SA58646 Datasheet, PDF (10/42 Pages) NXP Semiconductors – UHF 900 MHz transceiver IC

NXP Semiconductors
SA58646
UHF 900 MHz transceiver IC
7.3 Transmitter part
The transmitter architecture is of the direct modulation type. The transmit VCO will be
frequency modulated by either speech or data (see Figure 4).
Before the VCO, an amplifier sums the modulating signal and the data TX signal. VCO
varicaps are integrated. External inductors that are in series with bonding wires and lead
frame are needed to obtain the right frequency. The power amplifier is capable of driving
50 Ω. The output level is programmed through the serial bus interface.
data TX
TXO
MODI
MODO
summator
amplifier
VB
XTAL
DUAL PLL
FREQUENCY
SYNTHESIZER
TXPD
VCO
TX
PA
PAO
PA_OUT
[2:0]
TXLOX
TXLF TXLOY
VREG
TX
VTX
Fig 4. Transmit part
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7.4 Synthesizer
The crystal local oscillator and reference divider provide the reference frequency for the
RX and TX PLLs. The 10-bit programmed divider value for the reference divider is
selected based on the crystal frequency, the desired RX and TX reference frequency
values. The crystal frequency of 16.348 MHz is chosen to provide to the microcontroller
the standard 4.096 MHz frequency when programming the clock divider value to 4. Then
the 16.384 MHz crystal frequency is proposed. The clock divider value will be
programmed to 1, 2, 2.5, 4 and 128. The clock divider value of 128 is chosen to place the
SA58646 in Sleep mode which enables current saving in the microcontroller. The clock
output is an emitter follower type.
The 16-bit TX counter is programmed for the desired transmit channel frequency. The
16-bit RX counter is programmed for the desired local oscillator frequency. The counters
are built with a 6-bit prescaler (divider value R from 64 to 127) and a 10-bit divider (divider
value C from 8 to 1023). The full counter then provides a divider value from 512 to 65535.
To calculate the settings of the two counters, the following procedure is used:
C = int (M / 64)
R = M − C × 64
where M being the division ratio between the VCO frequency and the reference frequency.
SA58646_1
Product data sheet
Rev. 01 — 8 February 2007
© NXP B.V. 2007. All rights reserved.
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