# LO and Calibration Signal Distribution in a Multi-Antenna Satellite Navigation Receiver +
## 3 Distributed GNSS Receiver Concept with Two 2 \(\times\) 1 Antenna Sub-Arrays
### System Calibration  
#### 3.2.1 LO Signal Distribution  
There are two options to provide the LO signal; the first and most obvious one is to generate the LO signal in a common PLL-based synthesizer using a single crystal-based reference oscillator, to split the signal in two identical signals by means of a power divider and to distribute it to both sub-arrays with low-loss coaxial cables. A second option is to distribute the reference signal (REF) instead, and employ two PLLs at each sub-array to synthesize the LO signal locally. Since both signals are derived from the same reference signal, the output signals of both PLLs remain phase-coherent. The schematic in Figure 4. illustrates the two options. In Figure 4, one common PLL module synthesizes the LO signal using the 10 MHz REF signal. The LO signal is split into two identical signals and distributed to the sub-arrays, where they are directly available for down-converting the input signals. In Figure 4, a common reference signal at a frequency of f\({}_{\mathrm{ref}}\) = 10 MHz is split into two identical copies and distributed to the sub-arrays. Each sub-array incorporates its own PLL-based LO signal generator. In both cases, cables of significant lengths are required to feed the sub-arrays with the REF and, respectively, LO  
Figure 3: Block diagram of the complete GNSS receiver consisting of two sub-arrays and a central base-band module. The LO (REF) and calibration signal for both frequency bands are split and distributed to the sub-arrays by cable connections. Figure 2: Principal block diagram of a dual-band sub-array of the distributed GNSS receiver. The four receiver channels are shown with their respective RF and LO inputs and IF outputs. Each sub-array contains a CAL and LO signal distribution between the two receiver chains at each frequency band. signal. The phase coherence required for the array signal processing is challenged by mechanical and thermal tolerances and variations, as well as other environmental effects. As one example, one side of the car can be subject to sunshine, the other side being shaded. The resulting temperature differences may lead to significant changes in the electrical length. The thermal phase sensitivity depends strongly on the type of cable. For teflon-insulated coaxial cables, maximum phase changes above 2500 ppm were reported [16]. For such a cable of 2 m length, as used in our study, this would result in phase changes of up to 17.5\({}^{\circ}\) at the LO frequency of 1648 MHz, causing errors in the DoA estimation algorithms. Phase-stable cables would come at the expense of increased costs. For the REF signal distribution, despite the lower phase imbalance at both sub-arrays due to the lower frequency, the multiplication by the frequency synthesizer leads to the same total phase imbalance as for the direct LO signal distribution. Therefore, the electrical properties in terms of phase imbalance of both concepts are similar. In the case of the REF signal distribution, the internal PLL of the ASIC is used to synthesize the LO signal. Using the internal PLL, the phase noise of the synthesized LO signal amounts to about \(-82\) dBc/Hz @ 10 kHz offset and \(-110\) dBc/Hz @ 1 MHz offset from the carrier frequency. In the case of the LO distribution, by using a dedicated PLL synthesizer IC (e.g., LTC6948 from Analog Devices, Wilmington, DE, USA), lower phase noise values of about \(-100\) dBc/Hz @ 10 kHz and \(-130\) dBc/Hz @ 1 MHz are achievable. The LO phase noise limits the carrier-to-noise ratio (CNR) of the output signal, narrowing the carrier tracking loop bandwidth and limiting the effective integration interval [17]. Therefore, the importance of this parameter for the system performance is an additional criterium to decide between both approaches.