WO2012151783A1 - 二次变频调制系统及变频方法 - Google Patents

二次变频调制系统及变频方法 Download PDF

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WO2012151783A1
WO2012151783A1 PCT/CN2011/077137 CN2011077137W WO2012151783A1 WO 2012151783 A1 WO2012151783 A1 WO 2012151783A1 CN 2011077137 W CN2011077137 W CN 2011077137W WO 2012151783 A1 WO2012151783 A1 WO 2012151783A1
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frequency
pass filter
fixed
local oscillator
fixed low
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PCT/CN2011/077137
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English (en)
French (fr)
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李文华
宋伯炜
夏劲松
范莹莹
王先勇
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苏州全波通信技术有限公司
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Priority to CN201180003631.3A priority Critical patent/CN102742240B/zh
Publication of WO2012151783A1 publication Critical patent/WO2012151783A1/zh

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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03D—DEMODULATION OR TRANSFERENCE OF MODULATION FROM ONE CARRIER TO ANOTHER
    • H03D7/00—Transference of modulation from one carrier to another, e.g. frequency-changing
    • H03D7/16—Multiple-frequency-changing
    • H03D7/165—Multiple-frequency-changing at least two frequency changers being located in different paths, e.g. in two paths with carriers in quadrature
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03D—DEMODULATION OR TRANSFERENCE OF MODULATION FROM ONE CARRIER TO ANOTHER
    • H03D2200/00—Indexing scheme relating to details of demodulation or transference of modulation from one carrier to another covered by H03D
    • H03D2200/0001—Circuit elements of demodulators
    • H03D2200/0017—Intermediate frequency filter
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03D—DEMODULATION OR TRANSFERENCE OF MODULATION FROM ONE CARRIER TO ANOTHER
    • H03D2200/00—Indexing scheme relating to details of demodulation or transference of modulation from one carrier to another covered by H03D
    • H03D2200/0041—Functional aspects of demodulators
    • H03D2200/0082—Quadrature arrangements

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  • the invention relates to a device and a method in the field of wireless communication technology, in particular to a secondary frequency conversion modulation system and a frequency conversion method.
  • the baseband signal needs to be upconverted to a radio frequency.
  • the baseband signal of MHz needs to be upconverted to the corresponding RF frequency, such as 48 MHz to 866 MHz.
  • the traditional up-conversion has: secondary frequency conversion, primary frequency conversion and direct frequency conversion, wherein: the traditional secondary frequency conversion method is to digitally convert the digital baseband signal through a digital frequency converter to obtain a digital low intermediate frequency signal, such as 36. MHz, 44 MHz, etc.; then converted to analog low-IF signal by digital-to-analog converter (DAC); analog filtering by fixed bandpass filter at low intermediate frequency, filtering out higher harmonics generated by digital-to-analog conversion; The IF signal is subjected to the first stage analog frequency conversion through the analog frequency converter, and is moved up to a relatively high high intermediate frequency, such as 1 GHz; bandpass filtering is performed at a high intermediate frequency by a fixed bandpass filter to filter out the image spectrum and the local oscillator frequency; then the high intermediate frequency signal is passed through an analog frequency converter for second-stage analog frequency conversion, and down to the desired modulation At the frequency; finally, through the low-pass filter, the image spectrum and the local oscillator spectrum are filtered out.
  • the advantage of this type of frequency conversion is that the local oscillator frequency is not within the required spectrum and can be filtered out well with the image spectrum.
  • the secondary frequency conversion system is quite complicated, requiring 1 level digital frequency conversion and 2 stage analog frequency conversion.
  • two sets of band pass filters are needed, especially the high medium frequency band pass filter, the frequency point is high, and the required spectrum and filtering are needed.
  • the image spectrum is not far from the local oscillator frequency, which increases the difficulty of the filter design and puts high demands on the system and circuit design.
  • the phase noise is worse than the primary and direct conversion.
  • the traditional method of single frequency conversion is to digitally convert the digital baseband signal through a digital frequency converter to obtain a digital low intermediate frequency signal, such as 36 MHz, 44. MHz, etc.; then converted to analog low-IF signal by DAC; simulated filtering by fixed bandpass filter at low intermediate frequency, filtering out higher harmonics generated by digital-to-analog conversion; then passing analog low-IF signal through analog inverter Perform a first-level analog frequency conversion and directly move up to the desired modulation frequency; finally, filter the image spectrum and the local oscillator spectrum through a band-pass filter. As shown in Figure 1 (b).
  • the phase noise is better than the double frequency conversion.
  • An analog bandpass filter with a bandwidth of 8 MHz from MHz to 866 MHz is extremely difficult.
