CN102510264A - Digital down converter and its realization method - Google Patents

Abstract

The invention belongs to the technical field of the communication, discloses a digital down converter and its realization method. The digital down converter comprises: an A/D sampler (21) for receiving an analog intermediate-frequency signal and converts the analog intermediate-frequency signal into a digital signal; a digital control oscillator (22) for receiving the digital signal, generating two paths of digital local oscillation signals, performing the digital frequency-mixing between the two paths of digital local oscillation signals and the digital signal, outputting the frequency-mixed signal to a finite impulse response low-pass filter (24), wherein the two paths of the digital local oscillation signals are respectively triangular wave signals and the finite impulse response low-pass filter (24) for receiving the digital frequency-mixed signal from the digital control oscillator (22) to accomplish the digital down conversion. Because the hardware resources needed to generate the triangular wave signals are very few and compared with the general digital down converter using sine, cosine signals as the local oscillation, the scale and the cost of the hardware are reduced.

Description

Digital down converter and its implementation

Technical field

The present invention relates to wireless communication field, be specifically related to a kind of implementation method of digital down converter.

Background technology

Software radio has become the developing direction of the generally acknowledged modern radio communication technology of industry owing to possess the unrivaled superiority of conventional wireless power technology.Desirable software radio system is stressed the opening and the programmability of architecture, stresses the flexibility of hardware circuit, the as close as possible antenna of digitized processing (ADC and DAC), changes configuration, the 26S Proteasome Structure and Function of hardware through the renewal of software.At present, the technology of directly radio frequency (RF) the being sampled universal commercialization of still being unrealized is the method that generally adopts and carry out digitlization with digital frequency converter at intermediate frequency; Its main thought is that digital mixer multiplies each other in multiplier with the single-frequency local oscillation signal and the input sample signal of discretization, again through interpolation or filtering extraction; Consequently; Input signal spectrum is moved required frequency band, and the also corresponding change of data rate is done further processing for subsequent module.And on the numeral/low-converter is exactly the critical component of software radio.

Traditional digital down converter consists of the following components: digital controlled oscillator, digital mixer, decimation filter, FIR (finite impulse response) low pass filter (LPF).As the digital controlled oscillator of the core component of digital down converter, its implementation has two kinds: look-up table and CORDIC (rotation of coordinate numerical calculation method) algorithm.Though look-up table realizes that principle is fairly simple, consume bigger memory resource, and permissible accuracy is high more, the memory resource of consumption increases exponentially.Cordic algorithm utilization plus-minus and shift operation are through repeatedly iterative manner realization.This algorithm use pipeline organization, its range of application often receive the restriction of the pipeline organization that is adopted again.

The structure of shown in Figure 1 is traditional digital down converter; Its course of work is following: receiver is received analog if signal; Behind A/D sampler 11 ', the digital local oscillation signal that produces with digital controlled oscillator 12 ' (just, cosine wave), completion digital mixing process in digital mixer 13 '; The output signal of digital mixer 13 ' will obtain two ways of digital signals I/Q through FIR low pass filter 14 ', accomplish the process of Digital Down Convert.

The present invention propose to adopt the triangular wave of 90 ° of two-way phase phasic differences replace original just, cosine wave, carry out digital mixing then.With respect to traditional digital down converter, the present invention is simple, is easier to circuit and realizes.

Summary of the invention

Technical problem: the object of the present invention is to provide a kind of hardware to realize simple numerical low-converter and its implementation.

Technical scheme: digital down converter provided by the invention, this digital down converter comprises the A/D sampler, is used to accept analog if signal and converts thereof into digital signal; Digital controlled oscillator; Be used to accept said digital signal; Produce two-way numeral local oscillation signal; And said two-way numeral local oscillation signal carried out digital mixing with digital signal, and the signal behind the digital mixing is exported to the finite impulse response low pass filter, the digital local oscillation signal of said two-way is respectively a triangular signal; The finite impulse response low pass filter is used to accept the signal after the word mixing of digital controlled oscillator output, accomplishes Digital Down Convert; The A/D sampler, digital controlled oscillator, digital mixer, the finite impulse response low pass filter is formed receiver.

