CN101833100B - Method for constructing fully-digital GNSS compatible navigation receiver - Google Patents
Method for constructing fully-digital GNSS compatible navigation receiver Download PDFInfo
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Abstract
The invention relates to a method for constructing a fully-digital GNSS compatible navigation receiver, which comprises four steps: I, constructing a receiving link in accordance with actual needs by taking the characteristics of satellite signals, performance indexes and current device level into comprehensive consideration; II, choosing a proper A/D chip and a proper time clock source to complete direct radio frequency sampling and constructing a sampling rate in accordance with bandwidth sampling requirements; III, constructing a filter decimation network to reduce the sampling rate, completing the down-conversion of the radio frequency, converting the frequency of the bandwidth satellite navigation signals near 1.2G and 1.5G two frequency points down to a low medium frequency under a condition of a high sampling rate, and finally outputting a sampling time clock and a GNSS medium frequency signal; and IV, constructing digital radio frequency front end and rear end medium frequency receiver interfaces for making the receiver compatible with other medium frequency receiver. The thought of using software radio realizes the collective receiving of full-range GNSS satellite navigation signals; and the medium frequency receivers can complete navigation solution and output measurement values, so multi-system and multi-frequency point compatible navigation is realized. The method has an application and development prospect in the technical field of communication.
Description
(1) technical field:
The present invention relates to a kind of construction method of fully-digital GNSS compatible navigation receiver, it is relevant with GPS, satellite navigation receiver and software radio research direction, belongs to communication technical field.
(2) background technology:
In recent years, the construction of GNSS satellite navigation system has had very big development, in every field to low cost, high-performance, the demand of easy-to-use GNSS compatible receiver increases greatly flexibly.General navigation neceiver all adopts the multistage down coversion structure of simulation, just carries out digitizing and signal Processing after downconverting to intermediate frequency, and sort signal receive mode very flexible is introduced error easily.And need not be signal structure, system upgrading exchange device more again based on the GNSS receiver of software radio thought; Only need unified hardware platform be provided for the user; Mode with software upgrading updates to navigation neceiver, has that cost is low, function upgrading is expanded advantage easily.
The key idea of software radio is with the as close as possible antenna of A/D, accomplishes signal processing function as much as possible with software.And utilized this idea just based on the GNSS receiver structure of radio frequency Direct Sampling, it has, and required device is few, cost is low, low in energy consumption, obtain high performance advantage easily, be a kind of full digital navigation neceiver framework.Than the radio-frequency front-end of the multistage down coversion of conventional analogue, the radio-frequency front-end of radio frequency Direct Sampling does not need complicated mixing conceptual design; For new satellite-signal system, only need to select suitable sampling rate and BPF.; GNSS receiver for the many constellations of multifrequency does not need multichannel radio-frequency front-end, has reduced device and has taken, and has eliminated the potential interference of interchannel; Can not look like superheterodyne receiver and introduce extra phase error.Research both at home and abroad shows that compare with traditional receiver architecture, digitized radio-frequency front-end has identical performance.For the sampling of certain or certain several narrow band signals, sampling rate is eligible lower, and signal is accomplished down coversion through the mode of sampling; But for Sampling for Wide-Band Signal, like whole navigation frequency band signals, the sampling rate of choosing is often than higher, but present A/D and subsequent device level are difficult to reach very high clock rate.The full digital navigation neceiver need be taken all factors into consideration sampling rate and existing device level.
Idea of Software Radio is proposed by Mi Tela (J.Mitola) the earliest.And be proposition first in 1994 such as Blang (Brown) people of etc.ing based on the GNSS receiver mentality of designing of direct bandpass sampling technology.The receiver of design was used to accomplish the space measurement task at that time, like gravimetry and atmospheric envelope monitoring etc.These tasks have proposed new requirement to the consistance of the phase stability of the tracking performance of receiver and L1 and L2 wave band, pseudorange and carrier wave measurement etc.Designed a kind of totally digitilized front end and replaced the simulation frequency conversion front end that constitutes by mixing device and frequency synthesizer etc. in the Current GPS receiver in order to reach people such as this Blang of requirement; The A/D sampling rate of 800MHz has been proposed in the literary composition; Receive the restriction of semiconductor technology at that time; Do not have commercial A/D device to use, the collection of data is realized by the AD chip of special use, can only realize at that time that 1bit quantized.
