number not smaller than 3), and an adder35. A reference numeral identical to that of the first embodiment is assigned to a component having a similar function as that of the first embodiment and an overlapping description is omitted. Hereinafter, a point different from that of the first embodiment is described.

In this embodiment, the duty adjusting circuit34generates N local signals with the duty of 1/N of which phases are different from each other and inputs them to the mixers33-1to33-N, respectively. A function of each of the mixers33-1to33-N is similar to that of the mixer33of the first embodiment. The mixers33-1to33-N frequency-convert the input signal (for example, the TV broadcast signal) based on the input local signal and output the signals after the frequency conversion to the adder35. The adder35adds up the N signals output from the mixers33-1to33-N to output.

FIG. 7is a view of an example of the local signal with the duty of 1/N generated by the duty adjusting circuit34of this embodiment. The duty adjusting circuit34of this embodiment generates N local signals to be input to the mixers33-1to33-N, respectively. The duty of each of the generated local signals is 1/N, and they are generated such that the phases of which are shifted from each other by 2π/N. For example, the local signal to be input to the lowest mixer33-1inFIG. 7and the local signal to be input to the second lowest mixer33-2inFIG. 7are shifted from each other by 1/N of one period (T/N). The duty adjusting circuit34of this embodiment generates the local signals of which phases are sequentially shifted from each other by 2π/N in this manner.

Operation of this embodiment other than the above-description is similar to that of the first embodiment. Also, the differential signal may be used as in the case illustrated inFIG. 2of the first embodiment also in the frequency converting device of this embodiment. Also, as described in the first embodiment, the oscillator31may also have the function of the duty adjusting circuit34.

As described in the first embodiment, when performing the frequency conversion using the local signal with the duty of 1/N, the N-th harmonic of the local signal may be suppressed; however, noise property might be decreased than in the case in which the duty is ½. This is because a switching rate increases and the like as the duty becomes smaller. On the other hand, in this embodiment, the noise property may be improved by adding up the local signals of which phases are shifted from each other.

Meanwhile, although the N mixers are provided to generate the N local signals with the duty of 1/N in this embodiment, it is also possible that N2(N2≠N, N2is an integral number not smaller than 2) mixers are provided to generate N2local signals with the duty of 1/N. Then, the local signals with the duty of 1/N of which phases are different from each other by 2π/2N are generated, for example. In this case also, the noise property is expected to be improved as compared to the first embodiment.

As described above, the duty adjusting circuit34generates the N local signals with the duty of 1/N of which phases are different from each other by 2π/N to input to the mixers33-1to33-N, and the mixers33-1to33-N frequency-convert the input signal based on the local signal to output to the adder35. Then, the adder is configured to add up the signals output from the mixers33-1to33-N; Therefore, it is possible to improve the noise property as compared to the first embodiment while suppressing the N-th harmonic of the local signal.

Third Embodiment

FIG. 8is a view of a functional configuration example of the frequency converting device according to a third embodiment. The frequency converting device of this embodiment is composed of the oscillator31, a duty adjusting circuit36, mixers37-1to37-2, and the adder35. A reference numeral identical to that of the first or second embodiment is assigned to a component having a similar function as that of the first or second embodiment and an overlapping description is omitted. Hereinafter, a point different from that of the first or second embodiment is described.

In this embodiment, the duty adjusting circuit36generates two local signals with the duty of 1/N (N is the integral number not smaller than 3) of which phases are different from each other by π/m (M is an integral number not smaller than 2, M≠N) to input to the mixers37-1and37-2, respectively.

FIG. 9is a view of an example of the local signal with the duty of 1/N generated by the duty adjusting circuit36of this embodiment. For example, the local signal to be input to the lowest mixer37-1inFIG. 9and the local signal to be input to the second lowest mixer37-2inFIG. 9are shifted from each other by 1/M of half a period. That is to say, the phases thereof are shifted from each other by π/M. The duty adjusting circuit36of this embodiment generates the local signals of which phases are shifted from each other by π/M in this manner.

By using the two local signals of which phases are different from each other by π/M, an M-th harmonic of the local signal may be suppressed. For example, regardless of a value of N, the local signal f(t) with the duty of 1/N may be expressed by a following equation (6), in general.