  • analog filters can not be achieved. Therefore, this variable frequency analog bandpass filter can neither guarantee good out-of-band rejection nor guarantee good band-pass characteristics, that is, the in-band flatness cannot be guaranteed.
  • the direct conversion that is, the zero intermediate frequency modulation method is to convert the complex digital baseband signals I and Q through two DACs into a complex analog baseband signal, that is, a complex zero intermediate frequency signal; at zero intermediate frequency, the two paths of I and Q are passed through two.
  • a low-pass filter performs simple analog low-pass filtering to filter out higher harmonics generated by digital-to-analog conversion; and then passes the analog complex-zero IF signals I and Q through orthogonal converters to directly orthogonally modulate On the required frequency.
  • the multiple harmonic spectrum of the desired frequency is filtered out by a simple fixed low-pass filter. As shown in Figure 1 (c).
  • Direct conversion has its own advantages. Since the system has only one frequency conversion, the phase noise is better than the double frequency conversion and the third frequency conversion. Secondly, the baseband analog filter is a simple low-pass filter with simple design and easy performance. Finally, because of the zero-IF structure, the RF Some only need to design a general fixed low-pass filter to filter out the high-order harmonic spectrum of the required frequency, which greatly simplifies the hardware design and optimizes the performance.
  • the simple direct conversion modulation method has the following problems:
  • the orthogonality of the local oscillator signal has very strict requirements in the entire modulation frequency range.
  • the non-orthogonal signal of the local oscillator signal the introduced image spectrum will be directly superimposed on the required spectrum, which seriously affects the modulation performance and the quality of the transmitted signal.
  • the first category is simply dependent on the improved analog performance of the upconverter components.
  • the absolute orthogonality of the local oscillator signal is guaranteed, and very high requirements are placed on the device, which greatly increases the cost of the device, and even for the same batch of devices, due to its analog characteristics.
  • the second type of method adding orthogonal calibration to the direct conversion structure.
  • the accuracy of the current correction algorithm is limited, the processing time is long, and the phase is repeatedly oscillated, and its performance is not suitable for high-standard real-time television broadcast communication.
  • the problem with this patent is mainly that if the frequency band to be covered needs to be expanded, for example, the modulation range from the VHF segment to the UHF segment needs to be satisfied at the same time, the high harmonic of the local oscillator signal generated when the low-frequency RF signal is generated, It will fall directly into the effective bandwidth of the high-frequency RF signal, and the fixed low-pass filter in the direct-conversion structure will not be able to filter out such local oscillator higher-order harmonics, thus failing to meet the out-of-band spurious requirements specified by the standard.
  • the modulation band is required to range from 200 MHz to 866 MHz, 200 MHz second harmonic 400 MHz, third harmonic 600 MHz, fourth harmonic 800 MHz, both falling within the modulation spectrum (both less than the highest modulatable frequency of 866MHz), will greatly affect the performance of the RF.
  • the present invention provides a secondary frequency conversion modulation system and a frequency conversion method according to the above-mentioned deficiencies of the prior art, and utilizes the advantages of direct frequency conversion, thereby eliminating the need to design a high frequency analog band pass filter and effectively satisfying orthogonality. At the same time, it supports a wide range of modulatable frequency bands, greatly reducing the requirements for device and circuit design.
  • the invention relates to a secondary frequency conversion modulation system, comprising: a dual port digital-to-analog converter, a dual-port fixed low-pass filter, a fixed local oscillator orthogonal frequency converter, a first fixed low-pass filter, and a variable local oscillator.
  • the frequency converter and the second fixed low-pass filter wherein: the input end of the dual-port digital-to-analog converter receives the I baseband data and the Q baseband data respectively, and outputs the I road analog signal and the Q road analog signal to the dual port fixed low pass
  • the filter is filtered by the dual-port fixed low-pass filter to output the high-order harmonics and then output to the fixed local oscillator orthogonal frequency converter.
  • the fixed local oscillator orthogonal frequency converter is converted to the high intermediate frequency by the zero-IF quadrature modulation method and is first.
  • the fixed low-pass filter performs high-order harmonic filtering and outputs to the variable local oscillator, and the local oscillator changes the output of the modulated RF signal to the second fixed low-pass filter and outputs the final RF.
  • the dual-port fixed low-pass filter is two parallel low-order fixed low-pass filters whose distance between the effective spectrum and the harmonic is an integer multiple of the sampling frequency of the dual-port digital-to-analog converter.