This people such as (Dennis M.Akos) of Acker in 1999 a kind of Front-end Design method that can receive two or more separate signal GNSS receivers has been proposed on people's such as Blang basis; Promptly use selectivity bandpass sampling technology; Only select interested useful signal to fold into final Nyquist frequency band the inside; What adopted at that time is TRW AMAD-7 chip, and quantization digit is up to 4bit, has realized the collection of GPS L1 signal and GLONASS L1 signal.This people such as grade of Acker had built complete GPS L1 and L3 waveband radio frequency Direct Sampling data acquisition system (DAS) through the MAX104 ADC evaluation board and the FPGA development board of commercialization in 2002; And through catching of signal that the real gps data that collects is finished dealing with; Verified the superiority of radio frequency Direct Sampling technology again; Data acquisition board adopts the DMA transmission interface, and flank speed is 132MB/s.The Zi Yaji of Cornell University in 2003 people such as (Mark L.Psiaki) uses business-like component-assembled to realize the data acquisition system (DAS) that can receive L1C/A sign indicating number and L2 band signal based on selectivity bandpass sampling technology; And use the sampling clock of different frequency and different degree of stability to gather actual gps data; Then through handling these data, comparative analysis SF this how to select and clock stability to the receiver Effect on Performance.This has done the good and bad contrast of performance of directly adopting front end about down coversion front end and radio frequency again with people such as Zi Yaji Acker in 2004, and deep research is also being done by the L-3 communication synthesis system house of the Gefle university of Hispanic subsequently Univ Catalunya Politecnica, Australian University of New South Wales, Sweden, the Tampere Polytechnics of Finland, the U.S. and the Ecole Polytechnique Federale de Lausanne of Switzerland aspect the technological GNSS receiver of radio frequency Direct Sampling.
The research of domestic radio frequency Direct Sampling is started late, and research institution mainly concentrates in each universities and colleges.Like University of Electronic Science and Technology, Beijing Institute of Technology, BJ University of Aeronautics & Astronautics.The research of University of Electronic Science and Technology mainly is used in the radio communication wideband digital receiver with the radio frequency Direct Sampling, and function and complex structure realize that difficulty is big.Its method also is not suitable for the navigation signal processing.North science and engineering, Beijing Institute of Aeronautics also developed in recent years similar Acker this system and obtained good effect.
The present invention is directly near the GNSS satellite navigation signals of two frequency ranges 1.2G and 1.5G frequency, and band occupancy 187MHz through calculating, can not accomplish direct bandpass sampling and aliasing not with low sampling rate.Use for reference the multi-rate digital signal treatment technology of wireless communication system, design is to the filtering extraction separated network of satellite-signal.Through selecting suitable chip, solved the problem of system handles high sampling rate.
(3) summary of the invention:
1, purpose: the construction method that the purpose of this invention is to provide a kind of fully-digital GNSS compatible navigation receiver; It utilizes idea of Software Radio; Realize the integrated reception of full frequency band GNSS satellite navigation signals; Can accomplish navigation calculation and export observed reading by intermediate-frequency receiver, realize multisystem multifrequency point compatible navigation.
2, technical scheme:
The construction method of a kind of fully-digital GNSS compatible navigation receiver of the present invention, it mainly comprises following several steps:
Step 1: the ground received power of satellite navigation signals is lower than noise power, carry out Direct Sampling to it, and the performance requirement of analog to digital converter (A/D) is than higher, and the simulation of its front end receives link and needs particular design.At first, the present invention adopts three grades of low noise amplifiers (LNA) to amplify noise power, makes it satisfy the signal lowest power requirement of A/D.Through a broadband prefilter, leach the navigation frequency band signals behind the first order LNA; Be divided into two paths of signals through shunt after the third level LNA,, leach the navigation frequency band signals of the bandwidth 136MHz of 1.2G near and the navigation signal of the bandwidth 51MHz of 1.5GHz near respectively through a BPF.; Close the road through combiner behind the wave filter, form the signal that supplies the A/D sampling through automatic gain control (AGC) again.The noise figure of entire front end receiving circuit is by noise figure (2dB) decision of first order LNA.The input bandwidth of A/D need be greater than the input signal bandwidth.The window shake of A/D and the shake of sampling clock must be as far as possible little, otherwise will influence the signal to noise ratio (S/N ratio) after the sampling.
