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In recent years, GPS global positioning system In departments such as China?s military affairs, traffic, post and telecommunications, ground ore, colliery, petroleum, building and agriculture, meteorological phenomena, land management, finance, public security men,etc. and trade, and in aviation boat very much and time service of minuting, physics prospect and posture determine field used widely. The material base of its application is GPS receivers of different types. The material base of its application is GPS receivers of different types

Rough statistics, China applies to GPS receivers of different types of each field in several million every year at present, among them rely mainly on importing in foreign countries more. Though a lot of units enter this field in China, review GPS receiver that China produces, GPS core used orients the products and mainly comes from U.S.A., Japan, South Korea and Taiwan and other places. In order to get rid of the present passive state under one?s control, design the navigation receiver with independent intellectual property right to be imperative. It is a key part which receives flexible attitude performance as GPS front circuit of radio frequency of receiver of important component of receiver. A lot of index of it, the intersection of noise and coefficient, dynamic range, mirror inhibit from frequently and 1dB compress some and the intersection of phase place and noise,etc., influence the performance of the receiver directly. So, it is very important to estimate the accuracy * of GPS receiver performance the accurate measurement of the radio frequency index. There should be receiver of the independent intellectual property right, there must be the methods of testing of a set of intact and effective module indexes of radio frequency

Basic demand that GPS signal tests

GPS signal generally uses the wave band of two radio frequencies: A signal frequency is 1575.42MHz L1 wave band ,Frequency 1227.6MHz of another signal L2 wave band . Generally speaking, the wave band that the commercial GPS receiver uses is L1 wave band. Receiver receive minimum the intersection of signal and consumption arrive 130dBm, very much faint signal this for 133dBm, flood in the noise. Measuring instrument and equipment and fittings it that the mould group of GPS radio frequency should use can want the signal frequency of five times higher satellite to be above with the frequency, the most basic distorted measurement of harmonic wave that could be satisfied. Characteristic of 50? for use with axis, piecing, the intersection of person who support and all characteristic impedance measure, could match well. Meanwhile, it tests tools auxiliarily except that impedance should have apt to correct, with little error and connecting convenient, high dependability and repeated characteristics to match well. It is very important to correct the tester regularly too, and will measure all fittings such as connecting wire, piecing, attenuator,etc. together after connecting while correcting.

Method that every index of GPS radio frequency tests

Some test schemes of GPS radio frequency are numerous, among them more important index is as follows, gain, controlled to gain the range, input and compress some, miscellaneous news coefficient, the mirror is inhibited frequently, segregation degree that originally shook the signal, originally shake and make an uproar etc..

Gain measurement

Gaining the enlarged intensity that the signal referred to inputting to ADC compares with signal that GPS aerial receives of front of GPS radio frequency. GPS the intersection of receiver and the intersection of radio frequency and front gain generally in about 110dB. Gain, can use spectral analysis instrument come on measurement.

The gaining of devices such as low in noise amplifier, mixer,etc. can measure S21 to get with networking analysis instrument of vector quantity, notice 50? of the port matching. Connect it like Fig. 2.

Two systematic function parameters reflect linearity degree of the receiver, hand in and adjust some and 1dB to compress and click in three steps. Three steps hand in, transfer characteristic, can hand in and adjusting one to mix the intersection of neighbor and a signal in the useful signal, cause the degradation of signal quality. But to GPS, in only one pieces of passway under strip, have strong the intersection of neighbor and dishes of interfering signal, so compress performance come on the intersection of linearity and degree to consider system from 1dB mainly. The real its output power of amplifier is and unable to have kept the linear proportion amplifying all the time with increase of the input power, always reach the saturation finally, the input power when gaining more linear ideal value of the amplifier reduce 1d? is called input 1d? and compress some see Fig. 3 . Like after the connection of Fig. 1 at the time of measurement, do the power scanning with the signal generator, thus found input 1dB and compressed some

2.2 Noise coefficient

GPS the intersection of receiver and radio frequency itself can produce the noise too, express with noise coefficient inside the front, it is the better that its value is the smaller.

Si and Ni are input signals and input the miscellaneous news, So and No output signals and output the miscellaneous news.

One grade of low miscellaneous news amplifiers low noise amplifier, LNA used in front of GPS receiver If you can?t mix, report by coefficient importance especially. If the miscellaneous news coefficient is too great, will influence the systematic sensitivity to GPS receiver. There are two kinds of commonly used miscellaneous news coefficient methods of measurement: Use the miscellaneous news coefficient tester to measure, gain the law directly. The miscellaneous news coefficient tester measures the sketch map such as Fig. 4 directly.