  • the fixed local oscillator orthogonal frequency converter is composed of a fixed local oscillator generator, a phase shifter and two multipliers, wherein: the input ends of the two multipliers respectively receive the I channel simulation of the dual port fixed low pass filter output The signal and the local frequency of the fixed local oscillator generator and the Q analog signal of the dual port fixed low-pass filter output and the local frequency of the 90-degree phase shift by the phase shifter. The outputs of the two multipliers are superimposed and output to The first fixed low pass filter.
  • the first fixed low pass filter is a low order fixed low pass filter, and the distance between the effective spectrum and the higher harmonics is the center frequency of the high intermediate frequency signal or an integral multiple thereof.
  • the second fixed low pass filter is a low order fixed low pass filter whose distance between the effective spectrum and the image spectrum is twice the difference between the fixed local oscillator and the required modulation frequency. Note that the high local frequency of the fixed local oscillator is f H , and the required modulation frequency is f C , then the distance between the effective spectrum and the image spectrum is 2 ⁇ (f H - f C ).
  • the invention relates to a frequency conversion method of the above system, comprising the following steps:
  • the first step the I baseband data and the Q baseband data are sequentially converted into analog signals by a dual port digital-to-analog converter, and then the high-order harmonics are filtered through a dual-port fixed low-pass filter to obtain an I-channel of the analog spectrum at the baseband. Analog signal and Q analog signal.
  • the second step is to use the fixed local oscillator orthogonal frequency converter to perform the first direct frequency conversion on the I road analog signal and the Q channel analog signal, that is, after the zero intermediate frequency frequency conversion, the spectrum is moved to the high intermediate frequency of the center frequency fH, corresponding
  • the image spectrum is directly offset and its higher harmonics are respectively located at integer multiples of the center frequency.
  • the third step is to filter out the higher harmonics generated by the first direct conversion by the first fixed low-pass filter
  • the local oscillator signal of frequency f H -f C is generated by changing the local oscillator, and the analog signal obtained in the third step is subjected to the second frequency conversion, and the spectrum is respectively moved to f C and 2f H -f C ;
  • the image spectrum 2f H -f C is filtered by the second fixed low-pass filter, and the remaining spectrum f C is the desired modulation frequency.
  • the invention greatly simplifies the requirements for analog filters.
  • the conventional frequency conversion technology does not require a multi-stage intermediate frequency and high frequency band pass filter.
  • the present invention only needs the simplest fixed low pass filter, and the filter does not need to change with the required modulation frequency variation, and thus can be widely used.
  • the same general-purpose fixed low-pass filter is used within the modulation range. Therefore, the circuit and system design is greatly simplified, the requirements on the device are reduced, the area, power consumption and cost are reduced, and the integration is facilitated.
  • the invention satisfactorily solves the influence of orthogonality on performance, and greatly reduces the cost of the device and the difficulty of circuit and system design.
  • the traditional direct conversion modulation method faces the problem of orthogonality. If it is guaranteed by the device, it will greatly increase the difficulty of circuit design and the cost of the device while increasing the modulatable frequency range of the system. Therefore, the radio frequency performance is difficult to obtain. Guarantee.
  • the invention is a novel secondary frequency conversion combined with a direct conversion modulation method. Compared with the conventional direct conversion modulation method, it only needs to perform orthogonal modulation at a certain frequency, and the requirements on the device are greatly reduced.
  • the invention is capable of corresponding to a wide range of modulatability. It can be seen that as long as the frequency value of the high frequency frequency f H is designed to be higher than the required maximum modulation frequency (f H >f CMAX ), the frequency modulation range that the system of the present invention can satisfy is completely unrestricted, that is to say While not increasing the system design difficulty and device cost, it can greatly expand the supportable modulation range, thus greatly simplifying the circuit and system design, reducing the requirements on the device, reducing power consumption and cost, and facilitating integrated.
  • Figure 1 is a schematic view of a prior art
  • (a) is the system structure of the traditional double conversion; (b) is the system structure of the traditional primary frequency conversion; (c) is the system structure of the traditional direct conversion.
  • FIG. 2 is a schematic structural view of a secondary frequency conversion system of the present invention.
  • FIG. 3 is a schematic diagram of a system spectrum of an embodiment.
  • the embodiment includes: a dual port digital-to-analog converter, a dual-port fixed low-pass filter, a fixed local oscillator orthogonal frequency converter, a first fixed low-pass filter, and a variable local oscillator frequency converter.
  • a second fixed low-pass filter wherein: the input end of the dual-port digital-to-analog converter receives the I baseband data and the Q baseband data respectively, and outputs the I analog signal and the Q analog signal to the dual-port fixed low-pass filter After the high-order harmonics are filtered by the dual-port fixed low-pass filter, the output is output to the fixed local oscillator orthogonal frequency converter, and the fixed local oscillator orthogonal frequency converter is converted to the high intermediate frequency by the zero intermediate frequency quadrature modulation method and is fixed by the first fixed low.
