TI GPS PPS Timing Application Note

Transcription

1 Application Note Version 0.6 January

2 Contents Table of Contents 1 INTRODUCTION PPS CHARACTERISTICS TEST SETUP PPS TEST RESULTS... 6 Figures Figure 1 - Simplified GPS Receiver 1PPS Output... 3 Figure 2 - Hardware Test Setup diagram... 4 Figure 3 - Test Procedure Timeline... 5 Figure 4 - Jitter Distribution for Device Figure 5 - Jitter Distribution for Device Figure 6 - Jitter Distribution for Device

3 1 Introduction Texas Instrument s 65nm family of GPS chips support one pulse-per-second (1PPS) timing as part of its many features, it provides a high precision 1ms wide pulse whose rising edge is aligned to GPS time (or UTC time) second boundary. This feature enables a wide array of precision timing applications. The pulse is present on the PPS_OUT pin of TI GPS chips. The purpose of this document is to explain the test procedure and to summarize 1PPS accuracy test results. Nominal test results are provided for a sample of 3 devices that have been tested using the method detailed in this application note. 2 1PPS Characteristics 1PPS signal suffers a certain jitter with respect to the ideal GPS second boundary which is generated by the atomic clocks of the satellites. GPS satellite geometry, signal conditions, TCXO s drift and variation, as well as the internal circuitry of a GPS receiver all contribute to this jitter. Cables and other interconnections also add to the delay and should be taken into consideration during characterization and in the final application. Figure 1 - Simplified GPS Receiver 1PPS Output Figure 1 show a simplified version of the 1PPS output with respect to the ideal GPS second boundary and indicated the offset present. The offset component could be simply eliminated by calibrating it out of the system. However, the 1PPS signal typically shows jitter which depends on several factors, as previously discussed. This component is more difficult to eliminate given its non-deterministic nature. 3

4 3 Test Setup Figure 2 - Hardware Test Setup diagram Figure 2 shows the connections of the test setup. The Spirent GSS6700 is used as the reference since it can provide a highly accurate 1PPS pulse. The simulator s RF output is connected to the GPS device through a 15dB attenuator. The 15dB attenuator is used to lower the power output of the simulator and provide open-sky level of signal input to the GPS receiver. The 1PPS output of the GPS simulator and of the GPS receiver are respectively connected to Channels 1 and 2 of the oscilloscope. The PC running LabVIEW reads 1PPS data and analyzes the data to derive statistical output. The characterization of the 1PPS is performed by measuring the jitter of the TI GPS receiver PPS_OUT when tracking for extended periods of time. Below are the test conditions. 1. A static GPS simulator scenario is used as described in Table Tests are done at Room Temperature and nominal conditions. 3. Once the GPS receiver is tracking all the visible satellites and has fully decoded the navigational data the test is started to capture the PPS pulses. 4. GPS TCXO clock frequency of 26 MHz used for all PPS tests. 5. All PPS tests were done using the Texas Instruments CC4000 SW v1.10 (which is equivalent to NL5500 GPS Firmware v31.82). 4

5 Initial Geodetic Navigation State Latitude degree (North ) Longitude degree (West ) Altitude m (WGS-84), m (MSL) Satellite Configuration Number of SVs <= 10 (equal power) PDOP <= 3 Horizontal Mask Angle Dedicated Multi-path Channels Miscellaneous Simulation Length 5 degree 0 channels 10 hour Table 1 GPS simulator scenario The timeline of the characterization is shown in Figure 3. After acquisition is started, a fix will be obtained in 30 to 40 seconds. Once this occurs, a wait time is allowed to ensure that the receiver fully decodes the navigational data before the PPS data acquisition begins. Figure 3 - Test Procedure Timeline 5

6 4 PPS Test Results Table 2 below summarizes the PPS test data captured with three Texas Instruments GPS devices. For each device data was captured for 6 hours. PPS Error Parameter Device#1 Device#2 Device#3 Mean (ns) Standard Deviation (ns) PPS Jitter (ns) ±29.7 ±29.1 ±29.9 Table 2 Summary from 1PPS tests The jitter distribution for each device is shown in Figures 4-6 below. Figure 4 - Jitter Distribution for Device 1 6

7 Figure 5 - Jitter Distribution for Device 2 Figure 6 - Jitter Distribution for Device 3 7