It is the most direct method to measure noise coefficient to use the noise coefficient analysis instrument. The result is most accurately too. And the analysis instrument can reveal gaining and miscellaneous news coefficient at the same time. But the analysis instrument has frequency that is limited, and as measuring very at the time of the miscellaneous news coefficient of the height, for example the noise coefficient exceeds 10dB, it is very inaccurate to measure the result.

It is a definition based on noise coefficient to gain the law. The noise is produced from two factors. One is to reach the interference that the radio system inputs, different from useful signal needed. Second the intersection of radio system and random perturbation of signal carrier. The second kind of situation is resulted from Brownian movement, apply to the thermal balance in any electronic device, the usable noise power of the device is: PNA = KT? F, ? F = Noise bandwidth Hz . At the room temperature 290? K When, the density PNAD of power spectrum of the noise = -174dBm/Hz.

Therefore we have the following formula:

NF = PNOUT ? -174dBm/Hz 20 * log10BW Gain

In the formula, PNOUT is the noise power of the output altogether already examined, the density of power spectrum of ambient noise when ? 174dBm/Hz is 290 degrees of K. BW is the frequency bandwidth of interest. Gain is the systematic gaining. NF is a noise coefficient of DUT. It is a formula of reduction, we can measure and output the density of power spectrum of the noise dBm/Hz directly ,The formula turns into at this moment:

NF = PNOUTD 174dBm/Hz ? Gain

In this way so long as we measure and gain the value calculating the noise coefficient again.

The mirror image is inhibited

The existence of the mirror image signal will influence the SNR of the signal. In order to inhibit this kind of influence, the receiver must have certain inhibiting rate of mirror image. Namely inhibit the signal of the position of mirror image as to sure before the mixer, mix with the impact on intermediate frequency signal of signal of the strip in order to reduce. The instrument measuring the mirror image and inhibiting needing using has signal generator, attenuator, spectral analysis instrument. The signal generator produces the mirror image signal of certain power, send to the radio frequency input terminus of GPS receiver through a fixed attenuator, then the power of the mirror image signal in front of seeing the mixer with the spectral analysis instrument.

2.4 Originally shake and isolate degree to the radio frequency

Last three items segregation degree on between every of receivers frequency port,signal and shake originally during segregation degree, segregation degree during signal and intermediate frequency, originally shake the segregation degree with intermediate frequency. It is originally power and original power proportion that shaking or signal lets out the other ports to isolate one degree of definitions, the unit is dB. In GPS in the receiver, important index to reach radio frequency isolate degree, shake to well the intersection of radio frequency and segregation degree of signal originally, may carry reverse radiation or transmit from the aerial instead from the receiver signal while originally shaking the power, cause and interfere with other electric apparatuses. On the other hand, as to GPS receiver making the structure of frequency conversion directly, will cause from mix, cause and run off the question of adjusting etc. directly. Measurement originally shakes and sees Fig. 6 to the connection method isolated degree among the radio frequency.

Originally shaking the end to put into the signal generator in module of GPS radio frequency, the attenuator can guarantee 50? impedance is matched. The end of radio frequency is adding the spectral analysis instrument analysis result. The intermediate frequency not needed will connect 50? to match and support when being carried. Measure other segregation degree, measurement method is totally similar.

2.5 Originally shake the phase place noise

The phase place noise that originally shakes the signal that the comprehensive device of frequency offers is too large, will reduce the front SNR of GPS radio frequency, thus influence GPS to receive the sensitivity. The phase place noise originally shaken is expressed as:

Lf =Pnf /Pc dBc/Hz

Lf in the type It is the table density of phase place noise of single sideband, yuan fetch dBc/Hz in the unit; Pnf For the intersection of f and bandwidth of for single the intersection of sideband and power of 1Hz in carrier frequency of deviating from, regard dBm as unit; Pc is the power of the signal carrier, regard dBm as the unit.

The miscellaneous news of phase place originally shaken can be measured with spectral analysis instrument directly. Think the measurement value is accurate, must be according to the skew amount of carrier frequency of making an uproar measured, choose the values of SPAN and RBWS of the spectral analysis instrument rationally. Because these two values have determined the bandwidth of wave filter of intermediate frequency within the spectral analysis instrument, if the bandwidth is too big, the ones that measure make an uproar and flood under the connection through one?s female relatives of the wave filter, will cause and measure the result mistake. See Fig. 7.

Some spectral analysis instruments of new type will offer some options now, can be according to simply choosing the values of appropriate SPAN and RBWS automatically frequently, help us to measure the phase place noise more accurately. Such as series Agilent PSA, there is the following that select and help to optimize measurement.

Optimize phase noise for frequency offsets <50kHz from the carrier???????? 50kHz ?????????

Optimize phase noise for frequency offsets

This entry was posted on Sunday, July 10th, 2011 at 10:10 am and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

Source: http://hujk.org/principle-and-method-of-penetrating-front-of-%E9%A0%BB-to-test-that-gps-receives/

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