Step 2:, need to select suitable sample rate f for the broadband navigation signal
sWith the not mutual aliasing of the frequency spectrum that guarantees sampling back signal.By the bandpass sampling principle, the span of the sampling rate of a plurality of bandpass signals is minimum to be its all bandpass signal bandwidth sums, and the highest is the twice of highest signal frequency.Fall into [f after the sampling
s/ 2, f
s/ 2] signal spectrum was not crossed over 0 after the signal in interval need satisfy following three condition: a. sampling; B. aliasing not between the signal of sampling back; C. the sampling back highest frequency of signal in interval is no more than f
s/ 2.According to above condition, can make up a function, in Matlab, be that independent variable is asked functional value and the sample rates values that satisfies condition of record with above-mentioned sampling rate scope.Through emulation, the about 536MHz of minimum sampling rate that satisfies condition.The present invention is chosen as 744MHz with sampling rate, the requirement that can satisfy condition, and it also is convenient selecting this sampling rate and existing radio-frequency module is made performance comparison.
Step 3: since the sampling rate of selecting for present most devices (like FPGA (field programmable gate array); DSP (digital signal processor)) is difficult to handle; Therefore designed the filtering extraction network and reduced sampling rate and accomplish radio frequency down-conversion, this part can be realized in FPGA.A/D is mainly with near the broadband satellite navigation signal 1.2G and two frequencies of 1.5G; Under high sampling rate, downconvert to Low Medium Frequency, through the filtering extraction network; In conjunction with A/D half rate output characteristics, accomplish separation, down coversion and the reduction sampling rate of target frequency signal.Get signal after the sampling and can isolate the 1.5G frequency band signals through a low-pass filter; Carry out the twice extraction then and make sampling rate be reduced to the half the of 744MHz, such speed can adapt to for FPGA, and the Treatment Design that low-pass filter adds 2 times of extractions becomes efficient configuration; Earlier low-pass filter is carried out heterogeneous decomposition; Can be put into wave filter by 2 times of withdrawal devices of equivalence principle and form efficient configuration before, utilize the A/D output speed simultaneously, can two withdrawal devices be saved for the half the characteristic of sampling clock speed; In FPGA, only need make two wave filters, major clock has become f
s/ 2 rather than the 744MHz that is difficult to realize.In 1.5G frequency range interested target navigation signal times with corresponding carrier frequency accomplished down coversion thereafter; LPF leaches echo signal; The target of this moment is that signal rate is reduced to 62MHz; Also need carry out 6 times of extractions, this is to realize through first 3 times of two-stage cascade modes that extract back 2 times of extractions, and frequency overlapped-resistable filter before 3 times of extractions and low-pass filter are before merged into one and realized; 2 times of preceding frequency overlapped-resistable filters of extraction adopt half-band filter, and the signal after having extracted is through a low pass filter filters out out-of-band noise.Signal for the 1.2G frequency range then at first is to after wherein interested frequency is down-converted to base band earlier; LPF leaches echo signal; 2 times of extractions then utilize aforementioned principles can multiplication and filtering all be put into after 2 times of extractions equally, form structure efficiently.Thereafter part and similar to the processing of 1.5G frequency band signals repeats no more.
Step 4: the introduction by step 3 can know that the filtering extraction network can directly be exported the base band navigation signal and supply zero intermediate frequency reciver to handle; Can be through baseband digital signal be upconverted to intermediate frequency, the output digital medium-frequency signal supplies digital if receiver to use; Digital medium-frequency signal through D/A digital-to-analog conversion (D/A) output analog if signal, can be the analog intermediate frequency receiver signal is provided.The interface mode of digital RF fore-end and rear end intermediate-frequency receiver is flexible, can be easily compatible other intermediate-frequency receiver.The sampling rate of digital RF front end can flexible configuration with the output intermediate frequency.The present invention accomplishes the signal processing function and the ephemeris parsings such as acquisition and tracking of signal through digital if receiver, the information processing functions such as the compatible positioning-speed-measuring of the many constellations of multifrequency, and the output observed result is to the PC host computer.
