SA58646 Datasheet, PDF (13/42 Pages) NXP Semiconductors – UHF 900 MHz transceiver IC

NXP Semiconductors
SA58646
UHF 900 MHz transceiver IC
7.6 TX baseband
This section covers the TX audio path from pins MICI to TXO. The input signal at pin MICI
is AC-coupled. The microphone amplifier output is also AC-coupled.
The microphone amplifier is an inverting operational amplifier whose gain can be set by
external resistors. The non-inverting input is connected to the internal VB reference
voltage. External resistors are used to set the gain and frequency response.
The TX baseband has a compressor with the characteristic shown in Figure 9. The
Automatic Level Control (ALC) provides a ‘soft’ limit to the output signal swing as the input
voltage increases slowly (that is, a sine wave is maintained at the output). A hard limiter
clamps the compressor output voltage at 1.26 V (p-p). The ALC and hard limiter can be
disabled via the microcontroller interface. The hard limiter is followed by a mute. The TX
gain is digitally programmable with 32 steps of 0.5 dB.
MICO
MICI ^VB
microphone
amplifier
Fig 8. TX baseband part
CMPI
ALC
ALC
COMPRESSOR
HARD
LIMITER
CCAP
HD_LIM
TX gain
MUTE
TX_GAIN
TX_MU ^[4:0]
TXO
001aaf623
V_TXO
(dBV)
0
−10
−20
−30
−40
−60
y = 0.5x − 5
−50
−40
−30
Fig 9. Compressor characteristic
hard limiting signals:
V_CMPI = −4 dBV
V_TXO = 1.26 V (p-p)
V_CMPI = −2.5 dBV
V_TXO = −11.5 dBV
slowly changing
ALC signals:
V_CMPI = −16 dBV
V_TXO = −13 dBV
−20
−10
0
V_CMPI (dBV)
001aaf624
SA58646_1
Product data sheet
Rev. 01 — 8 February 2007
© NXP B.V. 2007. All rights reserved.
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