What preferably, digital controlled oscillator produced is that two-way numeral local oscillation signal is the triangular signal of 90 ° of phase phasic differences.

The present invention also provides a kind of digital down converter implementation method; This method comprises the steps: that the A/D sampler receives analog if signal and convert thereof into digital signal; Digital controlled oscillator receives said digital signal, and produces two-way numeral local oscillation signal, and this two-way numeral local oscillation signal digital signal is accomplished the digital mixing process in digital mixer; The signal of digital mixer after with digital mixing exported to the finite impulse response low pass filter; To obtain two ways of digital signals through the finite impulse response low pass filter, and accomplish the process of Digital Down Convert, said two-way numeral local oscillation signal is respectively a triangular signal.

What preferably, digital controlled oscillator produced is that two-way numeral local oscillation signal is the triangular signal of 90 ° of phase phasic differences.

Beneficial effect: the digital controlled oscillator of digital down converter disclosed by the invention adopts the triangular wave of 90 ° of two-way phase phasic differences as local oscillation signal; Because available the adding of generation of triangular wave sample value/down counter is realized; Compare the greatly big complexity that has reduced the circuit realization with traditional look-up table and cordic algorithm.

Description of drawings

The structure chart of the digital down converter that Fig. 1 is traditional;

The structure chart of Fig. 2 digital down converter of the present invention.

Among the above figure:

A/D sampler 21, digital controlled oscillator 22, digital mixer 23, FIR low pass filter 24.

Embodiment

The present invention will be described below with reference to accompanying drawings.

1, referring to Fig. 2, digital down converter, this digital down converter comprises

A/D sampler 21 is used to accept analog if signal and converts thereof into digital signal;

Digital controlled oscillator 22; Be used to accept said digital signal; Produce two-way numeral local oscillation signal; And said two-way numeral local oscillation signal carried out digital mixing with digital signal, and the signal behind the digital mixing is exported to finite impulse response low pass filter 24, the digital local oscillation signal of said two-way is respectively a triangular signal;

Finite impulse response low pass filter 24 is used to accept the signal after the word mixing of digital controlled oscillator 22 outputs, accomplishes Digital Down Convert;

A/D sampler 21, digital controlled oscillator 22, digital mixer 23, finite impulse response low pass filter 24 is formed receiver.

What digital controlled oscillator 22 produced is that two-way numeral local oscillation signal is the triangular signal of 90 ° of phase phasic differences.

The digital down converter implementation method; This method comprises the steps: that A/D sampler 21 receives analog if signal and convert thereof into digital signal; Digital controlled oscillator 22 receives said digital signal, and produces two-way numeral local oscillation signal, and this two-way numeral local oscillation signal digital signal is accomplished the digital mixing process in digital mixer 23; Digital mixer 23 is exported to finite impulse response low pass filter 24 with the signal behind the digital mixing; To obtain two ways of digital signals through finite impulse response low pass filter 24, and accomplish the process of Digital Down Convert, said two-way numeral local oscillation signal is respectively a triangular signal.

What digital controlled oscillator 22 produced is that two-way numeral local oscillation signal is the triangular signal of 90 ° of phase phasic differences.

Shown in Figure 2 is the structure of digital down converter of the present invention; Its work is constructed as follows: receiver is received analog if signal; After A/D sampler 21; The digital local oscillation signal that the digital signal that obtains and digital controlled oscillator 22 produces (two-way is the triangular wave of 90 ° of phasic differences mutually); In digital mixer 23, accomplish the digital mixing process, the output signal of digital mixer 23 will obtain two ways of digital signals I/Q through FIR (finite impulse response) low pass filter 24, accomplish the process of Digital Down Convert.