3, the beneficial effect that compared with prior art has of the present invention:
1. through the radio-frequency front-end of design digitalization, realized the integrated reception of navigation frequency range satellite-signal, rather than only handled to the navigation signal of certain navigational system or some navigation signal frequency.
2. than the radio-frequency front-end of the multistage down coversion of conventional analogue, saved complicated mixing conceptual design; For the GNSS receiver of the many constellations of multifrequency, do not need traditional multichannel radio-frequency front-end, reduced device and taken, eliminated the potential interference of interchannel; Can not look like superheterodyne receiver and introduce extra phase error.
3. sampling rate is configurable flexibly with each satellite-signal IF spot.
4. for new satellite-signal system, only need to select suitable sampling rate and BPF., good extendability is arranged.
5. the filtering extraction network that separates of design multifrequency point is accomplished the multifrequency point sampling rate and is reduced, and considers the realizability of hardware in the design, the taking of resource; Considered the input and output signal to noise ratio (S/N ratio) of filtering extraction network; Reached with the perfectly straight identical performance of sampling that connects of traditional analog down and arrowband band.
(4) description of drawings:
Fig. 1. the overall design block diagram of invention
Fig. 2. navigation frequency range synoptic diagram
Fig. 3. three grades of cascade system synoptic diagram
Fig. 4. navigation frequency range nearby radio frequency range synoptic diagram
Fig. 5. the multiband bandpass sampling is synoptic diagram as a result
Fig. 6. sampling rate selection result synoptic diagram
Fig. 7. sampling back navigation signal frequency range synoptic diagram of living in
Fig. 8. filtering extraction network in general block diagram
Fig. 9 .module_1 modular design block diagram
Figure 10 .module_2 modular design block diagram
(5) embodiment:
The general frame of the present invention is as shown in Figure 1.At first (3db bandwidth 1.15～1.65GHz) receives satellite-signal, amplifies through three grades of LNAs (LNA), and wherein behind the first order LNA, through a broadband prefilter, bandwidth range approximately is the big broadband of 1.2～1.5GHz through broad-band antenna; Behind the afterbody LNA,, be divided into two-way, leach the navigation frequency band signals of the bandwidth 136MHz of 1.2G near and the navigation signal of the bandwidth 51MHz of 1.5GHz near respectively through a BPF. immediately following a shunt; Close the road through combiner behind the wave filter, frequency band such as Fig. 2 of closing behind the road show; Signal is accomplished gain control through AGC subsequently, by the A/D chip sampling of the highest support of sampling rate 1GHz, and the signal of half sampling clock speed of A/D output two-way; This signal will be accomplished and fall sampling rate and multifrequency point Signal Separation through the filtering extraction network of realizing at FPGA; FPGA output comprises data clock, can export baseband digital signal in addition, and digital medium-frequency signal is through D/A and the exportable analog if signal of filtering; At last, the digital intermediate frequency signal that intermediate-frequency receiver receives FPGA output carries out navigation calculation, and the result is uploaded to PC by serial ports.
1.A/D preceding reception link design
Because the GNSS signal adopts the spread-spectrum signal system; Signal is submerged in the noise, so the general relatively wireless communication link of reception link design before the A/D is more special, mainly considers signal gain; Noise figure; Nonlinear characteristic, the characteristic of wave filter, factors such as the dynamic range of A/D and input bandwidth.
(1) signal gain
Because the GNSS navigation signal is all covered by noise, guarantee that A/D can sample navigation signal, must signal be amplified with the LNA amplifying circuit of certain gain.
As shown in Figure 2, the total bandwidth of two navigation frequency ranges is 187MHz, and noise power N is approximately:
N＝-174+10log(187·10
6)＝-91dBmW
The maximum 800mV of the A/D chip full scale voltage chosen, the received power of lowest signal approximately-0.9dBm, signal demand amplifies about 90dB, link adopts three grades of amplifications among the present invention, every grade is amplified 30dB, full gain 90dB.