  • the pass filter performs high-order harmonic filtering and outputs to the variable local oscillator, and the local oscillator changes the output RF signal to the second fixed low-pass filter and outputs the final RF.
  • the dual-port fixed low-pass filter is two parallel low-order fixed low-pass filters whose distance between the effective spectrum and the harmonic is an integer multiple of the sampling frequency of the dual-port digital-to-analog converter.
  • the fixed local oscillator orthogonal frequency converter is composed of a fixed local oscillator generator, a phase shifter and two multipliers, wherein: the input ends of the two multipliers respectively receive the I channel simulation of the dual port fixed low pass filter output The signal and the local frequency of the fixed local oscillator generator and the Q analog signal of the dual port fixed low-pass filter output and the local frequency of the 90-degree phase shift by the phase shifter. The outputs of the two multipliers are superimposed and output to The first fixed low pass filter.
  • the first fixed low pass filter is a low order fixed low pass filter, and the distance between the effective spectrum and the higher harmonics is the center frequency of the high intermediate frequency signal or an integral multiple thereof.
  • the second fixed low pass filter is a low order fixed low pass filter whose distance between the effective spectrum and the image spectrum is twice the difference between the fixed local oscillator and the required modulation frequency. Note that the high local frequency of the fixed local oscillator is f H , and the required modulation frequency is f C , then the distance between the effective spectrum and the image spectrum is 2 ⁇ (f H - f C ).
  • the frequency conversion method of the above system in this embodiment includes the following steps:
  • the first step the I baseband data and the Q baseband data are sequentially converted into analog signals by a dual port digital-to-analog converter, and then the high-order harmonics are filtered through a dual-port fixed low-pass filter to obtain an I-channel of the analog spectrum at the baseband. Analog signal and Q analog signal.
  • the second step is to use the fixed local oscillator orthogonal frequency converter to perform the first direct frequency conversion on the I road analog signal and the Q channel analog signal, that is, after the zero intermediate frequency frequency conversion, the spectrum is moved to the high intermediate frequency with the center frequency being f H , corresponding
  • the image spectrum is directly offset and its higher harmonics are respectively located at integer multiples of the center frequency.
  • the third step is to filter out the higher harmonics generated by the first direct conversion by the first fixed low-pass filter
  • the local oscillator signal with the frequency f H -f C is generated by changing the local oscillator, and the analog signal obtained in the third step is subjected to the second frequency conversion, and the spectrum is respectively moved to f C and 2 (f H - f C );
  • the image spectrum 2 (f H -f C ) is filtered out by the second fixed low-pass filter, and the remaining spectrum f C is the desired modulation frequency.
  • the traditional two-frequency conversion technology requires two sets of band-pass filters, especially high-frequency band-pass filters, which have high frequency points (such as 1). GHz), and the required spectrum is not far from the image spectrum to be filtered and the local oscillator frequency (eg the first low IF conversion is often at 36 MHz, or 44 MHz, then the distance between the required spectrum and the mirror is low) Double the intermediate frequency, such as 72 MHz, or 88 MHz).
  • the filter design is difficult, bulky, and high in power consumption, and has high requirements for heat dissipation and circuit design.
  • the present invention utilizes direct conversion to complete the first frequency conversion operation, and the distance between the required spectrum and the higher order spectrum to be filtered is the center frequency of the high intermediate frequency signal or an integral multiple thereof (for example, the high intermediate frequency is used when the first frequency conversion is used) At GHz, the higher harmonics to be filtered out are at 2 GHz, 3 GHz, 4 GHz). Such a wide distance makes it possible to design a low-order, simple-tapped fixed low-pass filter, which enables full-band coverage and greatly reduces design difficulty.
  • the high intermediate frequency frequency f H when the first frequency conversion is selected is greater than the highest modulation frequency f CMAX (f H >f CMAX ) that needs to be supported.
  • f H is 1 GHz, it can support the entire UHF band and VHF band (no need to change the crystal and filter parameters), and only need this frequency point (f H ) to ensure orthogonality (no need to support)
  • the entire band only needs to be met by a single frequency point, which greatly reduces the requirements on the device, and is very economical and practical.
  • the invention can be applied to an existing platform to realize two frequency conversions, and under the requirement of greatly reducing the difficulty of hardware and system design, the useful signal shoulder width can exceed 55 decibels, and the out-of-band spurs in the adjacent frequency band exceed 55 decibels in the whole frequency band. And ultra-high modulation error rate (MER) performance.
