ADF7011_15 datasheet(19/24 Pages) AD

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ADF7011

–19–

MODULATION SCHEMES

Frequency Shift Keying (FSK)

Frequency shift keying is implemented by setting the N value

for the center frequency and then toggling this with the TxDATA

line. The deviation from the center frequency is set using Bits

D1–D7 in the modulation register. The deviation from the

center frequency in Hz is

FSK

PFD Frequency

Modulation Number

DEVIATION

() =

The modulation number is a number from 1 to 127 (Bits D1–

D7 in modulation register). FSK is selected by setting Bits S1

and S2 to zero in the modulation register.

PFD/

CHARGE

PUMP

INTEGER-N

FRACTIONAL-N

THIRD ORDER

MODULATOR

–FDEV

+FDEV

TxDATA

FSK DEVIATION

FREQUENCY

INTERNAL VCO USING

SPIRAL INDUCTORS

GAIN 70 MHz/V–90 MHz/V

PA STAGE

CHEAP AT CRYSTAL

VCO

Figure 13. FSK Implementation

Gaussian Frequency Shift Keying (GFSK)

Gaussian frequency shift keying reduces the bandwidth occupied

by the transmitted spectrum by digitally prefiltering the TxDATA.

A TxCLK output line is provided from the ADF7011 for syn-

chronization of TxDATA from the microcontroller. The TxCLK

line may be connected to the clock input of an external shift

SHIFT

ADF7011

DATA FROM

MICROCONTROLLER

TxDATA

TxCLK

ANTENNA

Figure 14. TxCLK Pin Synchronizing Data for GFSK

Setting Up the ADF7011 for GFSK

To set up the frequency deviation, set the PFD and the mod

control Bits MC1 to MC3.

GFSK

PFD Frequency

DEVIATION

() =

× 2

where m is mod control (Bits MC1 to MC3 in the modulation

To set up the GFSK data rate

Data Rate bits s

PFD Frequency

Divider Factor

Index Counter

() =

Amplitude Shift Keying (ASK)

Amplitude shift keying is implemented by switching the output

stage between two discrete power levels. This is implemented by

toggling the DAC, which controls the output level between two

7-bit values set up in the modulation register. A zero TxDATA

bit sends Bits D1–D7 to the DAC. A high TxDATA bit sends

Bits P1–P7 to the DAC. A maximum modulation depth of 30 dB

is possible. ASK is selected by setting Bit S2 = 1 and Bit S1 = 0.

On-Off Keying (OOK)

On-off keying is implemented by switching the output stage to a

certain power level for a high TxDATA bit and switching the

output stage off for a zero. Due to feedthrough effects, a maximum

modulation depth of 33 dB is specified. For OOK, the transmitted

power for a high input is programmed using Bits P1–P7 in the

modulation register. OOK is selected by setting Bits S1 and S2

to 1 in the modulation register.

CHOOSING CHANNELS FOR BEST SYSTEM

PERFORMANCE

The fractional-N PLL allows the selection of any channel within

868 MHz to 870 MHz to a resolution of <l00 Hz, as well as

facilitating frequency hopping systems.

Careful selection of the RF transmit channels must be made to

achieve best spurious performance. The architecture of frac-

tional-N results in some level of the nearest integer channel

moving through the loop to the RF output. These “beat-note”

spurs are not attenuated by the loop if the desired RF channel

and the nearest integer channel are separated by a frequency of

less than the loop BW.

The occurrence of beat-note spurs is rare, as the integer frequen-

cies are at multiples of the reference, which is typically >4 MHz.

The beat-note spurs can be significantly reduced in amplitude

by avoiding very small or very large values in the fractional

quency, a 100 kHz loop filter will reduce the level to < –45 dBc.

When using an external VCO, the Fast Lock (bleed) function will

reduce the spurs to < –60 dBc for the same conditions above.

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