(2) dynamic range and input bandwidth
The peak signal input bandwidth of A/D is 1.7GHz, and the about 1.6G of the maximum value bandwidth of echo signal can guarantee that the full range band of signal receives.
The dynamic range of gain is mainly considered the variation of the signal power that 3 factor: a. are caused by temperature, and assumed temperature changes at-45 °～85 °, changes about 2dB; B. antenna part change in gain, approximately 10dB; C. because the about 6dB of change in gain of the radio frequency front end chip that design, technology, temperature and mains voltage variations cause; Consider the 5dB allowance, gain dynamic range is 2+10+6+5=23dB, as the AGC range of control.
(3) noise figure
Three grades of cascade systems as shown in Figure 3, its total noise figure is expressed from the next:
We can find out from top expression formula: if the gain G of first subsystem
1Very high, the influence of second and third subsystem is not a particular importance so, perhaps is eliminated the noise figure F of first subsystem
1Whole receiver noise factor F is played decisive influence; If the gain of first order subsystem is not very big, and the gain G of second level subsystem
2Very big, the noise effect of first and second grade subsystem is very big so, is determining whole receiver noise factor.In most of radio receivers, the noiseproof feature of total system is all by preceding what subsystem controls, because the noise of preceding face system can be amplified by back level subsystem.Among the present invention, noise figure depends primarily on the noise figure of first order LNA, approximately 2dB.
(4) filter characteristic
Do not have big undesired signal in the GNSS frequency band in theory, but outside band, have high reject signal, as shown in Figure 4,1.8GHz PCS signal is used GNSS and is strong undesired signal.Usually, wave filter is to satisfying more than the 30dB in the decay of the GNSS of 0-900MHz and 1800-5000MHz out-of-band-signal.If without any filtering, produce the gain of nearly 100dB, then can ADC is saturated, influence the circuit operate as normal; And out-of-band-signal can exert an influence to the whole noiseproof feature of receiver through behind the receiver link, therefore, in the receiver link, has added the double preselection wave filter among the design out-of-band-signal is suppressed.
The outer signal of wave filter among Fig. 1 behind the LNA1 main filtering navigation frequency range; Wave filter behind the shunt then leaches the broadband navigation signal of two frequency ranges.Need satisfy maximum value bandwidth 1.6G during the parameter designing of wave filter to the good characteristic of only being with is arranged between A/D input bandwidth 1.7G.
(5) jittering noise
Usually the noise source among the ADC comprises the quantizing noise (perhaps exchanging the differential nonlinearity mistake) of A/D converter, converter inside thermonoise and thrashing (jitter).The GNSS satellite signal carrier frequency is very high, thereby when signal is sampled by ADC, sampling instant inconsistent, and such as the window shake of clock jitter and ADC, the phase noise of introducing can make the end of making an uproar of ADC output signal increase, and signal to noise ratio (S/N ratio) descends.Thrashing is brought by sampling clock shake and window shake jointly
The hole ambiguity is also claimed in the window shake, is meant the uncertain of aperture time.The window shake representative ADC sampling time at random change, is by sampling and the thermonoise of holding circuit brings.The window shake is the dominance error source that restriction can reach signal to noise ratio (S/N ratio).Subsidiary its window shake explanation of the technical manual of most of ADC products.The window shake is described with root-mean-square value (rms) usually, and root-mean-square value (rms) is represented the standard deviation of window time.
The window jitter limits sinusoidal signal can be by the accurate maximum frequency of sampling of ADC.The window shake has brought the temporal uncertainty of sampled signal, has reduced the noise grade of ADC, has increased the possibility of intersymbol interference.These influences ratio direct and signal moment change level adapts, such as the slope of signal.Therefore, the Signal-to-Noise of higher frequency can be because window shake and than lower signal degradation more serious of frequency.Provide the window Jitter Calculation formula that maximum is allowed according to the frequency of input signal and the resolution of ADC.As follows:
N is a quantization digit, f
MaxBe maximum input carrier frequency, N gets 8bit here, f
MaxGet 1610MHz, can obtain maximum and allow the window jitter sigma
a=0.77ps.And the window of ADC is dithered as 0.4ps, meets the demands.