  • MER ultra-high modulation error rate

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Abstract

一种无线通信技术领域的二次变频调制系统及变频方法，包括：依次连接的双口数模转换器、双口固定低通滤波器、固定本振正交变频器、第一固定低通滤波器、变化本振变频器以及第二固定低通滤波器。本发明利用了直接变频的优点，从而无需设计高频率模拟带通滤波器，又能在有效满足正交性的同时支持大为广泛的可调制频段，极大的降低对器件与电路设计要求。

Description

二次变频调制系统及变频方法 技术领域
本发明涉及的是一种无线通信技术领域的装置及方法，具体是一种二次变频调制系统及变频方法。
背景技术
在数字信号传输系统中，需要将基带信号上变频到射频。以数字电视传输系统为例，8 MHz的基带信号需要被上变频到相应的射频频率，如48 MHz到866 MHz。
传统的上变频有：二次变频、一次变频和直接变频，其中：传统二次变频的方法是将数字基带信号通过数字变频器进行数字变频，得到数字低中频信号，如36 MHz，44 MHz，等；再通过数模转换器（DAC）转换为模拟低中频信号；在低中频处通过固定带通滤波器进行模拟滤波，滤除数模转换产生的高次谐波；然后将模拟低中频信号通过模拟变频器进行第一级模拟变频，上移到比较高的高中频，如1 GHz；在高中频处通过固定带通滤波器进行带通滤波，滤除镜像频谱和本振频率；然后再将该高中频信号通过模拟变频器进行第二级模拟变频，下移到所需调制的频率上；最后通过低通滤波器，滤除镜像频谱和本振频谱。如图1(a)所示。
这种变频方式，好处是本振频率不在所需要的频谱之内，可以和镜像频谱一起很好地滤除。但是二次变频系统相当复杂，需要1级数字变频和2级模拟变频，同时需要两组带通滤波器，尤其是高中频带通滤波器，其频率点高，且所需频谱与要滤除的镜像频谱和本振频率相隔不远，增加了滤波器设计的难度，对系统和电路设计都提出了较高的要求。同时，相位噪声比一次变频和直接变频的要差。
传统一次变频的方法是将数字基带信号通过数字变频器进行数字变频，得到数字低中频信号，如36 MHz，44 MHz，等；再通过DAC转换为模拟低中频信号；在低中频处通过固定带通滤波器进行模拟滤波，滤除数模转换产生的高次谐波；再将模拟低中频信号通过模拟变频器进行一级模拟变频，直接上移到所需调制的频率上；最后通过带通滤波器滤除镜像频谱和本振频谱。如图1(b)所示。
这种变频方式，相位噪声比二次变频好，但是，需要随所需调制频率变化，设计不同模拟带通滤波器以滤除镜像频谱和本振频谱。举例以数字电视的频段来说，设计一个范围从48 MHz到866 MHz，带宽为8 MHz的模拟带通滤波器是极端困难的，例如，在48 MHz处，8 MHz带宽和该频率点的比值为8/48=0.17，但是在866 MHz处，8 MHz带宽和该频率点的比值为8/866=0.01。如此大范围的变化，模拟滤波器无法实现。因此这种变频率的模拟带通滤波器既不能保证很好的带外抑制，也不能保证很好的带通特性，即带内的平坦度无法保证。
直接变频，即零中频的调制方式是将复数字基带信号I、Q两路通过两个DAC，转换为复模拟基带信号，即复零中频信号；在零中频处将I、Q两路通过两个低通滤波器进行简单的模拟低通滤波，滤除数模转换产生的高次谐波；再将该模拟复零中频信号I、Q两路通过由正交变频器，直接正交调制到所需的频率上。最后通过简单的固定低通滤波器，滤除掉所需频率的多次谐波频谱。如图1(c)所示。
直接变频有自己的优势。由于系统只有一次变频，相位噪声优于二次变频和三次变频；其次，基带的模拟滤波器是简单的低通滤波器，设计简单，性能容易得到保证；最后，因为零中频的结构，在射频部分只需要设计通用的固定低通滤波器，用于滤除所需频率的高次谐波频谱即可，极大的简化了硬件设计，优化了性能。
但是简单的直接变频调制方式存在以下问题：在整个调制频率范围内，对本振信号的正交性，有着非常严格的要求。本振信号的不正交，引入的镜像频谱，将直接叠加在所需要的频谱之中，从而严重影响了调制性能以及发射信号的质量。目前常见的正交误差消除有两类方式。第一类是简单的依赖于上变频器件的模拟性能的提高。然而要在很大范围的频段内，保证本振信号的绝对正交性，对器件提出了非常高的要求，相应会大大的提高了器件的成本，而且即使是同一批器件，由于其模拟特性不一致，最终调制性能也很难保证一致性。第二类方法：在直接变频结构中加入正交校准。但目前的校正算法的精度有限，处理时间长，容易引起相位反复震荡，其性能并不合适于高标准的实时电视广播通信使用。
