Ap p l i c At i o n no t e

Overview

Global Positioning Technology is finding its way into more and more wireless products. This is especially true of cell phones. E911 has mandated the integration of GPS functionality into these handsets and as consumers become better acquainted with the technology, their de-mand for GPS data and services is sure to increase. While cell phone manufacturers are eager to add GPS, they are also placing restrictive requirements on device size and power consumption. This has led to the development of GPS receiver/processor ICs that incor-porate on-board low noise amplifier front ends, or LNAs. Unfortunately, the noise performance and resultant sys-tem sensitivity of these integrated LNAs are not always adequate. To reduce front-end noise and improve sensi-tivity, manufacturers are turning to external LNA devices. Discrete LNAs can be located near the antenna so trace losses are reduced. This, coupled with tuning and filter-ing, can improve noise performance by more than 1.5dB over on-chip, integrated LNAs. Key desirable features for a discrete GPS LNA are low noise, high gain, small size, and low power con-sumption. A programmable power-save function that shuts down the device to conserve power is also es-sential. NEC’s UPC8232T5N MMIC LNA is engineered to meet all these requirements. Designed specifically for the implementation of GPS in battery-powered hand-sets, the SiGe-Carbon UPC8232T5N also features inte-grated matching circuits, ESD protection, and a robust

AN1050

Using the UPC8232T5N Discrete LNA to Improve GPS Signal Performance in Mobile Handsets

UPC8232T5N SiGe:C MMIC LNA

Performance @ 1.575GHz

Noise Figure 0.95dB

Gain 17.5dB

Input Return Loss –15dBm

Output Retun Loss –17dBm

Input P1dB –20dBm

IIP3 –10dBm

ICC 3.2mA

An Evaluation Board, P/N UPC8232T5N-EVAL-A, is available from CEL

Figure 1

bandgap regulator. The UPC8232T5N delivers a noise figure of 0.9dB with 17.5dB gain (Figure 1), and power consumption of just 3.2mA at 2.7 – 3.3V. Ideal for today’s ever-smaller phones, its miniature size — 1.5 x 1.5 x 0.37 mm — and the lack of need for a complex external active bias network make it the smallest discrete LNA solution currently available.

Out-of-Band Signals and Desensitization

1.575GHz GPS LNAs, especially in highly-integrated phones, are subject to a variety of out-of-band (OOB) signals: 900MHz phone transmission, 1800-1900MHz PCS signals, 2.4GHz WLAN and Bluetooth, and 2.5-2.7GHz WiMAX. If the strength of these signals is suf-ficiently high, it can have the undesirable effect of re-ducing the LNA gain in the GPS band, an effect called desensitization. Measurements taken at California East-ern Laboratories (CEL) show that an OOB signal strength of -15dBm can decrease the gain in an LNA by as much as 1dB, 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 1dB — then recording the input power level that caused the desensitization.

Tuning to Mitigate Desensitization

Desensitization can be mitigated through tuning and filtering. The tuning that yields the best performance employs a low-loss input series L-C network as shown

in Figure 2 (next page). Essentially a band-pass filter, it enables the UPC8232T5N to deliver 17.46dB gain (Figure 3). By adding a second capacitor in a “T” network, Figure 4, a high-pass filter is created. This circuit will decrease the sensitivity of the LNA to 900MHz signals, but has no favorable effect on signals above the 1575MHz GPS band. It also has a slightly negative effect on the noise figure. The table in Figure 5 compares the sensitivity to out-of-band signals for the two different configurations.

Distributed Filtering:The Optimal Solution

The most common and effective way of dealing with de-sensitization is filtering. The most optimal is distributed filtering. In distributed filtering, a low-loss SAW band-pass filter is placed ahead of the LNA, and a high-rejec-tion SAW band-pass filter just after it. In this “real world” configuration, Figure 6, interfering signals are rejected prior to the LNA, improving desensitization by more than 20dB. This pre-filter also reduces any intermodulation of out-of-band signals in the LNA. All this is achieved with a noise penalty of less than 0.5dB — which in turn can be mitigated by employing an extremely low noise LNA like the UPC8232T5N.