Clock jitter provides the characteristic to the clock generator of ADC clock signal.It is produced by oscillator phase, and has brought extra ADC device sampling time error.Most of high-speed communication systems comprise that radio frequency receives and transmitter, all uses phase-locked loop to carry out frequency synthesis.These systems can receive the influence of clock jitter.Clock jitter is the time range with sampled clock signal phase place random variation, and the frequency range of phase noise defines in other words.External clock can also can obtain through the outer signal source through combining chip frequently.It is little that the shake of external clock will be tried one's best, and should satisfy the electrical specification requirement of A/D simultaneously.
2. sampling rate confirms
The present invention is the navigation signal sampling to full frequency band, and as shown in Figure 2, echo signal comprises two frequency ranges, and we regard near the signal the 1.2G as a passband, and centre frequency is 1.232G, bandwidth 136MHz; 1.5G near signal is regarded a passband as, centre frequency is 1.5845G, bandwidth 51MHz.
The purpose of radio frequency Direct Sampling is with suitable sampling rate high-frequency signal to be adopted intermediate frequency, supposes to have M frequency band signals, and carrier frequency is f
Cj, j=1 ... .., M.IF-FRE after the sampling is:
Signal intermediate frequency after the sampling will be positioned at [f
s/ 2, f
s/ 2] interval, the carrier frequency that has possibly be mixed to negative value, and by symmetry, we get on the occasion of frequency and as intermediate frequency value are
As shown in Figure 5, when some frequency band signals is adopted [f
s/ 2, f
s/ 2] in the time of in the interval, can cause several kinds of situation: like f
If2Such frequency band is crossed over 0; Like f
If4Such frequency band has been crossed over f
s/ 2; Like f
If1And f
If3That kind, the mutual aliasing of frequency band.These all are that we do not hope to see.Derived several constraint conditions thus:
Here, j=1 ..., M, k=(j+1) ..., M, B represents the bandwidth of j frequency range.Various non-negative and more than must guaranteeing all more than or equal to 1, can guarantee that just each frequency range is the phase aliasing not with after the fs sampling.Above 3 formulas are joined together and can be write as:
d(f
s)＝min[a
1(f
s)，....，a
M(f
s)，b
1(f
s)，....，b
M(f
s)，
c
12(f
s)，c
13(f
s)....，c
M-1，M(f
s)]
Operational sample rate f s will be such set { f
s: d (f
s)>=1}.D (f
s) will be piecewise linear.And the scope of fs must satisfy following two formulas:
f
s≤2max(f
cj)
According to the fs of above scope, d (f can draw
s) figure and record d (f
sThe value of)>=1, as shown in Figure 6.Can see the about 536MHz of minimum sampling rate that satisfies condition.The present invention is chosen as 744MHz with sampling rate, satisfies condition equally, and selecting this sampling rate in addition also is to contrast for the performance with existing radio-frequency module.
3. filtering extraction network design
Signal spectrum through the 744MHz sampling is as shown in Figure 7, and the signal of two navigation frequency ranges is all sampled low frequency range.Through calculating, the B1 in the 1.5G frequency spectrum, the L1 frequency is sampled to 87.42MHz, and the B2 frequency in the 1.2G frequency range is adopted to 280.86MHz, and the B3 frequency is adopted to 219.48MHz, and other navigation signal sampling back frequency can calculate according to sampling rate.
Filtering extraction network in general block diagram of the present invention is as shown in Figure 8.Major function is to accomplish two navigation of 1.2G and 1.5G frequency band signals to separate and reduce sampling rate, and the signal of its generation can directly supply rear end intermediate-frequency receiver use.
The main effect of module 1 (module_1) is to leach the 1.5G frequency band signals among Fig. 8, and its principle is as shown in Figure 9.Among the last figure of Fig. 9, according to Spectrum Relationship, designing a low-pass filter can filtering 1.2G frequency band signals, after the filtering directly twice extract; If the data of A/D output 744MHz speed are difficult to handle so data of two-forty for present device, the filtering extraction of back is difficult to realize.Two width of cloth figure of Fig. 9 are of equal value on principle.In Fig. 9 figure below, carry out heterogeneous decomposition to low-pass filter earlier, then with the efficient drawing-out structure of the preposition formation of withdrawal device, the multiply-add operation that filtering is required is placed on low extraction yield one end.The A/D chip of the present invention's sampling is the design of radio frequency Direct Sampling specially, is output as the signal of two-way 1/2 speed, A in the corresponding diagram, and B 2 points, in fact module_1 just comprises 2 multiphase filters, has saved 2 withdrawal devices.