经过对现有技术的检索发现，中国专利号CN 101795252 A 直接变频调制方法及其调制装置中，提出了一种直接变频调制方式及其调制装置，通过高倍率本振信号的引入，在相当大的频段内都能保证本振信号的完美正交。然而该专利存在的问题主要是，如果需要对覆盖的频段进行扩展，例如需要同时满足从VHF段到UHF段的调制范围的话，生成低频率射频信号时所产生的本振信号的高倍谐波，就会直接落入高频率射频信号的有效带宽内，而直接变频结构中的固定低通滤波器将无法滤除这样的本振高次谐波，从而无法满足标准规定的带外杂散要求。例如要求调制频段范围从200 MHz到866 MHz，则200 MHz的二次谐波400 MHz、三次谐波600 MHz、四次谐波800 MHz，都落在调制频谱范围内（皆小于最高可调制频率866MHz），这将极大的影响了射频的性能指标。
 发明内容
本发明针对现有技术存在的上述不足，提供一种二次变频调制系统及变频方法，利用了直接变频的优点，从而无需设计高频率模拟带通滤波器，又能在有效满足正交性的同时支持大为广泛的可调制频段，极大的降低对器件与电路设计要求。
本发明是通过以下技术方案实现的：
本发明涉及一种二次变频调制系统，包括：依次连接的双口数模转换器、双口固定低通滤波器、固定本振正交变频器、第一固定低通滤波器、变化本振变频器以及第二固定低通滤波器，其中：双口数模转换器的输入端分别接收I路基带数据和Q路基带数据并输出I路模拟信号和Q路模拟信号至双口固定低通滤波器，经双口固定低通滤波器滤除高次谐波后输出至固定本振正交变频器，固定本振正交变频器通过零中频正交调制的方式变频至高中频并由第一固定低通滤波器进行高次谐波滤除并输出至变化本振变频器，变化本振变频器输出调制射频信号至第二固定低通滤波器并输出最终射频。
所述的双口固定低通滤波器为两个并列的低阶固定低通滤波器，其有效频谱与谐波之间的距离为双口数模转换器的采样频率的整数倍。
所述的固定本振正交变频器由固定本振发生器、移相器和两个乘法器组成，其中：两个乘法器的输入端分别接收双口固定低通滤波器输出的I路模拟信号和固定本振发生器输出的本地频率以及双口固定低通滤波器输出的Q路模拟信号和经移相器进行90度移相的本地频率，两个乘法器的输出端叠加后输出至第一固定低通滤波器。
所述的第一固定低通滤波器为低阶固定低通滤波器，其有效频谱与高次谐波之间的距离为高中频信号中心频率或其整数倍。
所述的第二固定低通滤波器为低阶固定低通滤波器，其有效频谱与镜像频谱之间的距离为固定本振与所需的调制频率的差的2倍。记固定本振的高中频为fH，所需的调制频率为fC，则有效频谱与镜像频谱之间距离为2×（fH-fC）。
本发明涉及上述系统的变频方法，包括以下步骤：
第一步、I路基带数据和Q路基带数据依次经过双口数模转换器转换成模拟信号，再经过双口固定低通滤波器滤除高次谐波后得到模拟频谱位于基带的I路模拟信号和Q路模拟信号。
第二步、采用固定本振正交变频器对I路模拟信号和Q路模拟信号进行第一次直接变频，即零中频变频后，将频谱搬移到中心频率为fH的高中频处，对应的镜像频谱被直接抵消且其高次谐波分别位于中心频率的整数倍。
第三步、通过第一固定低通滤波器滤除第一次直接变频产生的高次谐波；
第四步、通过变化本振变频器产生频率为fH-fC 的本振信号并对第三步得到的模拟信号进行第二次变频，将频谱分别搬移到fC以及2fH-fC ；
第五步、通过第二固定低通滤波器，滤除镜像频谱2fH-fC ，剩余频谱fC为所需的调制频率。
本发明与目前已有的技术相比的优点在于：
本发明极大的简化了对模拟滤波器的要求。传统的变频技术无不需要多级中频、高频带通滤波器，本发明只需要最简单的固定低通滤波器，而且滤波器无需随所需的调制频率变化而变化，因此可以在相当广泛的调制范围内使用同一个通用型的固定低通滤波器。因此极大的简化了电路与系统的设计，降低了对器件的要求，减少了面积、功耗和成本，有利于集成。
