Improving OP1dB in GNSS/GPS Receivers

Transcription

1 Application Note AN-0088 Improving OP1dB in GNSS/GPS Receivers Abstract Mobile wireless communications devices are getting smaller while the number of radio receivers and transceivers operating simultaneously within these devices is increasing. Nearly all of these devices rely on location based service (LBS) supplied data for fast, accurate, and reliable position data collected and updated from a global navigation satellite system (GNSS) receiver. These GNSS receivers operate using signals at a comparatively microscopic level, even in the presence of multiple radios transmitting and receiving only a few millimeters away. This application note (AN) explains the GNSS receiver figure of merit (FOM) for linearity known as compression point, more accurately known as output power at a one-decibel compression point, OP1dB, and how the employment of a high linearity low noise amplifier (LNA) external to any modern GNSS system-on-chip (SoC) can improve OP1dB. The result of improved OP1dB is explained: a GNSS receiver s ability to provide a good user experience is improved even in the presence of in-band and out-of-band interferers operating a few millimeters away. The benefits of the Parsec part number PT1233D low noise amplifier (LNA) are summarized with a comparison of its abilities versus competitor offerings. Compression Point Explained An important figure of merit (FOM) for GNSS receivers is the one-decibel (1-dB) compression point, known as the RF power output measured at the one-decibel (1-dB) compression point, OP1dB. At low RF input power levels an RF system or device behaves linearly that is, the RF output power is equal to the small signal power gain (Gss) times the RF input power: RFout = Gss * RFin. Parsec Technologies, Inc. November

2 RF Power Output, RFout Ideal Linear Response RF Power Input, RFin Illustration. Ideal Linear Gain Response At higher RF input levels higher order harmonics appear in the signal spectrum causing a reduction in large signal, or power gain. The 1-dB compression point of an RF component or circuit is defined as the RF input power where the measured power gain is 1-dB smaller than the small signal power gain. (Barth, 29 Oct 2008). Illustration. OP1dB the 1-dB Compression Point, courtesy of Receiver linearity is an important FOM for a receiver out-of-band (OOB) interference or electromagnetic (EM) energy can saturate the receiver low noise amplifier (LNA) this can create spurious harmonic mixing products which can ultimately lead to stopping the effective operation of the receiver sometimes referred to as desensing and/or jamming. OOB interference is filtered out by Parsec Technologies, Inc. November

3 the intermediate frequency (IF) filter in a super heterodyne receiver architecture or within the digital filtering section of a homodyne (low or zero IF) receiver architecture, if the receiver front-end is not saturated. (Loy, 1999). The receiver can be made more resilient against OOB interference/em energy by increasing the linearity of the front end (FE). Additional measures are needed to improve desensing and/or jamming resistance against in-band interference. In-band interference is passed through the IF filtering circuit or low or zero IF digital filtering and can de-sense or jam the receiver back-end. Desensing or jamming can occur at interference signal levels as low as -110 dbm. (Barth, 29 Oct 2008). Higher bit quantization can improve the resilience of the GNSS receiver against in-band interference, as can certain narrow band filtering techniques used in the receiver FE (called notch filtering). Input referred third order intercept (IIP3) point is often used as a measure of linearity for GNSS/global positioning system (GPS) receivers. (Young, 29 July 2013). IIP3 is typically about 10-dB above the 1-dB compression point, usually measured in dbm referred to a 50-ohm system. High Linearity LNAs in the GNSS Receiver Front-End (FE) Today s GNSS receiver SoCs offer a combination of compelling features aimed at meeting good user experience: Broadband design to cover the frequency range from 1560 to 1610 MHz (50-MHz BW) High integration to have a small form factor Gain optimization to help achieve state-of-the-art sensitivity Low noise figure to help achieve state-of-the-art sensitivity Achieving these goals alone does not guarantee good sensitivity, fast time-to-first-fix (TTFF), accurate position reporting, or reliable position data over time and changing receiver conditions. Why? Another concern regarding low power satellite signals is the presence of high power radio interferers/jammers co-located and operating simultaneously in the GNSS equipped device. Today s highly integrated smart phones are subject to a variety of wireless links that are transmitting or receiving signals with high power ( two (2) watts, +33 dbm, transmit power), such as 3GPP compliant cellular pentaband transceivers operating from 800 MHz to 3.8 GHz, Bluetooth and WLAN operating in up to a 90-MHz total occupied bandwidth centered at about GHz and WiMAX operating from 2.5 to 2.7 GHz. The illustration below shows a simplified block diagram of a mobile phone with a 3GPP cellular main path and a GNSS receiver path. (Bachu, 11 Jun 2011). If the strength of these signals is high enough the GNSS LNA is driven into saturation and GNSS receiver gain is reduced. The lower gain and increased total noise figure of the system is called desensitization. (CEL, 22 Feb 2007). Depending on the isolation conditions in the smart phone a certain OOB compression point is required. An OOB signal strength of -15dBm can decrease the gain in an LNA by as much as 1-dB, thereby reducing GPS system sensitivity. These measurements were made by injecting OOB signals into test LNAs and monitoring the gain (at 1.575GHz) until it was decreased by 1-dB then recording the input power level that caused the desensitization. (CEL, 22 Feb 2007). Parsec Technologies, Inc. November

