GPS receiver HF channel simulation

At present, the GPS system has been widely applied to people in all spheres of life.

With the GPS location of theoretical research, as well as hardware improves, GPS positioning systems are increasingly sophisticated. This article from the software point of view of the experimental analysis of high frequency channel GPS receiver works; on this basis, the design of a gain distribution programmes, analysis of the noise characteristics of frequency circuit, while giving high frequency channel circuit in System View platform system simulation results.

　　

　　

1 receiver antenna and transmission loss

GPS signal due to the use of code rate fb1 = 1.023MHz of spreading code (C/A-code), the modulation signal will occupy after 2.046MHz bandwidth.

L1-band (1575.42MHz) signal power spectral density diagram shown in Figure 1. Spread spectrum signal bandwidth in fL1 = 1575.42MHz center frequency bandwidth of the receiver antenna 2.046MHz bandwidth coverage at least meet fL1fb1 =

1 575.421.023MHz。

Receiver receive minimum power must be greater than-160dBW (-130dBm), in order to guarantee this, C/A-code modulation for satellite transmission power L1 carrier must meet 21.9W (13.4dBW namely 43.4dBm).

If the transmitter output power, minimal 43.4dBm receiver input level-130dBm, induction in the receiver antenna signal level on minimum-130dBm, behind the experiment simulation data is in assuming that the transmission loss at for. 173.4dB

2 receiver frequency channel design

2.1 receive channel works

GPS receiver antenna receiving L1 band rough code (C/A-code) modulation of spread spectrum signal filtering, pre-and-drop, to the rear amplifier and then zoom in, mixing is a low-frequency signal.

So, after a down-conversion, the center frequency from fL1 moved if fIF, but spectrum between the various components and do not change compared to the relationship. To three levels down-conversion, for example, the receive channel of the module diagram as shown in Figure 2, antenna, filters and amplifier 1 form part dynamically antenna.

　　

　　

RF carrier down conversion formed by the if signal center frequency:

fIF3=fL1-fLO1-fLO2-fLO3=1575.42-1400-140-31.111=4.309MHz。

Of these, fLO1, fLO2 and fLO3 to local oscillator frequency. Mixing of frequency as shown in table 1.

　　

2.2 receive channel gain design

Now calculated from the input to the second total gain after mixing.

When the third mixing input sine voltage values are valid for up to 20mV hard limiting when you reach. The smallest receiver input level about-160dBW (-130dBm) calculation, 50 Ω input impedance voltage is on:

　　

　　

From input to channel limiter total gain greater than before, levels, gain 109dB are distributed as follows:

Front amplifier gain: 19dB;

2m cable loss:-2.5dB;

Rear amplifier gain: 50dB;

The second mixing gain:-10 + (-7) =-17dB;

Intermediate frequency amplifier gain: 80dB;

Total total gain: 129.5dB.

Taking into account the dynamic signal strength under the receiver down 8dB, this input to the limiter input total gain as 121.5dB. So design of channel total gain meet unit sensitivity requirements.

Most of the mixing transform gain (75dB) occurs in the third mixing — the second 35.42MHz IF signal transform into IF output frequency 4.3MHz Department.

Therefore SAW (surface acoustic wave) filters the output is IF link on the most sensitive to external interference. Third mixing 60dB gain control range.

2.3 channel noise analysis and calculation

The GPS receiver in the noise characteristics of RF front-end (NF)-(1) as shown:

　　

　　

Where: F 1: dynamic antenna LNA noise characteristic (dB);

F 2: RF-medium frequency conversion module (except IF all filter circuit module) noise characteristic (dB);

G1: dynamic antenna LNA RF signal gain (dB);

L1: LNA RF filter after due and cable entry of RF signal loss.

Here, take the dynamic antenna LNA gain + 26dB noise characteristics,; take RF 1.5dB — intermediate frequency conversion module (for example GP2015) noise characteristics to 9dB; from dynamic antenna to RF front-end (including additional RF Ceramic band-pass filter) for the introduction of loss coaxial cable length is variable.