Receiver is by A/D sampler 21, digital controlled oscillator 22, and digital mixer 23, FIR low pass filter 24 is formed.

Through relatively can finding, the present invention is with respect to traditional digital down converter, the innovation part be digital controlled oscillator produce triangular wave replace original just, cosine wave.In the circuit of reality, the realization of triangular wave will more than just, cosine wave is simply many, therefore can reduce the complexity that circuit is realized, economize on hardware resource.

Embodiment

The using value of digital down converter of the present invention will be described with the instance of a Digital Down Convert below.

Digital down converter provided by the invention; The digital controlled oscillator (NCO) that adopts the triangular signal sample value of 90 ° of two-way phase phasic differences to replace original traditional digital down converter produce just, the cosine signal sample value; Through FIR (finite impulse response) low pass filter higher harmonic components is filtered out then, only keep baseband signal component.

Suppose to receive analog if signal

Frequency domain representation is Obtain signal through the A/D sampling

ω wherein _cRepresent carrier frequency,

Represent phase place, T _sRepresent the sampling interval.Traditional digital controlled oscillator produces be two-way just, cosine signal, this two paths of signals can be with duplicate signal

expression of shaking.Carry out digital mixing; Be real signal y (n) with just, the duplicate that constitutes of cosine wave shakes after signal multiplies each other, obtain exporting signal frequency-domain and represent:

ω in the following formula _cRepresent carrier frequency, ω _sRepresent sample frequency, Represent phase place, T _sRepresent the sampling interval.

Complex baseband signal frequency domain representation through obtaining behind the ideal low-pass filter is:

ω in the following formula _sRepresent sample frequency, ω represents frequency,

Represent phase place, T _sRepresent the sampling interval.

More than be the workflow of traditional digital down converter, will introduce the workflow of digital down converter of the present invention below.

Digital controlled oscillator of the present invention produces triangular wave, is used for the two-way triangular wave f of mixing ₁(t), f ₂(t) phase difference is 90 °.

$f_1\left(t\right)=\left\{\begin{array}{cc}4f_{c}t& 0\&le;t<\frac{1}{4}T\\ -{4f}_{c}t+2& \frac{1}{4}T\&le;t<\frac{3}{4}T\\ {4f}_{c}t-4& \frac{3}{4}T\&le;t<T\end{array}\right.---\left(3\right)$

$f_2\left(t\right)=\left\{\begin{array}{cc}-{4f}_{c}t+1& 0\&le;t<\frac{1}{2}T\\ {4f}_{c}t-3& \frac{1}{2}T\&le;t<T\end{array}\right.---\left(4\right)$

F in the following formula _cRepresent carrier frequency, T represents triangle wave period.

These two signals are carried out Fourier expansion:

$f_1\left(t\right)=\underset{n=1}{\overset{\&infin;}{\mathrm{\&Sigma;}}}{b}_n\sin \left({\mathrm{n\&omega;}}_{c}t\right)=\frac{8}{{\mathrm{\&pi;}}^2}[\sin \left({\mathrm{\&omega;}}_{c}t\right)-\frac{1}{3^2}\sin \left({3\mathrm{\&omega;}}_{c}t\right)+\frac{1}{5^2}\sin \left({5\mathrm{\&omega;}}_{c}t\right)+\&CenterDot;\&CenterDot;\&CenterDot;]---\left(5\right)$

$f_2\left(t\right)=\underset{n=1}{\overset{\&infin;}{\mathrm{\&Sigma;}}}{a}_n\cos \left({\mathrm{n\&omega;}}_{c}t\right)=\frac{8}{{\mathrm{\&pi;}}^2}[\cos \left({\mathrm{\&omega;}}_{c}t\right)+\frac{1}{3^2}\cos \left({3\mathrm{\&omega;}}_{c}t\right)+\frac{1}{5^2}\cos \left({5\mathrm{\&omega;}}_{c}t\right)+\&CenterDot;\&CenterDot;\&CenterDot;]---\left(6\right)$

Wherein,

N is an odd number; ω _c=2 π f _cRepresent carrier frequency.