A/D output signal is through having only the signal of 1.5G frequency band after the module_1, the 1.2G signal is by filtering.Processing afterwards; To carry out mixing to the interested signal in the 1.5G frequency band, be mixed to base band (being divided into I, the Q two-way) after; Earlier through a low pass; The filtering out-of-band noise is because 3 times of extractions thereafter still need the front to add a frequency overlapped-resistable filter, so be merged into one to two low-pass filters here.2 times of the back are extracted the use half-band filter, thereafter with a FIR low-pass filter, and the signal of filtering out-of-band noise and other frequency.At last, the IQ two paths of signals respectively up-conversion to intermediate frequency export.Signal is through 12 times of extractions, and output sampling rate is 62MHz.
Take to be mixed to earlier base band (I, Q two-way) for the signal in the 1.2G frequency range, low pass leaches out-of-band noise, carries out twice earlier and extracts, and principle and module_1 are similar, and the function of module_2 is shown in figure 10.The last figure of Figure 10 needs a multiplier to accomplish mixing; But the highest clock rate of choosing of its multiplier of fpga chip is 500MHz; The thought here remains a multiplication filtering and is placed on low sampling rate one end, carries out heterogeneous decomposition to the low-pass filter behind the multiplier, withdrawal device reach one-level; Constitute efficient configuration, shown in figure among Figure 10.Figure 10 figure below is after moving on to withdrawal device to multiplier, can see A, 2 output signals that can directly utilize A/D 1/2 speed again of B.The design of filtering extraction behind the Module_2 with above similar, repeat no more.
Processing for the 1.2G frequency band signals can also be passed through Hi-pass filter filtering 1.5G low-band signal, and direct then 2 times of extractions are difficult to form efficient configuration but this Hi-pass filter adds the form of withdrawal device, so do not adopt this mode.Accomplished the separation of navigation frequency band signals through the filtering extraction Network Design; Down coversion and reduction sampling rate; Insert in can extracting through fraction and accomplish any sampling rate signal output,, accomplish the configurable flexibly of IF spot through the different frequency of up-conversion is set.
Realize among the FPGA of filtering extraction network in Fig. 1,, in the design of attention in design low-pass filter, must prevent too much noise aliasing for later chipization lays the foundation, can not aliasing noise in the band, to guarantee output signal-to-noise ratio.
4. with the interface of intermediate-frequency receiver and other function
The present invention selects digital if receiver to receive as navigation signal; Separate the terminal of mediation information processing, it receives digital medium-frequency signal, can merge a plurality of systems and frequency; Test result is through the PC storage and show that the while can be made performance comparison with existing analog radio frequency front end easily.In addition, FPGA can directly export digital baseband signal, supplies zero intermediate frequency reciver to use; Can also data elder generation buffer memory be stored to PC through bus, supply the poster processing soft to use.