Low-Loss Input Series L-C Network

Part Symbol Value Source Part Number

Capacitor C1 2.0pF Murata GRM15 Series

Capacitor C2 1.0pF Murata GRM15 Series

Capacitor C3 1000pF Murata GRM15 Series

Capacitor C4 1000pF Murata GRM15 Series

Inductor L1 12.0nH Murata LQW15 Series

UPC8232T5N

VCC

RFIN

RFOUT

PowerSaveB

IAS

C1

C3

C4

C2

L1

17.46dB @ 1.575GHz

Power GainUsing Low LossMatching Circuit

STOP 3100MHzSTART 100MHz

Figure 2

Figure 3

The high-rejection post-LNA SAW filter is a high-er-loss device, but it eliminates any strong signals outside the GPS band which might be amplified by the LNA. Some designs put this high-rejection filter ahead of the LNA, but this configuration can add 1.5dB to the overall noise figure; performance that is not acceptable in most situations. The performance of a distributed line-up is illus-trated in the tables in Figures 7 and 8. Note the overall noise figure, which is maintained at 1.5dB including all the associated filter losses, and the improvement in desensitization, which is now above +5dBm.

Implementation of a distributed filtering circuit requires close attention to component specifica-tion and interaction. The LNA should be tuned for best noise figure and input match. A poor input match will induce ripple loss in the SAW filter ahead of the LNA, which in turn will degrade over-all noise performance. Since the LNA is particu-larly sensitive to the Q of the input inductor, the components used to construct the input match-ing circuit should include high-Q devices. Traces should also be adequately sized, as excessively narrow traces will add to the loss. Figure 9 illus-trates a UPC8232T5N evaluation board that em-ploys distributed filtering.

High Pass Matching Circuit

Part Symbol Value Source Part Number

Capacitor C1 1.0pF Murata GRM15 Series

Capacitor C2 1.0pF Murata GRM15 Series

Capacitor C3 1000pF Murata GRM15 Series

Capacitor C4 1000pF Murata GRM15 Series

Capacitor C5 2.7pF Murata GRM15 Series

Inductor L 1 6.2nH Murata LQW15 Series

UPC8232T5N

VCC

RFIN

RFOUT

PowerSaveB

IAS

C1 C5

C3

C4

C2

L1

Figure 4

4590 Patrick Henry Drive, Santa Clara, CA 95054-1817

Telephone 408-919-2500 • FAX 408-988-0279 •Telex 34/6393

www.cel.com

Information and data presented here is subject to change without notice. California Eastern Laboratories assumes no responsibility for the use of

any circuits described herein and makes no representations or warranties, expressed or implied, that such circuits are free from patent infingement.

© California Eastern Laboratories 2/22/2007

Conclusion

With the right components and careful circuit design, engineers implementing GPS functionality can ad-dress the challenges posed by out-of-band signals. With the UPC8232T5N MMIC LNA and distributed fil-tering, they can do it without sacrificing real estate or power consumption.

Matching Circuit Series L-C “T” High Pass

Noise Figure @ 1.575dB 0.95dB 1.05dB

Desensitization:

@ 900MHz –15dBm –5dBm

@ 1710MHz –15dBm –18dBm

@ 1850MHz –15dBm –17dBm

@ 2400MHz –12dBm –14dBm

Sensitivity — Matching Circuits

1.575GHz

UPC8232T5NSiGe:C MMIC LNA

Low-LossPre–Filter

High RejectionPost–Filter

Figure 6 Distributed Filtering

Figure 5

UPC8232T5N with Pre- and Post-Filtering

Noise Figure @ 1.575dB 1.5dB

Gain@ 1.575dB 15.6dB

Desensitization:

@ 900MHz >+10dBm

@ 1710MHz +5dBm

@ 1850MHz +8dBm

@ 2400MHz >+10dBm

Sensitivity — Distributed Filtering

1. 15.60dB @ 1.575GHz

2. –76.25dB @ 900MHz 3. –50.57dB @ 1.9GHz

Swept GainDistributed Filtering

STOP 4000MHzSTART 500MHz

Figure 9 Distributed filtering: UPC8232T5N with pre- and post-filters

Figure 7

Figure 8