GPS World - January 2008 - (Page 31)

Receiver Design | SYSTEM DESIGN & TEST 3 implicit in GPS signal processing, and the incor2 rect gain can subtly aﬀect testing results, particularly 1.5 near the limits of receiver 1 performance. In cases where the goal 0.5 of a particular test is to 0 evaluate a receiver’s per- 8 - 7 - 6 - 5 - 4 - 3 - 2 -1 0 1 2 3 4 5 6 7 formance at a given C/N0 p FIGURE 9 Histograms corresponding to various examples of overall gain (4-bit ADC) level, the use of an external LNA may also require well below the thermal noise ﬂoor, the an adjustment in the power level of the expected input to the AGC/ADC ordi- simulator. Assuming the LNA gain is narily has nearly constant power den- suﬃciently large, the eﬀect of inserting sity within the signal band. A histogram the ampliﬁer is to roughly substitute its of such a signal should have a roughly noise ﬁgure for the noise contributions Gaussian shape, as shown in blue in FIG- of all downstream elements (the exact URE 9 (4-bit ADC). value may be quantiﬁed by using the By comparison, too little or too much Friis formula). To preserve a constant end-to-end gain results in the histograms C/N0 in this case, it is necessary to adjust shown in green and red, respectively. the simulator’s signal level to account e resulting signals at these power for this eﬀect. As a simple numerical exlevels violate a number of assumptions ample, assume the test setup described 2.5 Expected distribution Too much gain Not enough gain above, with 12.1 dB attenuator loss and 6.3 dB splitter insertion loss. Neglecting the receiver’s front-end characteristics for simplicity, the eﬀect of inserting an LNA with 18 dB of gain and a noise ﬁgure of 1.8 dB is to reduce the degradation in C/N0 between simulator output and receiver input from 12.1 + 6.3 = 18.4 dB to 12.1 + 1.8 + 0.2 = 14.1 dB. In this case, the signal levels on the simulator should be reduced by 4.3 dB to preserve a constant C/N0 at the receiver input. Finally, it is worth noting that this approach can be used to directly verify that appropriate signal levels are being delivered to the input of each RUT, as follows. First, one or more of the tests described above are performed without the LNA in place, and the results are noted. e LNA is then inserted, the simulator’s signal levels are adjusted to maintain a constant C/N0 as described above, and the tests are repeated. In theory, receiver performance should More for less NovAtel’s precise thinking makes it possible. NovAtel’s SPAN Technology now delivers more for less with a tightly integrated GNSS receiver and an IMU. for less, visit www.novatel.com Precise thinking | www.gpsworld.com | www.novatel.com January 2008 | GPS World 31 http://www.novatel.com http://www.novatel.com http://www.gpsworld.com

Table of Contents Feed for the Digital Edition of GPS World - January 2008

GPS - January 2008 Contents Out in Front Expert Advice The Money-Go-Round u-Nav Latest Acquisition Apples to Apples Global SBAS 2008 GPS Receiver Survey Advertisers Index & Company Directory The Manufacturer's Road Year of the Who Working Indoor Up and Down Good, Better, Best Marketplace Classifieds Seen + Heard

GPS World - January 2008

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