CMX649 Wireless Voice Link Design Guide
©
2004 CML Microcircuits
Page 11 of 42
a dramatic reduction in current consumption, thereby enhancing battery life and talk-
time.
4.2.2 Receiver
When receive mode is selected, the MICRF505 control registers are adjusted to
cause the following actions to occur:
•
Receive mode selected.
•
Synchronous data mode selected.
•
LNA not bypassed.
•
Sallen-Key filter bandwidth set to 340kHz.
•
Main channel filter disabled.
•
Lock detect enabled.
The antenna input is amplified by a LNA that drives a quadrature mixer. The mixer
output is amplified and presented to a Sallen-Key low-pass filter, whose bandwidth is
set to 340kHz to prevent attenuation in the signal passband.
The main channel filter is bypassed in this application by setting bit 6 of Control
Register 0x02 to 1. The reason for doing this involves a tradeoff between current
consumption, voice quality and BER performance, and is explained as follows.
The formula for estimated receive bandwidth is:
f
BW
= f
offset
+ f
dev
+ (Baudrate/2)
…where f
dev
and
f
offset
are as previously defined,
and f
BW
is the required receive
bandwidth. Substituting previously calculated values yields the estimated receive
bandwidth:
f
BW
= 20kHz + 186.7kHz + (166.6kHz/2)
≅
290kHz
The bandwidth of the main channel lowpass filter must be higher than the required
receive bandwidth to prevent attenuation of energy in the signal passband.
A single 8MHz crystal translates to a maximum main channel lowpass filter cutoff
frequency of 200kHz, significantly lower than the required bandwidth of 290kHz.
This situation would cause significant attenuation of energy in the passband, with a
corresponding degradation of BER.
Three potential remedies for this situation exist:
1. Use two crystals to increase main channel lowpass filter cutoff frequency.
2. Reduce the baud rate to reduce the required bandwidth.
3. Bypass the main channel lowpass filter and keep the same baudrate.
Option 1 requires an extra crystal that presents additional cost and board space.
Option 2 is undesirable because a baudrate reduction would jeopardize either the
voice quality or the TDD nature of the application. Empirical data suggests that
option 3 is an acceptable compromise so long as the Sallen-Key filter bandwidth is
