ADF7021 datasheet(27/64 Pages) AD

27 / 64 page

Data Sheet

ADF7021

Rev. B | Page 27 of 64

Setting the Transmit Data Rate

In all modulation modes except oversampled 2FSK mode, an

accurate clock is provided on the TxRxCLK pin to latch the data

from the microcontroller into the transmit section at the required

data rate. The exact frequency of this clock is defined by

DATA CLK =

DIVIDE

CLK

CDR

DIVIDE

CLK

DEMOD

XTAL

where:

XTAL is the crystal or TCXO frequency.

DEMOD_CLK_DIVIDE is the divider that sets the demodulator

clock rate (R3_DB[6:9]).

CDR_CLK_DIVIDE is the divider that sets the CDR clock rate

(R3_DB[10:17]).

Refer to the Register 3—Transmit/Receive Clock Register

section for more programming information.

Setting the FSK Transmit Deviation Frequency

In all modulation modes, the deviation from the center

frequency is set using the Tx_FREQUENCY_DEVIATION bits

(R2_DB[19:27]).

The deviation from the center frequency in Hz is as follows:

For direct RF output,

fDEV [Hz] =

DEVIATION

FREQUENCY

PFD ×

For RF_DIVIDE_BY_2 enabled,

fDEV [Hz] =

DEVIATION

FREQUENCY

PFD×

where Tx_FREQUENCY_DEVIATION is a number from 1 to

511 (R2_DB[19:27]).

In 4FSK modulation, the four symbols (00, 01, 11, 10) are

transmitted as ±3 × fDEV and ±1 × fDEV.

Binary Frequency Shift Keying (2FSK)

Two-level frequency shift keying is implemented by setting the

N value for the center frequency and then toggling it with the

TxDATA line. The deviation from the center frequency is set

using the Tx_FREQUENCY_DEVIATION bits, R2_DB[19:27].

2FSK is selected by setting the MODULATION_SCHEME bits

(R2_DB[4:6]) to 000.

Minimum shift keying (MSK) or Gaussian minimum shift

keying (GMSK) is supported by selecting 2FSK modulation

and using a modulation index of 0.5. A modulation index of

0.5 is set up by configuring R2_DB[19:27] for a FREQDEVIATION =

0.25 × transmit data rate.

3-Level Frequency Shift Keying (3FSK)

In 3-level FSK modulation (also known as modified Duobinary

FSK), the binary data (Logic 0 and Logic 1) is mapped onto

three distinct frequencies, the carrier frequency (fC), the carrier

frequency minus a deviation frequency (fC − fDEV), and the

carrier frequency plus the deviation frequency (fC + fDEV).

A Logic 0 is mapped to the carrier frequency while a Logic 1 is

either mapped onto frequency fC − fDEV or fC + fDEV.

fC – fDEV

fC + fDEV

RF FREQUENCY

–1

Figure 41. 3FSK Symbol-to-Frequency Mapping

Compared to 2FSK, this bits-to-frequency mapping results in a

reduced transmission bandwidth because some energy is removed

from the RF sidebands and transferred to the carrier frequency.

At low modulation index, 3FSK improves the transmit spectral

efficiency by up to 25% when compared to 2FSK.

Bit-to-symbol mapping for 3FSK is implemented using a linear

convolutional encoder that also permits Viterbi detection to be

used in the receiver. A block diagram of the transmit hardware

used to realize this system is shown in Figure 42. The convolu-

tional encoder polynomial used to implement the transmit

spectral shaping is

P(D) = 1 − D2

where:

P is the convolutional encoder polynomial.

D is the unit delay operator.

A digital precoder with transfer function 1/P(D) implements an

inverse modulo-2 operation of the 1 − D2 shaping filter in the

transmitter.

PRECODER

1/P(D)

CONVOLUTIONAL

ENCODER

P(D)

FSK MOD

CONTROL

AND

DATA FILTERING

Tx DATA

0, 1

0, +1, –1

0, 1

N DIVIDER

fC + fDEV

fC – fDEV

Figure 42. 3FSK Encoding

The signal mapping of the input binary transmit data to the

3-level convolutional output is shown in Table 10. The

convolutional encoder restricts the maximum number of

sequential +1s or −1s to two and delivers an equal number of

+1s and −1s to the FSK modulator, thus ensuring equal spectral

energy in both RF sidebands.

ADF7021 Datenblatt(PDF) 27 Page - Analog Devices

ADF7021 Datenblatt(HTML) 27 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

Datenblatt Download

Link URL