GPS World, March 2013

Receiver Design GNSS MODERNIZATION 25Hz PLL Bandwidth 1 2 3 4 5 6 3100 3150 3200 3250 Time s Carrier Frequency Hz FIGURE 4 GPS carrier loop loss of lock convert input GNSS RF signals to a low IF signals FIGURE 6 illustrates the daughter card implementation The drawback of this architecture is that it may bring in an extra frequency shift by the unstable oscillator The configuration of the daughter card oscillator is implemented by an internal TCXO clock which is on the motherboard Unfortunately the internal TCXO clock has coarse resolution in terms of frequency adjustments This extra frequency offset multiplies the corresponding factor that eventually provides mixer functionality to the daughter card This approach can directly lead to a large frequency offset to the mixer which is brought into the IF signals Finally when we conduct the tracking operation through the software receiver this large frequency offset is beyond the lock range of a narrow typically desirable GNSS carrier tracking loop as shown in Figure 4 In general a TCXO is preferred when size and power are critical to the application An oven controlled crystal oscillator OCXO is a more robust product in terms of frequency stability with varying temperature Therefore for the USRP N210 onboard oscillator issue it is favorable to use a high quality external OCXO as the basic reference clock when using USRP N210 for GNSS applications Front End Daughter Card Options A variety of daughter card options exist to amplify mix and filter RF signals TABLE 2 lists comparison results of three daughter cards BURX DBSRX and DBSRX2 to supply some guidance to researchers when they are faced with choosing the correct daughter board The three daughter cards have diverse properties such as the primary ASIC frequency coverage range filter bandwidth and adjustable gain BURX gives wider radio frequency coverage than DBSRX and DBSRX2 DBSRX2 offers the widest filter bandwidth among the three options To better compare the performance of the three daughter cards we conducted another three experiments In the first we directly connected the RF port with a terminator on the USRP N210 platform to evaluate the noise figure on 100Hz PLL Bandwidth 1 2 3 4 5 6 2800 2900 3000 3100 3200 3300 Time s Carrier Frequency Hz FIGURE 5 GPS carrier loop lock tracking BURX DBSRX DBSRX2 Primary ASIC SKY73012 MAX2118 MAX2112E Frequency 800MHz Coverage 245GHz 4MHz 50MHz 8MHz 66MHz 8MHz 80MHz 485dB 88dB 88dB MIMO Cable TABLE 2 Front end daughter card options Mixer Adjustable amplifier Oscillator Bandwidth Adjustable RQ JXUDWLRQ Direct Down Conversion Amplifier Filter Gain I2C Antenna 300MHz 4GHz Adjustable amplifier Adjustable amplifier 800MHz 24Ghz Filter Filter Mixer Daughter Card I Q FIGURE 6 Daughter card tuning implementation the three daughter cards From FIGURE 7 we can draw some conclusions BURX has a better sensitivity than DBSRX and DBSRX2 when the gain is set below 30dB DBSRX2 observes feedback oscillation when the gain set is higher than 70dB The second experimental setup configuration used a USRP N210 platform an external OCXO oscillator to provide stable reference clock and a GPS simulator to evaluate the C N 0 performance of the three daughter www gpsworld com March 2013 GPS World 39