adf7023-j Analog Devices, Inc., adf7023-j Datasheet - Page 69

• Manufacturer Part Number: adf7023-j
• Description: High Performance, Low Power, Ism Band Fsk/gfsk/msk/gmsk Transceiver Ic
• Manufacturer: Analog Devices, Inc.

2FSK/GFSK/MSK/GMSK DEMODULATION
A correlator demodulator is used for 2FSK, GFSK, MSK, and GMSK
demodulation. The quadrature outputs of the IF filter are first
limited and then fed to a digital frequency correlator that performs
filtering and frequency discrimination of the 2FSK/GFSK/MSK/
GMSK spectrum. Data is recovered by comparing the output
levels from two correlators. The performance of this frequency
discriminator approximates that of a matched filter detector, which
is known to provide optimum detection in the presence of additive
white Gaussian noise (AWGN). This method of 2FSK/GFSK/MSK/
GMSK demodulation provides approximately 3 dB to 4 dB better
sensitivity than a linear frequency discriminator. The 2FSK/GFSK/
MSK/GMSK demodulator architecture is shown in Figure 80.
The ADF7023-J is configured for 2FSK/GFSK/MSK/GMSK
demodulation by setting DEMOD_SCHEME = 0 in the
RADIO_CFG_9 register (Address 0x115).
To optimize receiver sensitivity, the correlator bandwidth and
phase must be optimized for the specific deviation frequency,
data rate, and maximum expected frequency error between the
transmitter and receiver. The bandwidth and phase of the
discriminator must be set using the DISCRIM_BW bits in the
RADIO_CFG_3 register (Address 0x10F) and the DISCRIM_
PHASE[1:0] bits in the RADIO_CFG_6 register (Address 0x112).
The discriminator setup is performed in three steps.
Step 1: Calculate the Discriminator Bandwidth
Coefficient K
The Discriminator Bandwidth Coefficient K depends on the
modulation index (MI), which is determined by
where
FSK_Dev is the 2FSK/GFSK/MSK/GMSK frequency deviation
in hertz (Hz), measured from the carrier to the +1 symbol
frequency (positive frequency deviation) or to the −1 symbol
frequency (negative frequency deviation).
Data Rate is the data rate in bits per second (bps).
RFIO_1N
RFIO_1P
MI
=
LNA
2 ×
Data
FSK
SYNTHESIZER
Rate
_
MIXER
(LO)
RF
Dev
(ADDRESS RADIO_CFG_9[7:6])
MAX_AFC_RANGE[7:0]
IFBW[1:0]
IF FILTER
AFC SYSTEM
IF
RANGE
Figure 80. 2FSK/GFSK/MSK/GMSK Demodulation and AFC Architecture
AFC_LOCK_MODE[1:0]
LIMITERS
Q
I
AFC_KI[3:0] (ADDRESS RADIO_CFG_11[7:4])
DISCRIM_PHASE[1:0]
CONTROL
PI
CORRELATOR
FREQUENCY
AFC_KP[3:0]
DISCRIM_BW[7:0]
Rev. 0 | Page 69 of 100
POST_DEMOD_BW[7:0]
AVERAGING
FILTER
POST-DEMOD
The value of K is then determined by
MI ≥ 1, AFC off: K = Floor
MI < 1, AFC off: K = Floor
MI ≥ 1, AFC on: K = Floor
MI < 1, AFC on: K = Floor
where:
MI is the modulation index.
K is the discriminator coefficient.
Floor[x] is a function to round down to the nearest integer.
IF_Freq is the IF frequency in hertz (200 kHz or 300 kHz).
FSK_Dev is the 2FSK/GFSK/MSK/GMSK frequency deviation
in hertz.
Freq_Error_Max is the maximum expected frequency error, in
hertz, between Tx and Rx.
Step 2: Calculate the DISCRIM_BW Setting
The bandwidth setting of the discriminator is calculated based
on the Discriminator Coefficient K and the IF frequency. The
bandwidth is set using the DISCRIM_BW[7:0] setting
(Address 0x10F), which is calculated according to
2
T
FILTER
DISCRIM_BW[7:0] = Round
AFC LOCK
DATA_RATE[11:0]
CLOCK AND
RECOVERY
DATA
RxDATA/
RxCLK
⎡
⎢
⎢
⎢
⎣
⎡
⎢
⎢
⎢
⎣
⎡
⎢
⎣
⎡
⎢
⎢
⎣
Data
Data
FSK
FSK
IF
IF
_
_
2
2
Freq
_
_
Freq
Rate
Rate
⎡
⎢
⎣
Dev
Dev
COMMUNICATIONS PROCESSOR
K
×
IF
⎤
⎥
⎥
⎥
⎦
⎤
⎥
⎦
+
+
IF
IF
. 3
PREAMBLE_MATCH = 0
SYNC WORD
Freq
PREAMBLE
Freq
_
25
_
DETECT
DETECT
_
Freq
Freq
SPORT MODE
GPIOS
Freq
MHz
_
_
ADF7023-J
Error
Error
⎤
⎥
⎦
_
_
Max
Max
⎤
⎥
⎥
⎦
⎤
⎥
⎥
⎥
⎦