EVAL-ADF7021DBZ5 Datasheet, PDF (28/64 Pages) Analog Devices – High Performance Narrow-Band Transceiver IC

ADF7021
Table 10. 3-Level Signal Mapping of the Convolutional Encoder
TxDATA
1 0 1 1 00 1 00 1
Precoder Output 1 0 0 1 0 1 1 1 1 0
Encoder Output +1 0 −1 +1 0 0 +1 0 0 −1
Another property of this encoding scheme is that the transmitted
symbol sequence is dc-free, which facilitates symbol detection
and frequency measurement in the receiver. In addition, there
is no code rate loss associated with this 3-level convolutional
encoder; that is, the transmitted symbol rate is equal to the data
rate presented at the transmit data input.
3FSK is selected by setting the MODULATION_SCHEME bits
(R2_DB[4:6]) to 010. It can also be used with raised cosine
filtering to further increase the spectral efficiency of the
transmit signal.
4-Level Frequency Shift Keying (4FSK)
In 4FSK modulation, two bits per symbol spectral efficiency is
realized by mapping consecutive input bit-pairs in the Tx data
bit stream to one of four possible symbols (−3, −1, +1, +3). Thus,
the transmitted symbol rate is half of the input bit rate.
By minimizing the separation between symbol frequencies,
4FSK can have high spectral efficiency. The bit-to-symbol
mapping for 4FSK is gray coded and is shown in Figure 43.
Tx DATA 0 0 0 1 1 0 1 1
f
+3f_DEV
SYMBOL
FREQUENCIES
+f_DEV
–f_DEV
–3f_DEV
t
Figure 43. 4FSK Bit-to-Symbol Mapping
The inner deviation frequencies (+fDEV and − fDEV) are set using
the Tx_FREQUENCY_DEVIATION bits, R2_DB[19:27]. The
outer deviation frequencies are automatically set to three times
the inner deviation frequency.
Data Sheet
The transmit clock from Pin TxRxCLK is available after writing
to Register 3 in the power-up sequence for receive mode. The
MSB of the first symbol should be clocked into the ADF7021 on
the first transmit clock pulse from the ADF7021 after writing to
Register 3. Refer to Figure 6 for more timing information.
Oversampled 2FSK
In oversampled 2FSK, there is no data clock from the TxRxCLK
pin. Instead, the transmit data at the TxRxDATA pin is sampled
at 32 times the programmed rate.
This is the only modulation mode that can be used with the UART
mode interface for data transmission (refer to the Interfacing to
Microcontroller/DSPsection for more information).
SPECTRAL SHAPING
Gaussian or raised cosine filtering can be used to improve
transmit spectral efficiency. The ADF7021 supports Gaussian
filtering (bandwidth time [BT] = 0.5) on 2FSK modulation.
Raised cosine filtering can be used with 2FSK, 3FSK, or 4FSK
modulation. The roll off factor (alpha) of the raised cosine filter
has programmable options of 0.5 and 0.7. Both the Gaussian
and raised cosine filters are implemented using linear phase
digital filter architectures that deliver precise control over the
BT and alpha filter parameters, and guarantee a transmit spectrum
that is very stable over temperature and supply variation.
Gaussian Frequency Shift Keying (GFSK)
Gaussian frequency shift keying reduces the bandwidth occupied
by the transmitted spectrum by digitally prefiltering the transmit
data. The BT product of the Gaussian filter used is 0.5.
Gaussian filtering can only be used with 2FSK modulation. This
is selected by setting R2_DB[4:6] to 001.
Raised Cosine Filtering
Raised cosine filtering provides digital prefiltering of the transmit
data by using a raised cosine filter with a roll-off factor (alpha)
of either 0.5 or 0.7. The alpha is set to 0.5 by default, but the
raised cosine filter bandwidth can be increased to provide less
aggressive data filtering by using an alpha of 0.7 (set R2_DB30
to Logic 1). Raised cosine filtering can be used with 2FSK,
3FSK, and 4FSK.
Raised cosine filtering is enabled by setting R2_DB[4:6] as
outlined in Table 11.
Rev. B | Page 28 of 64