Assuming that the cable length 2m, band-pass filter insertion loss (considering the overall loss L1) 2.5dB. Therefore by type (1) must:

　　

　　

The receiver noise characteristics in high frequency channel is 1.

6dB。

For a set of dynamic antenna LNA noise characteristics, LNA gain higher-frequency RF-converted part of the whole of the received noise characteristics of independence.

From the noise, it is best to use a GPS receiver with low noise amplification characteristics of antennas, antenna LNA with suitable high-gain (> 19dB) and very small cable loss (<-2dB).

3 high frequency channel circuit of system simulation

According to figure 2-frequency channel of schematics, as well as the preceding analysis of gain and noise characteristics, build its System View system simulation circuit.

3.1 signal source

Here the signal source adopts the simple way, only emulates a C/A-code, and then contact data signal D code for die 2 together, then the modem to L1 carrier, transmission loss arrived at the receiving antenna and receiving antenna to receive signals is to introduce noise spread spectrum signals.

Noise and interference simulation through the join to the simulation system assumes that the noise or interference signals. Here, the noise using temperature noise: impedance = 50 Ω, noise temperature = 300K. C/A-code and the C/A-code modulation D codes graphics as shown in Figure 3. Of these, the C/A-code modulation delay D code for 5 μ s.

　　

　　

The L1 carrier spread spectrum signals and receiver RF front-end to receive RF signal shown in Figure 4.

The figure shows that the RF front-end receive GPS signal drowning in noise.

3.2 received if signal

The first level of intermediate frequency mixing output signal spectrum diagram as shown in Figure 5.

Among them, the RF filter passband center frequency setting in 1575.42MHz, 2MHz band (-3dB); 1st IF filter center frequency, in 175.42MHz poles of chebyshev response, 0.1dB ripple. Mixer for active double-balanced mixer.

　　

　　

Figure 5 shows the first level-if output frequency in 175.42MHz nearby.

The second level of intermediate frequency mixing output signal spectrum graph shown in Figure 6 (a) below.

Which, in a second IF filter band-pass filter for the SAW. Center frequency 35.42MHz, band 2MHz (1dB), insertion loss 14 ~ 18dB, only with > 10dB (2M), group delay ripple < 300ns (34.62 ~ 36.22MHz), maximum group delay < 1.7ns. SAW band ripple 0.8dB; SAW filter frequency response characteristics with the usual limited impact response (FIR) filters to simulation.

The figure 6 partial enlarged the spectrum graph shown in Figure 6 (b), visible level 2 intermediate frequency output frequency in 35.42MHz nearby.

　　

　　

　　

　　

Third-level mixing before limiter will be third-level mixing input voltage limit in 20mV.

Third-level mixing resulting intermediate frequency output signal spectrum diagram as shown in Figure 7. The figure shows that the third-level-if output frequency in 4.309MHz nearby. 3 IF the center frequency to filter 4.3MHz.

This article from the software point of view of the experimental analysis of the GPS receiver frequency channel works; design of a high-frequency channel allocation of gain, and analysis of its noise characteristics; on this basis, the high-frequency channel circuit for system simulation.

Actually used according to the needs of interference tolerance and gain adjust, as appropriate, such requirement. This study on the development of the related fields of work have significance. To receive a link in the channel characteristics, the author will require further attention.

Reference documents

1 Datasheet of GP2015:GPS receiver RF front end. http://www.zarlink.com

2 GP2000: GPS receiver hardware design application note. http://www.zarlink.com

3 datasheet of QuickMount micro GPS patch antenna AN-PC-131.http://www.macom.com

4-guards, Zhang Hua Chang-System View dynamic system analysis and design of communication systems simulation: Xidian University Publishing House, XI¡¯an, 2001

5 Kaplan E.D. (u), Qiu Wang million due and righteous principle and application of translation .GPS. Beijing: electronic industry press, 2002

6 King Pu Desai .GPS receiver RF channel and frequency synthesizer design. Navigation, 1992; (2)