A local signal f(t−T/(2M)) of which phase is shifted by π/M relative to f(t) may be expressed by a following equation (7). Meanwhile, Σ in the equation (7) represents a sum from n=1 to ∞.
f(t−T/(2M))=c0+Σcn(sinnω0(t−T/(2M)))  (7)

Therefore, when M is set to satisfy nω0T/(2m)=π, absolute values of n-th harmonic components of f(t) and f(t−T/(2m)) are equal to each other and signs thereof are opposite, so that the n-th harmonic component is deleted when f(t) and f(t−T/(2M)) are added up. That is to say, when the frequency conversion is performed using the two local signals, by adding up the two signals after the frequency conversion, an effect by the n-th harmonic component is suppressed. Since ω0=2π/T, a deleted order n=M. That is to say, by generating the identical local signals of which phases are different from each other by π/M, an M-th harmonic component of the local signal may be suppressed.

In this manner, the local signals with the duty of 1/N of which phases are different from each other by π/M are used in this embodiment, so that both of the N-th harmonic component and the M-th harmonic component may be suppressed. For example, when N=3 and M=2, both of the second and third harmonic components may be suppressed.

Meanwhile, in this embodiment, although the number of the mixers are set to 2 (mixers37-1and37-2) and the two local signals are generated in order to make the description simple in the above-description, the number of mixers K may be 2 or larger and it may be configured that the local signal is generated for each mixer.

As described above, this embodiment is provided with the mixers37-1and37-2and the duty adjusting circuit36is configured to input the local signals with the duty of 1/N of which phases are different from each other by π/M to the mixers37-1and37-2. Therefore, both of the N-th harmonic component and the M-th harmonic component of the local signal may be suppressed.

Fourth Embodiment

FIG. 10is a view of a functional configuration example of the frequency converting device according to a fourth embodiment. The frequency converting device of this embodiment is composed of the oscillator31, mixer units39-1and39-2, and the adder35. A reference numeral identical to that of the first or second embodiment is assigned to a component having a similar function as that of the first or second embodiment and an overlapping description is omitted. Hereinafter, a point different from that of the first, second or third embodiment is described.

Each of the mixer units39-1and39-2is composed of the mixers33-1to33-N. In this embodiment, although the duty adjusting circuit38generates the N local signals with the duty of 1/N of which phases are different from each other by 2π/N to input to the mixers33-1to33-N, respectively, as in the second embodiment, this generates the local signals such that there is phase difference by π/M between the local signal to be input to the mixer unit39-1and the local signal to be input to the mixer unit39-2. For example, the phase difference between the local signal to be input to the mixer33-1of the mixer unit39-2and the local signal to be input to the mixer33-1of the mixer unit39-1is π/M and the phase difference between the local signal to be input to the mixer33-2of the mixer unit39-1and the local signal to be input to the mixer33-2of the mixer unit39-2is π/M.

Then, all the signals output from the mixers33-1to33-N of the mixer units39-1and39-2are added up by the adder35. Therefore, in this embodiment, both of the effects of the second and third embodiments may be obtained.

Meanwhile, although the mixer units39-1and39-2are provided and each of the mixer units39-1and39-2is composed of the mixers33-1and33-N in the example illustrated inFIG. 10, this may be opposite and N mixer units composed of the mixers37-1and37-2similar to those of the third embodiment may be provided. In this case, the phase difference between the local signals to be input to the mixers37-1and37-2is set to π/M, the local signals input to the mixer37-1of each of the mixer units are such that the phases thereof are different from each other by 2π/N, and the local signals to be input to the mixer37-2of each of the mixer units are such that the phases thereof are different from each other by 2π/N.

As described above, in this embodiment, the mixer units39-1and39-2composed of the mixers33-1to33-N of the second embodiment are provided, the duty adjusting circuit38is configured to generate the N local signals with the duty of 1/N of which phases are different from each other by 2π/N to input to the mixers33-1to33-N, and to generate the local signals such that there is the phase difference of π/M between the local signal to be input to the mixer unit39-1and the local signal to be input to the mixer unit39-2. Therefore, both of the N-th harmonic component and the M-th harmonic component of the local signal may be suppressed and the noise property may be improved as compared to that of the third embodiment.