CMX649 Wireless Voice Link Design Guide
©
2004 CML Microcircuits
Page 7 of 42
reproduction of the original analog voice signal. The ADM decoder settings, such as
time constants and integrator step sizes, have been empirically determined to
provide outstanding voice quality at the selected sampling rate. The recovered voice
signal is filtered and amplified before it is sent out of the AUDIO OUT pin to the audio
amplifier.
The Texas Instruments TPA6203 differential audio amplifier is used to drive the
earpiece speaker with the audio output from the CMX649. The TPA6203 was
selected because of its low noise, low quiescent current, small form factor, and low
cost. A single-ended input configuration was used because of the single-ended
output from the CMX649 voice codec. The differential output capability of the
TPA6203 was used because:
•
No output DC blocking capacitor was required, saving cost and space,
minimizing “popping” noises during power up/down transients, and improving
low-frequency audio response.
•
Output signal is biased at Vdd/2, regardless of input signal biasing.
•
Higher output power (4x) can be realized, for the same power supply voltage
and load impedance, with differential drive as compared to single-ended
output.
The TPA6203 amplifier is supplied directly from the battery to prevent current-surge
induced noise from propagating through to the other components. This capability is
possible due to the wide range of power supply voltage that the TPA6203 can
accept. The “C
bypass
” capacitor was added to enhance the amplifier’s power supply
rejection ratio (PSRR). The recommended 10
µ
F C
bypass
capacitor is a relatively
expensive component, so the end customer should decide whether the improvement
in PSRR is worth the extra cost and board space required by that component. For
more information on this audio amplifier, please visit
http://focus.ti.com/docs/prod/folders/print/tpa6203a1.html
The speaker used for this design was the Prime Acoustics 20N10 32
Ω
device. This
product was chosen for its small size, low power consumption, and very low cost.
The board layout for this project was performed specifically for this product. For
more information on this speaker, please visit
http://www.speaker.co.kr/product/micro.html.
While this particular speaker was satisfactory for this project, our speaker selection
process did not fully consider real-world production issues. In other words, we did
not have to select a speaker that would fit within a particular plastic injection-molding
cast. The end customer is encouraged to consider their own headset housing as
they select a low cost, small form factor speaker
4.2 RF Transceiver – MICRF505
The RF transceiver circuitry is based on the Micrel MICRF505 ‘zero-IF’ single-chip
transceiver. This device was chosen because of its low external component cost,
device flexibility, and its ability to block co-channel interference.