本发明在完美的解决正交性对性能影响的同时，极大降低了器件的成本与电路和系统设计难度。传统的直接变频调制方式面对正交性问题，如果通过器件保证的话，那么在增加系统可调制频率范围的同时，将极大的增加电路设计的难度以及器件的成本，射频性能也因此难以得到保证。本发明是结合直接变频调制方式的新型二次变频，与传统的直接变频调制方式相比，只需要在某一个特定的频点完成正交调制，对器件的要求大为降低。
本发明可对应广泛的可调制范围。可以看出只要设计高频频率fH的频率值，高于所需要的最高调制频率（满足fH>fCMAX），则本发明系统所能满足的频率调制范围完全不受限制，也就是说在丝毫不增加系统设计难度和器件成本的同时，能够极大的扩展可支持的调制范围，因此大大的简化了电路和系统设计，降低了对器件的要求，减少了功耗和成本，有利于集成。
 附图说明
图1为现有结构示意图；
其中：（a）为传统二次变频的系统结构；（b）为传统一次变频的系统结构；（c）为传统直接变频的系统结构。
图2为本发明二次变频系统结构示意图。
图3为实施例系统频谱示意图。
 具体实施方式
下面对本发明的实施例作详细说明，本实施例在以本发明技术方案为前提下进行实施，给出了详细的实施方式和具体的操作过程，但本发明的保护范围不限于下述的实施例。
如图2所示，本实施例包括：依次连接的双口数模转换器、双口固定低通滤波器、固定本振正交变频器、第一固定低通滤波器、变化本振变频器以及第二固定低通滤波器，其中：双口数模转换器的输入端分别接收I路基带数据和Q路基带数据并输出I路模拟信号和Q路模拟信号至双口固定低通滤波器，经双口固定低通滤波器滤除高次谐波后输出至固定本振正交变频器，固定本振正交变频器通过零中频正交调制的方式变频至高中频并由第一固定低通滤波器进行高次谐波滤除并输出至变化本振变频器，变化本振变频器输出调制射频信号至第二固定低通滤波器并输出最终射频。
所述的双口固定低通滤波器为两个并列的低阶固定低通滤波器，其有效频谱与谐波之间的距离为双口数模转换器的采样频率的整数倍。
所述的固定本振正交变频器由固定本振发生器、移相器和两个乘法器组成，其中：两个乘法器的输入端分别接收双口固定低通滤波器输出的I路模拟信号和固定本振发生器输出的本地频率以及双口固定低通滤波器输出的Q路模拟信号和经移相器进行90度移相的本地频率，两个乘法器的输出端叠加后输出至第一固定低通滤波器。
所述的第一固定低通滤波器为低阶固定低通滤波器，其有效频谱与高次谐波之间的距离为高中频信号中心频率或其整数倍。
所述的第二固定低通滤波器为低阶固定低通滤波器，其有效频谱与镜像频谱之间的距离为固定本振与所需的调制频率的差的2倍。记固定本振的高中频为fH，所需的调制频率为fC，则有效频谱与镜像频谱之间距离为2×（fH-fC ）。
如图3所示，本实施例上述系统的变频方法，包括以下步骤：
第一步、I路基带数据和Q路基带数据依次经过双口数模转换器转换成模拟信号，再经过双口固定低通滤波器滤除高次谐波后得到模拟频谱位于基带的I路模拟信号和Q路模拟信号。
第二步、采用固定本振正交变频器对I路模拟信号和Q路模拟信号进行第一次直接变频，即零中频变频后，将频谱搬移到中心频率为fH的高中频处，对应的镜像频谱被直接抵消且其高次谐波分别位于中心频率的整数倍。
第三步、通过第一固定低通滤波器滤除第一次直接变频产生的高次谐波；
第四步、通过变化本振变频器产生频率为fH-fC 的本振信号并对第三步得到的模拟信号进行第二次变频，将频谱分别搬移到fC以及2(fH-fC )；
第五步、通过第二固定低通滤波器，滤除镜像频谱2(fH-fC )，剩余频谱fC为所需的调制频率。
本发明独特优势包括：
首先，传统两次变频技术实现上需要两组带通滤波器，尤其是高中频带通滤波器，其频率点高（如1 GHz），且所需频谱与要滤除的镜像频谱和本振频率相隔不远（如第一次低中频变换常在36 MHz，或44 MHz，则所需频谱与镜像之间的距离是低中频的两倍，如72 MHz，或者88 MHz）。要在如此高的频率设计一个模拟的带通滤波器，滤波器设计的难度大，体积大，功耗高，对散热和电路设计都提出了较高的要求。而本发明利用了直接变频完成第一次变频操作，所需频谱与要滤除的高次频谱之间距离是高中频信号中心频率或其整数倍（如第一次变频时高中频采用1 GHz，则其要滤除的高次谐波分别在2 GHz、3 GHz、4 GHz）。如此宽的距离使得只需设计一个低阶数的、简单抽头的固定低通滤波器，即能实现全频段的覆盖，大大降低了设计难度。
