MRF89XAM8A-I Datasheet, PDF (59/140 Pages) Microchip Technology – Ultra Low-Power, Integrated ISM Band Sub-GHz Transceiver

3.3 Transmitter
The MRF89XA is set to Transmit mode when the
CMOD<2:0> bits (GCONREG<7:5>) are set to ‘100’
(see Register 2-1).
The transmitter chain in the MRF89XA is based on the
same double-conversion architecture and uses the
same intermediate frequencies as the receiver chain.
3.3.1 BIT RATE SETTING
In Continuous Transmit mode, setting the bit rate
through the BRVAL<6:0> bits (BRSREG<6:0>) is
useful to determine the frequency of DCLK. As
explained in Section 3.9.1, TX Processing, DCLK will
trigger an interrupt on the host microcontroller each
time a new bit has to be transmitted, as shown in
Equation 3-4.
EQUATION 3-4:
BR
=
-------------------------------^f--^x--^t--^a---^l------------------------------
64^∗[1 + val(BRVAL<6:0>)]
3.3.2 ALTERNATIVE SETTINGS
Bit rate, frequency deviation, and TX interpolation filter
settings are a function of the crystal frequency (f_xtal) of
the reference oscillator. Settings other than those
programmed with a 12.8 MHz crystal can be obtained
by selecting the correct reference oscillator frequency.
3.3.3
f_dev SETTING IN FSK MODE
The frequency deviation, f_dev, of the FSK transmitter is
programmed through the FDVAL<7:0> bits
(FDEVREG<7:0>), as shown in Equation 3-5.
EQUATION 3-5:
f_dev
=
--------------------------------^f--^x--^t--^a--^l-------------------------------
32^∗[1 + val(FDVAL<7:0>)]
For correct operation, the modulation index β should
be equal to Equation 3-6.
EQUATION 3-6:
β = --2---^∗----^f--^d--^e---^v-- ≥ 2
BRVAL
For communication between a pair of MRF89XAs the
f_dev should be at least 33 kHz to ensure a correct
operation on the receiver side.
MRF89XA
3.3.4
f_dev SETTING IN OOK MODE
f_dev has no physical meaning in OOK Transmit mode.
However, due to the DDS baseband signal generation,
the OOK signal is always offset by “-f_dev” (see
Section 3.2.7, Frequency Calculation). It is suggested
that f_dev retains its default value of 100 kHz in OOK
mode.
3.3.5 INTERPOLATION FILTER
After digital-to-analog conversion, the I and Q signals
are smoothened by interpolation filters. Low-pass filters
in this block digitally generates the signal and prevent
the alias signals from entering the modulators. Its
bandwidth can be programmed with the
(TXIPOLFV<6:0> bits (TXPARCREG>7:1), and should
be calculated as shown in Equation 3-7.
EQUATION 3-7:
Where,
BW ≅ 3^∗
f_dev
+
B_-----R_--
2
f_dev is the programmed frequency deviation
as set in FDEVREG
BR is the physical bit rate of transmission
Note:
Low interpolation filter bandwidth will
attenuate the baseband I/Q signals, thus
reducing the power of the FSK signal.
Conversely, excessive bandwidth will
degrade spectral purity.
For most of the applications a BW of around 125 KHz
would be acceptable, but for wideband FSK
modulation, the recommended filter setting cannot be
reached. However, the impact on spectral purity will be
negligible due to the existing wideband channel.
© 2010–2011 Microchip Technology Inc.
Preliminary
DS70622C-page 59