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DW9255 DW9255
35.42MHz SAW Filter for GPS Receivers
DS3861 - 3.1 November 1997
OUTPUT+
OUTPUT-
The DW9255 is a Surface Acoustic Wave (SAW) bandpass filter for use with the GP2000 Global Positioning System (GPS) receiver chip-set, available from Mitel Semiconductor. It is pre-tuned to the exact 2nd IF filter requirements of the GP2010 & GP2015 RF front-end devices, with a centrefrequency of 35.42MHz. The response is tuned for a flat passband, steep stopband and uniform passband group-delay with 3 external inductors. The device is realised on a Lithium Tantalate substrate and housed in a small leadless ceramic Surface Mount package. The DW9255 gives significant improvement in correlated GPS Signal-to-Noise Ratio (SNR) performance compared to conventional LC bandpass filter schemes. This aids satellite signal acquisition and tracking capability from the GP2000 GPS chip-set. This device effectively filters out-of-band (unwanted) noise in the GPS signal. The Automatic Gain Control (AGC) within the GP2010 and GP2015 RF Front-end devices will then operate only on in-band noise for optimum gain and superior correlated GPS signal strength.
INPUT+
2 GND 3
INPUT1 12 GND 11
GND
4
GND
DW9255
GND 5 10 GND
GND
6 7 8
9
GND
LCS12/1
FEATURES
INPUT +
s Insertion Loss of 17dB 1dB (typical) s 1dB Bandwidth 1.9MHz (typical) s Passband Ripple 0.8dB (typical) s Low Profile Ceramic Surface Mount Package s Operating Temperature Range -40 to +85C
GND 3
2
INPUT -
s Centre Frequency of 35.42MHz
1 12 GND
GND
4
11
GND
DW9255
APPLICATION
s Commercial Global Positioning
GND
5
10
GND
GND
6 7
OUTPUT +
9 8
OUTPUT -
GND
RELATED PRODUCTS AND PUBLICATIONS
Part GP2010 GP2015 Description GPS receiver RF Front-end Miniature GPS receiver RF Front-end Data Reference DS4056 DS4374
LCS12/4
Fig.1 Pinout
1
DW9255
GPS Antenna
SAW 1st IF Filter LNA GP2010/15 RF Front End DW9255
Clock Crystal GP2021 12 channel Correlator + Support Functions
DATA BUS ADDRESS BUS
TO PROCESSOR
TCXO
2 x Serial Communications Ports (Programmable UARTS)
Fig.2 DW9255 used with GPS chipset
ELECTRICAL CHARACTERISTICS (Typ. @ 25C)
Parameter Centre Frequency 1dB Bandwidth Insertion Loss Amplitude Ripple (34.62 to 36.22MHz) Relative Attenuation <28MHz (relative to insertion loss) <31MHz <33.5MHz >37.5MHz >40MHz >50MHz >63MHz >73 - 110MHz Group Delay Ripple (34.62 to 36.22MHz) Maximum Group Delay (34.62 to 36.22MHz) Operating Temperature Range Min 1.6 16 35 30 21 21 25 30 28 40 -40 Typ 35.42 1.9 17 0.8 40 35 25 25 30 40 35 45 190 1.6 Max 18 1.6 300 1.7 +85 Units MHz MHz dB dB (pk to pk) dB dB dB dB dB dB dB dB ns s C
2
DW9255
DW9255 used as 2nd IF filter for GP2010/15 Centre Frequency Pass Band Insertion loss 3rd IF Image frequency at 2nd IF Source Impedance Load Impedance 35.42MHz 1.0MHz (within 1.0dB) 14-18dB 26.8MHz 500 typical 1000 typical than 20dB) beyond this bandwidth, and so should have a brickwall type response at these extremes. The DW9255 SAW filter provides a 1dB Bandwidth of typically 1.9MHz centred on 35.42MHz, with a typical pass band ripple of 0.8dB, when the SAW input and output capacitance is resonantly matched with inductors of optimum value. The out-of-band signal rejection is better than 21dB at 2.0MHz, and better than 35dB at 7.5MHz. The frequency response of the DW9255 SAW filter with matching components is shown in Fig. 3. The matching components used with the GP2010/15 device are shown in Fig. 4.
The second external IF filter is connected between the output of Stage 2 and input of Stage 3. It is required to define the bandwidth of the RF section of the GPS receiver, hence it is critical to the receiver performance. The filter should be flat across the 2MHz bandwidth of the GPS Coarse-Acquisition (C/A) code signal. It should also have high rejection (greater
5 0
Insertion Loss (dB)
-5 -10 -15 -20 -25 -30 -35 -40 32 33 34 35 FREQUENCY (MHz) 36 37 38
Fig.3 Typical frequency response of DW9255 SAW filter used as 2nd IF filter
3
DW9255
VCC
L1
L2 IP+ OP+
SOURCE IMPEDANCE = 500
MIX 2 O/P IP-
DW9255
OP-
2.2H
MIX 3 I/P
LOAD IMPEDANCE = 1K
COILCRAFT 1008
GP2010 GP2010 or GP2015
L1, L2 = 560nH COILCRAFT 1008
Fig.4 Typical matching components when used with GP2010 or GP2015 GPS Front-end IC
680nH IP+ COILCRAFT 1008 OP+
910nF COILCRAFT 1008
DW9255
15pF VITRAMON (VJ0805A150JXA) IPOP-
Pin 1 = IPPin 2 = IP+ Pin 7 = OP+ Pin 8 = OP-
Fig.5 50 Matching network
4
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