ADF7021 datasheet(28/64 Pages) AD

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ADF7021

Data Sheet

Rev. B | Page 28 of 64

Table 10. 3-Level Signal Mapping of the Convolutional Encoder

TxDATA

Precoder Output

Encoder Output

−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

SYMBOL

FREQUENCIES

fDEV

–

fDEV

–3

fDEV

001

1011

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.

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

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.

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