其次，为了保证直接变频（捷变频）的正交性，如引用专利CN 101795252 A 直接变频调制方法及其调制装置中所述的，需要使用频率范围为1536 MHz到3544 MHz的大范围、高功耗频率源，也只能覆盖到443 MHz到886 MHz的范围。该专利无法支持在同样硬件平台上同时支持整个UHF波段和整个VHF 波段（必须更换不同的本地晶振以及滤波器参数），否则就会出现需要低频率射频信号时，所产生的本振信号的高倍谐波，因为无法有效被系统内低通滤波器滤除，从而直接落入高频率射频信号的有效带宽内。而本发明中，只需要设计时，选择第一次变频时高中频频率fH，大于所需要支持的最高调制频率fCMAX（满足fH>fCMAX）即可。例如选择fH为1 GHz，则其可以支持整个UHF波段和VHF波段（无需更换晶振和滤波器参数），且只需要该频点（fH）能够保证正交性即可（无需所支持的整个波段，只需要单频点满足），从而大大降低了对器件的要求，具有很好经济性和实用性。
用本发明应用于已有平台实现两次变频，在极大降低对硬件及系统设计难度的要求下，能达到有用信号带肩超过55分贝，全频段内邻频带内带外杂散超过55分贝，以及超高的调制误差率（MER）性能。

Claims (7)

  1. 一种二次变频调制系统，其特征在于，包括：依次连接的双口数模转换器、双口固定低通滤波器、固定本振正交变频器、第一固定低通滤波器、变化本振变频器以及第二固定低通滤波器。
  2. 根据权利要求1所述的二次变频调制系统，其特征是，所述的双口数模转换器的输入端分别接收I路基带数据和Q路基带数据并输出I路模拟信号和Q路模拟信号至双口固定低通滤波器，经双口固定低通滤波器滤除高次谐波后输出至固定本振正交变频器，固定本振正交变频器通过零中频正交调制的方式变频至高中频并由第一固定低通滤波器进行高次谐波滤除并输出至变化本振变频器，变化本振变频器输出调制射频信号至第二固定低通滤波器并输出最终射频。
  3. 根据权利要求1所述的二次变频调制系统，其特征是，所述的双口固定低通滤波器为两个并列的低阶固定低通滤波器，其有效频谱与谐波之间的距离为双口数模转换器的采样频率的整数倍。
  4. 根据权利要求1所述的二次变频调制系统，其特征是，所述的固定本振正交变频器由固定本振发生器、移相器和两个乘法器组成，其中：两个乘法器的输入端分别接收双口固定低通滤波器输出的I路模拟信号和固定本振发生器输出的本地频率以及双口固定低通滤波器输出的Q路模拟信号和经移相器进行90度移相的本地频率，两个乘法器的输出端叠加后输出至第一固定低通滤波器。
  5. 根据权利要求1所述的二次变频调制系统，其特征是，所述的第一固定低通滤波器为低阶固定低通滤波器，其有效频谱与高次谐波之间的距离为高中频信号中心频率或其整数倍。
  6. 根据权利要求1所述的二次变频调制系统，其特征是，所述的第二固定低通滤波器为低阶固定低通滤波器，其有效频谱与镜像频谱之间的距离为固定本振与所需的调制频率的差的2倍。记固定本振的高中频为fH,所需的调制频率为fC，则有效频谱与镜像频谱之间距离为2×（fH-fC ）。
  7. 一种根据上述任一权利要求所述系统的变频方法，其特征在于，包括以下步骤：
    第一步、I路基带数据和Q路基带数据依次经过双口数模转换器转换成模拟信号，再经过双口固定低通滤波器滤除高次谐波后得到模拟频谱位于基带的I路模拟信号和Q路模拟信号；
    第二步、采用固定本振正交变频器对I路模拟信号和Q路模拟信号进行第一次直接变频，即零中频变频后，将频谱搬移到中心频率为fH的高中频处，对应的镜像频谱被直接抵消且其高次谐波分别位于中心频率的整数倍；
    第三步、通过第一固定低通滤波器滤除第一次直接变频产生的高次谐波；
    第四步、通过变化本振变频器产生频率为fH-fC 的本振信号并对第三步得到的模拟信号进行第二次变频，将频谱分别搬移到fC以及2（fH-fC )；
    第五步、通过第二固定低通滤波器，滤除镜像频谱2(fH-fC )，剩余频谱fC为所需的调制频率。
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