Texas Instruments TRF1500 - Manual (Page 20)

SWRA004A
TRF1500 Integrated Dual-Band RF Receiver User’s Guide 21
Figure 5. Low-Band LNA Output Configuration
Surface Acoustic Wave (SAW) Filter
The SAW filter is used primarily as an image-reject filter (IRF).
The image frequency (f
IM
) is located at the desired channel
frequency (f
CH
) plus two times the IF frequency (f
IF
); f
IM
= f
CH
+ (2 x
f
IF
). The image frequency acts as an interferer to the system.
During the down-conversion process, the image and the desired
channel are both converted to a common IF. Left unfiltered, the
image could completely mask the desired signal. The IRF rejects
this image before the RF signal is introduced to the mixer.
By minimizing the image before it reaches the mixer, the
sensitivity of the receiver is enhanced. To further minimize
potential interferers, a band-select filter is typically used at the
front of the receiver, before the LNA. The band-select filter passes
only those frequencies that fall within the system receive band. In
many TDMA systems, the duplexor acts as the band-select filter.
The off-chip SAW image-reject filter used on the TRF1500
applications board has a 3dB nominal insertion loss and a 25 MHz
bandwidth at a center frequency of 880 MHz as shown in Figure 6.
Figure 6. SAW Filter Insertion Loss
S AW F ilter In sertio n L os s
-60
-55
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
860 900
F req uen cy (M Hz )
Insertion Loss (dB)