WE2408 View Datasheet(PDF) - Unspecified
Part Name
Description
MFG CO.
'WE2408' PDF : 36 Pages View PDF
WiNEDGE & WiRELESS
Winceiver WE2408
2.4GHz Single Chip FM Transceiver
RECEIVER BANDWIDTH ADJUSTMENT
The simplified application circuit diagram in page 3 is a 2.4G receiver with a 150kHz Rx bandwidth.
The Rx bandwidth can be adjusted from 25kHz to 150kHz. There are 3 major elements related to the
receiver bandwidth:
Baseband Bandwidth
BWbb: the baseband bandwidth. This is defined by 8 pole low pass filters in both the I and Q
baseband channels. The filtering in each channel is achieved by a two pole Sallen Key filter
combined with a six pole Gm filter. The defining resistors and capacitors for the Sallen Key
filters are off chip; the Gm filter is totally on-chip except for one bandwidth trimming resistor.
Note that in this document, the bandwidth of the individual baseband I and Q channels is half
of the demodulator bandwidth. For example, a receiver with a bandwidth of 150kHz (i.e. r
75kHz), baseband bandwidth BWbb is 75kHz, demodulator bandwidth BWdm is 150kHz
minimum.
The DC offset correction feedback loop in the baseband section introduces an effective high
pass filter (100Hz) in the center of the RF band.
Demodulator Bandwidth
BWdm: the demodulator bandwidth. This is the bandwidth of the Period to Digital
demodulator. It is a ‘brick wall’ filter centered on the IF frequency. BWdm must be wider than
receive bandwidth.
IF Bandwidth
There is filtering in the IF section following the second mixer, designed to attenuate mixer
products prior to the demodulator. Filtering is defined by a 4 pole low pass filter followed by a
single pole high pass filter. The low pass filter is an on-chip Gm design with an off-chip
bandwidth trimming resistor and the high pass filter is on-chip and fixed frequency at 8kHz.
The following sections shows how the bandwidth of these sections can be adjusted.
BASEBAND FILTERS BANDWIDTH
A. Sallen Key baseband filters
The R and C components for this filter are off chip. To set the baseband bandwidth, BWbb, adjust the
C values. Choose values to give a bandwidth equal to or slightly greater than the required bandwidth
according to the following formulae.
The recommended component tolerances for resistors should be 1%, with capacitor tolerances of
5%, or better.
Capacitor attached to IFIL2/QFIL2 = 1.8nF x (65kHz/BWbb)
Capacitor attached to IFIL3/QFIL3 = 0.68nF x (65kHz/BWbb)
E.G., for a required 150kHz bandwidth, BWbb=75kHz, suggested values of 1.5nF and 560pF give a
bandwidth of 150kHz.
© 2003 WiNEDGE & WiRELESS PTE LTD
Rev Date: 2003 May 23
Page 15
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