The receiver is controlled byantennaThe received electromagnetic signal is sent to the receiver.The ideal receiver suppresses all unwantedNoise, including other signals, and do not add any noise or interference to the required signal.Regardless of the form or format of the signal, it can be transformed to suit the characteristics required by the detection circuit of the signal processor, and then sent to the intelligent user interface.[1]
The receiver has two basic structures, one is superheterodyne structure, and the other is direct conversion structure.
so-calledSuperheterodyne receiver, that is, the receivedRF signalConduct with local oscillator signal of a certain frequencymixing orDown conversionThen an intermediate frequency modulation signal with a lower frequency is output. The frequency of the intermediate frequency signal is the fixed frequency difference between the frequency of the LO signal and the received signal.The final signal demodulation is to filter and amplify the IF signalDemodulatorDone.
In the direct conversion receiver, its basic principle is the same as that of the heterodyne receiver. The difference is that the local frequency involved in the mixing is not arbitrarily given, but equal to the carrier frequency. In this way, the IF frequency is 0, so there is no image frequency and image frequency interference. This scheme is usually called zero IF scheme.The RF part of the scheme omits the mirror frequency filter, while the IF filter becomeslow pass filter , simplifies the composition of the system, reduces the difficulty of design and implementation, and saves costs.However, the direct conversion receiver has obvious disadvantages. For example, because the frequency of the local oscillator is the same as the carrier frequency, it is easy to cause local oscillator leakage;There is DC deviation, which has a serious impact on the subsequent circuit;It can only be used for demodulation of amplitude modulation signal, and cannot demodulate FM and phase modulation signal;The local oscillator requires high stability, so most of them are only used in the low frequency band.[2]
function
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The main function of the receiver is that there are manyelectromagnetic wave, choose what you needfrequencyIngredient, inhibiting or filtering unwantedsignalorNoiseAndinterference signalAnd then get the original useful information through amplification and demodulation.[2]
Characteristic test
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IEEE 802.16d.2004 and IEEE 802.16e.2005 define the receiver test requirements, including:
(2) Adjacent and alternate channel suppression of receiver;
(3) Maximum input signal of receiver;
(4) Maximum allowable signal of receiver;
(5) Receiver image suppression.
These five receiver test items can be used to measure the deviation of receiver sensitivity.The adjacent and alternate channel suppression test of the receiver requires an additional RF source as the interference signal source.This interference source must be a standard signal modulated by OFDM[3]
classification
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Use classification
1、Navigation receiver。This type of receiver is mainly used for the navigation of moving carriers, which can give the position and speed of the carrier in real time.This kind of receiver generally adopts C/A codePseudo range measurementThe single point real-time positioning accuracy is low, generally ± 25mm, and ± 100mm with SA effect.This kind of receiver is cheap and widely used.According to different application fields, such receivers can also be further divided into: vehicle mounted type - used for vehicle navigation and positioning;Navigation type - used for ship navigation and positioning;Aviation type - used for aircraft navigation and positioning.Due to the high speed of aircraft, receivers used in aviation are required to adapt to high-speed movement.Spaceborne type - used for navigation and positioning of satellites.Since the speed of the satellite is more than 7km/s, the requirements for the receiver are higher.
2、Geodesic receiver。Geodesic receivers are mainly used for precision geodesy andPrecision engineering survey。High positioning accuracy.The instrument is complex in structure and expensive.Time service receiverThis kind of receiver mainly uses the high-precision time standard provided by GPS satellites for time service, and is commonly used in observatories andradioIn communicationtime synchronization 。
Carrier frequency classification
1. Single frequency receiver.The single frequency receiver can only receive L1 carrier signal and measure the carrier phase observation value for positioning.Because the influence of ionospheric delay cannot be effectively eliminated, the single frequency receiver is only suitable for precise positioning of short baselines (<15km).
2. Dual frequency receiver.Dual frequency receiver can receive L1 and L2 carrier signals at the same time.The ionospheric pair can be eliminated by making use of the difference between the dual frequency and the ionospheric delayelectromagnetic waveBecause of the influence of signal delay, dual frequency receiver can be used for precise positioning of thousands of kilometers.
With channel classification
1. Multichannel receiver
2. Sequential channel receiver
3. Multi channel receiver
Operating principle classification
1. Code correlation receiver.Code correlation receiver uses code correlation technology to obtain pseudo range observations.
2、Square receiver。The square receiver usescarrierThe square technique of the signal removes the modulated signal to recover the complete carrier signal. The phase difference between the generated carrier signal and the received carrier signal in the receiver is measured by the phase meter to determine the pseudo range observation value.
3. Hybrid receiver.This instrument combines the advantages of the above two receivers and can obtain the code phasePseudo range, you can also getcarrier phase Observations.
4、Interferometric receiver。This receiver uses GPS satellite as radio source and interferometry method to measure the distance between two stations.
