MRF89XAM8A-I Datasheet, PDF(20/140 Page) Microchip Technology – Ultra Low-Power, Integrated ISM Band Sub-GHz Transceiver

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MRF89XAM8A-I Datasheet, PDF (20/140 Pages) Microchip Technology – Ultra Low-Power, Integrated ISM Band Sub-GHz Transceiver

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MRF89XA

2.9.1 TRANSMITTER ARCHITECTURE

Figure 2-6 illustrates the transmitter architecture block

diagram. The baseband I and Q signals are digitally

generated by a DDS whose Digital-to-Analog

Converters (DAC) followed by two anti-aliasing low-

pass filters transform the digital signal into analog in-

phase (I) and quadrature (Q) components whose

frequency is the selected frequency deviation, is set

using the FDVAL<7:0> bits from FDEVREG<7:0>.

In FSK mode, the relative phase of I and Q is switched

by the input data between -90Â° and +90Â° with continu-

ous phase. The modulation is therefore performed at

this initial stage, because the information contained in

the phase difference will be converted into a frequency

shift when the I and Q signals are up-converted in the

first mixer stage. This first up-conversion stage is dupli-

cated to enhance image rejection. The FSK convention

is such that:

DATA = 1 â frf + fdev

DATA = 0 â frf â fdev

In OOK mode, the phase difference between the I and

Q channels is kept constant (independent of the

transmitted data). Thus, the first stage of up-conversion

creates a fixed frequency signal at the low IF = fdev (this

explains why the transmitted OOK spectrum is offset by

fdev). OOK Modulation is accomplished by switching the

PA and PA regulator stages ON and OFF. By

convention:

DATA = 1 â PAon

DATA = 0 â PAoff

After the interpolation filters, a set of four mixers

combines the I and Q signals and converts them into a

pair of complex signals at the second intermediate

frequency, equal to one-eighth of the LO frequency, or

one-ninth of the RF frequency. These two new I and Q

signals are then combined and up-converted to the

final RF frequency by two quadrature mixers fed by the

LO signal. The signal is pre-amplified, and then the

transmitter output is driven by a final power amplifier

stage. The I and Q signal details are illustrated in

Figure 2-7.

FIGURE 2-7:

I(t), Q(t) Signals Overview

Fdev

I(t)

Q(t)

DS70622C-page 20

Preliminary

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