Make f (t)=f ₁(t)+jf ₂(t), frequency domain representation is:

$F\left(\mathrm{\&omega;}\right)=\frac{8}{{\mathrm{\&pi;}}^2}\underset{n=1}{\overset{\&infin;}{\mathrm{\&Sigma;}}}\frac{1}{n^2}\&CenterDot;2\mathrm{\&pi;}\&CenterDot;\mathrm{\&delta;}[\mathrm{\&omega;}-{(-1)}^{\frac{n+3}{2}}{\mathrm{N\&omega;}}_c]$ Wherein n is an odd number

(7)

$=\frac{8}{{\mathrm{\&pi;}}^2}\&CenterDot;2\mathrm{\&pi;}\&CenterDot;\underset{m=0}{\overset{\&infin;}{\mathrm{\&Sigma;}}}\frac{1}{{(1+2m)}^2}\mathrm{\&delta;}[\mathrm{\&omega;}-{(-1)}^m(1+2m){\mathrm{\&omega;}}_c]$

ω represents frequency in the following formula, ω _sRepresent sample frequency.

Carry out digital mixing then; Be real signal y (n) and duplicate shakes after signal

multiplies each other, the output signal frequency-domain is represented as follows:

ω in the following formula _sRepresent sample frequency, ω _cRepresent carrier frequency, ω represents frequency,

Represent phase place, T _sRepresent the sampling interval.

Complex baseband signal frequency domain representation through obtaining behind the ideal low-pass filter is:

ω in the following formula _sRepresent sample frequency, ω represents frequency,

Represent phase place, T _sRepresent the sampling interval.

Can find out from formula (2) and formula (7); Down-conversion adopts triangular wave and is just adopting, the output signal frequency domain expression formula during cosine wave only differs a constant

this is that capacity volume variance by two kinds of signals causes, and the part of the gain that the system that can be used as is total is revised.

Consider the imperfect of LPF, when assessment of system performance, Error Vector Magnitude (EVM) index evaluation commonly used is to the influence degree of signal.EVM is defined as the amplitude of the error vector between actual signal vector and ideal signal vector, is an overall target that characterizes actual signal and ideal signal difference.Error Vector Magnitude is generally represented with the percentage of the ratio of error vector and desirable amplitude of the vector:

${\mathrm{EVM}}_{\mathrm{RMS}}={\left[\frac{\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}[{|I_n-I_{0,n}|}^2+{|Q_n-Q_{0,n}|}^2]}{\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}[{I_{0,n}}^2+{Q_{0,n}}^2]}\right]}^{\frac{1}{2}}---\left(10\right)$

In following formula, I _{0, n}, Q _{0, n}Be the I/Q road signal amplitude value of ideal signal, I _n, Q _nI/Q road signal amplitude value for reality.In this article, I _{0, n}, Q _{0, n}For adopting the I/Q tributary signal range value of multiple sinusoidal signal as the Digital Down Convert output of local oscillator, I _n, Q _nFor adopting the I/Q tributary signal range value of multiple triangular signal as the Digital Down Convert output of local oscillator.

Short-wave radio set at 2MHz-30MHz is widely used, and its radio-frequency front-end passes through preselection filter, low noise amplifier, is mixed to the 10.7MHz intermediate frequency, and its signal bandwidth is 8kHz.Be without loss of generality, choose linear FM signal

As test signal, carrier frequency f wherein _c=10.7MHz, df=18KHz, T=0.01s represent the pulse duration of LFM (linear frequency modulation) signal, through A/D sampler 22, sample rate f _s=3.13MHz.Obtain digital signal T _sRepresent the sampling interval.Digital controlled oscillator 22 produces periodic triangular wave, 90 ° of two-way triangular wave phase phasic differences, and note is made f respectively ₁(t), f ₂(t).Carry out mixing through digital mixer 23, promptly be x (n) respectively with f ₁(n), f ₂(n) multiply each other, obtain digital signal Q (orthogonal signalling)/I (in-phase signal), respectively through identical FIR low pass filter 24, just can obtain baseband signal I/Q then.