4 If we assume a smart phone antenna isolation of 10 db between main cellular band antenna and the GNSS antenna, we can estimate the GNSS module compression point at 900 MHz as P1dB_900 = 33 dbm 10 db = 23 dbm. As shown in the table below, low energy usage GNSS SoCs and external LNAs cannot achieve this level of compression point performance. Therefore, the pre-filter needs to attenuate the 900 MHz signal. The Parsec PT1233D LNA has a compression point of >+7 dbm at 900 MHz therefore, the pre-filter needs to have a rejection of minimum 16-dB at 900 MHz. Note that in filter design the insertion loss (IL) and OOB rejection is a design tradeoff. The Parsec PT1233D LNA s compression point is at least 4-dB better than any silicon based FE option, taking some of the GNSS receiver FE design stress away from the pre-filter that is, making it easier to meet both expected IL and required OOB rejection. Illustration. Interference from Mobile Cellular Radio Path to GNSS Path. (Bachu, 11 Jun 2011). Recall the 1 db compression point of an amplifier is specified in terms of output power. Thus, when this amplifier saturates, the input power will be Pin = Pout/Gss, or, equivalently, Pin (dbm) = Pout (dbm) Gss (db). A silicon process based GNSS SoC without external LNA will saturate at about -40 dbm RF power input. (Maxim, 2005). A silicon process based GNSS external LNA will saturate at about -20 to -11 dbm RF power input. (CEL, 22 Feb 2007), (Jagjit Singh Bal, 21 Feb 2012). Compare these saturation levels to the Parsec part number PT1233D LNA constructed using GaAs process technology. Parsec Technologies, Inc. November

5 GPS L1 Receiver Part Number/Description: OP1dB in Fc = MHz Gain, small signal, in Fc = MHz Saturation Level in Fc = MHz Maxim MAX2741 silicon L1-band GPS receiver IC CEL UPC8232T5N SiGe:C MMIC LNA Infineon BGA925L6 LNA for GNSS in B7HF Silicon-Germanium (SiGe:C) technology Parsec PT1233D GaAs LNA MMIC Table. Saturation Levels Silicon GPS Receiver SoC, Silicon External LNA, Parsec PT1233D LNA Summary and Conclusion Modern GNSS receivers face ever more severe in-band and out-of-band (OOB) interference issues as a result of the constant move to ever smaller end product designs along with the trend to employ multiple wireless communications transceivers in extremely close proximity. OP1dB measured in dbm referenced to 50-ohm is a key GNSS receiver figure of merit (FOM) allowing designers and users to estimate the relative capability of a receiver to operate with good user experience in the presence of inband and OOB interference. GNSS receiver desensing as a result of close proximity radio transmissions is an issue directly impacting future micro miniature device and product user experience. Based on the known operating conditions, GNSS receiver SoCs require the use of both an external to the SoC high OP1dB rated LNA and a suitably designed pre-filter ahead of the SoC. Using an ultra linear, low energy usage ( 9 mw peak at Vcc = +1.8Vdc) LNA such as the Parsec PT1233D provides a GNSS receiver of any configuration an OP1dB FOM margin of 4-dBm versus any silicon process based option. This margin allows a GNSS receiver designer greater flexibility in specifying a suitable pre-filter by relaxing the tradeoff requirements between low IL and high OOB rejection (by relaxing OOB rejection requirements by 4- db or more). The message for today s and tomorrow s GNSS receiver and micro miniature multiple radio equipped device designers is: employ an ultra linear external LNA with low noise figure, high gain, low energy usage, small footprint, and AnyVoltage SM (1.4 to 6.0 Vdc) operating capability: the Parsec PT1233D LNA. Bibliography Bachu, D. K. (11 Jun 2011). New Generation High Linearity Navigation Front-end Devices Covering GPS and GLONASS. Microwave Journal. Barth, C. (29 Oct 2008). A Low-Power Subsampling GPS Receiver Front-End. Stanford, California USA: Stanford University. CEL. (22 Feb 2007). AN1050, Using the UPC8232T5N Discrete LNA to Improve GPS Signal Performance in Mobile Handsets. Santa Clara, CA USA: California Eastern Laboratories_CEL. Parsec Technologies, Inc. November

6 Jagjit Singh Bal, C.-I. L. (21 Feb 2012). BGA925L6 Optimizing Rejection of LTE Band-13 ( MHz) Jammers. Munich, Germany: Infineon. Loy, M. (1999). Understanding and Enhancing Sensitivity in Receivers for Wireless Applications. Dallas, Texas USA: Texas Instruments. Maxim. (2005). MAX2741 Integrated L1-Band GPS Receiver Datasheet. Sunnyvale, CA USA: Maxim. Young, C. (29 July 2013). Low Noise Amplifier (LNA) Linearity Impacts to Close Proximity Co-Located GPS L1 Receivers. Plano, TX USA: Parsec Technologies Inc. Parsec Technologies, Inc. November