Claims (1)
1. the construction method of a fully-digital GNSS compatible navigation receiver; It is characterized in that: these method concrete steps are following: step 1: the ground received power of satellite navigation signals is lower than noise power; To carry out Direct Sampling to it; Analog to digital converter be the performance requirement of A/D than higher, the simulation of its front end receives link and needs particular design; At first, adopting three grades of low noise amplifiers is that LNA amplifies noise power, makes it satisfy the signal lowest power requirement of A/D; Through a broadband prefilter, leach the navigation frequency band signals behind the first order LNA; Be divided into two paths of signals through shunt after the third level LNA,, leach the navigation frequency band signals of the bandwidth 136MHz of 1.2G near and the navigation signal of the bandwidth 51MHz of 1.5GHz near respectively through a BPF.; Closing the road through combiner behind the wave filter, is that AGC forms the signal that supplies the A/D sampling through automatic gain control again; The noise figure of entire front end receiving circuit is by the noise figure 2dB decision of first order LNA; The input bandwidth of A/D need be greater than the input signal bandwidth; The window shake of A/D and the shake of sampling clock must be as far as possible little, otherwise will influence the signal to noise ratio (S/N ratio) after the sampling;
Step 2:, need to select suitable sample rate f for the broadband navigation signal
sWith the not mutual aliasing of the frequency spectrum that guarantees sampling back signal; By the bandpass sampling principle, the span of the sampling rate of a plurality of bandpass signals is minimum to be its all bandpass signal bandwidth sums, and the highest is the twice of highest signal frequency; Fall into [f after the sampling
s/ 2, f
s/ 2] signal spectrum was not crossed over 0 after the signal in interval need satisfy following three condition: a. sampling; B. aliasing not between the signal of sampling back; C. the sampling back highest frequency of signal in interval is no more than f
s/ 2; According to above condition, make up a function, in Matlab, be that independent variable is asked functional value and the sample rates values that satisfies condition of record with above-mentioned sampling rate scope; Through emulation, the about 536MHz of minimum sampling rate that satisfies condition; Sampling rate is chosen as 744MHz, the requirement that can satisfy condition, it also is convenient selecting this sampling rate and existing radio-frequency module is made performance comparison;
Step 3: because the sampling rate of selecting is difficult to handle for present most devices, has therefore designed the filtering extraction network and reduced sampling rate and accomplish radio frequency down-conversion, this part at the scene programmable gate array be to realize among the FPGA; Modulus converter A/D is with near the broadband satellite navigation signal 1.2G and two frequencies of 1.5G; Under high sampling rate, downconvert to Low Medium Frequency, through the filtering extraction network; In conjunction with A/D half rate output characteristics, accomplish separation, down coversion and the reduction sampling rate of target frequency signal; Get signal after the sampling and isolate the 1.5G frequency band signals through a low-pass filter; Carry out the twice extraction then and make sampling rate be reduced to the half the of 744MHz, such speed adapts to for FPGA, and the Treatment Design that low-pass filter adds 2 times of extractions becomes efficient configuration; Earlier low-pass filter is carried out heterogeneous decomposition; Be put into wave filter by 2 times of withdrawal devices of equivalence principle and form efficient configuration before, utilize the A/D output speed simultaneously, two withdrawal devices are saved for the half the characteristic of sampling clock speed; In FPGA, only need make two wave filters, major clock has become f
s/ 2 rather than the 744MHz that is difficult to realize; In 1.5G frequency range interested target navigation signal times with corresponding carrier frequency accomplished down coversion thereafter; LPF leaches echo signal; The target of this moment is that signal rate is reduced to 62MHz; Also need carry out 6 times of extractions, this is to realize through first 3 times of two-stage cascade modes that extract back 2 times of extractions, and frequency overlapped-resistable filter before 3 times of extractions and low-pass filter are before merged into one and realized; 2 times of preceding frequency overlapped-resistable filters of extraction adopt half-band filter, and the signal after having extracted is through a low pass filter filters out out-of-band noise; Signal for the 1.2G frequency range then at first is to after wherein interested frequency is down-converted to base band earlier; LPF leaches echo signal; 2 times of extractions then; Utilize aforementioned principles that multiplication and filtering all are put into after 2 times of extractions equally, form structure efficiently, part thereafter is with identical to the handling principle of 1.5G frequency band signals;
Step 4: can know that by step 3 the filtering extraction network is directly exported the base band navigation signal and supplied zero intermediate frequency reciver to handle; Through baseband digital signal is upconverted to intermediate frequency, the output digital medium-frequency signal supplies digital if receiver to use; Digital medium-frequency signal is exported analog if signal through digital to analog converter D/A; For the analog intermediate frequency receiver provides signal; The interface mode of digital RF fore-end and rear end intermediate-frequency receiver is flexible; Compatible easily other intermediate-frequency receiver, the sampling rate of digital RF front end and output intermediate frequency flexible configuration; Accomplish the acquisition and tracking and the ephemeris of signal through digital if receiver and resolve, the compatible positioning-speed-measuring information processing function of the many constellations of multifrequency, the output observed result is to the PC host computer.
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