FIR low pass filter 24 will be enough to the higher harmonic components in the filtering formula (8).Therefore, with f _sWith f _cSubstitution formula (8) knows that the minimum non-zero base band component is positioned at the 20KHz place, himself decays to be-71.4dB; Inferior minimum non-zero base band component is positioned at the 50KHz place, himself decays to be-71.8dB; The 3rd minimum non-zero component is positioned at the 90KHz place, himself decays to be-68.7dB; Therefore, the FIR low pass filter that is designed is wanted and can be decayed at least at the 20KHz place-8.6dB, decays at least at the 50KHz place-8.2dB, decay at least at the 90KHz place-11.3dB ... Or the like.Therefore, can obtain the design parameter of FIR low pass filter: use kaiser window, cut-off frequecy of passband is 16KHz, and the stopband initial frequency is 78KHz, and stopband attenuation is-80dB that the FIR low pass filter exponent number that comprehensively goes out is 117.

Adopt foregoing LFM signal and 117 rank FIR low pass filters, two kinds of digital down converter schemes are analyzed emulation.With adopt multiple triangular signal as the output signal of local oscillator as I _n, Q _n, adopt multiple sinusoidal signal as the output signal of local oscillator as I _{0, n}, Q _{0, n}The digital baseband signal that substitution (10) formula calculates and the Error Vector Magnitude EVM of theoretic digital baseband signal _RMS=0.0951%, visible distortion is very little, can satisfy the technical requirement of actual engineering fully.

The above is merely preferred embodiments of the present invention; Protection scope of the present invention is not exceeded with above-mentioned execution mode; As long as the equivalence that those of ordinary skills do according to disclosed content is modified or changed, all should include in the protection range of putting down in writing in claims.

Claims (4)

1. digital down converter, it is characterized in that: this digital down converter comprises

A/D sampler (21) is used to accept analog if signal and converts thereof into digital signal;

Digital controlled oscillator (22); Be used to accept said digital signal; Produce two-way numeral local oscillation signal; And said two-way numeral local oscillation signal carried out digital mixing with digital signal, and the signal behind the digital mixing is exported to finite impulse response low pass filter (24), the digital local oscillation signal of said two-way is respectively a triangular signal;

Finite impulse response low pass filter (24) is used to accept the signal after the word mixing of digital controlled oscillator (22) output, accomplishes Digital Down Convert;

A/D sampler (21), digital controlled oscillator (22), digital mixer (23), finite impulse response low pass filter (24) is formed receiver.

2. digital down converter according to claim 1 is characterized in that: what digital controlled oscillator (22) produced is that two-way numeral local oscillation signal is the triangular signal of phase phasic difference.

3. digital down converter implementation method; It is characterized in that: this method comprises the steps: that A/D sampler (21) receives analog if signal and convert thereof into digital signal; Digital controlled oscillator (22) receives said digital signal; And generation two-way numeral local oscillation signal; This two-way numeral local oscillation signal digital signal is accomplished the digital mixing process in digital mixer (23), digital mixer (23) is exported to finite impulse response low pass filter (24) with the signal behind the digital mixing, will obtain two ways of digital signals through finite impulse response low pass filter (24); Accomplish the process of Digital Down Convert, said two-way numeral local oscillation signal is respectively a triangular signal.

4. digital down converter implementation method according to claim 3 is characterized in that: what digital controlled oscillator (22) produced is that two-way numeral local oscillation signal is the triangular signal of phase phasic difference.

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