ADF7012 datasheet(15/28 Pages) AD

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ADF7012

Rev. 0 | Page 15 of 28

The nonlinear characteristic of the output stage results in an

output spectrum containing harmonics of the fundamental,

especially the third and fifth. To meet local regulations, a low-

pass filter usually is required to filter these harmonics.

The output stage can be powered down by setting Bit PD2 in

the function register low.

GOOK MODULATION

Gaussian on-off keying (GOOK) represents a prefiltered form

of OOK modulation. The usually sharp symbol transitions are

replaced with smooth Gaussian-filtered transitions with the

result being a reduction in frequency pulling of the VCO.

Frequency pulling of the VCO in OOK mode can lead to a

wider than desired bandwidth, especially if it is not possible to

increase the loop filter bandwidth to > 300kHz.

The GOOK sampling clock samples data at the data rate:

IndexCount

tor

DividerFac

bps

DataRate

PFD

)

(

(9)

Bits D1 to D6 represent the output power for the system for a

positive data bit. Divider Factor = 0x3F represents the max-

imum possible deviation from PA at minimum to PA at

maximum output. An index counter setting of 128 is

recommended.

Figure 32 shows the step response of the Gaussian FIR filter.

An index counter of 16 is demonstrated for simplicity. While

the pre-filter data would switch the PA directly from off to on

with a low-to-high data transition, the filtered data gradually

increases the PA output in discrete steps. This has the effect of

making the output spectrum more compact.

PA SETTING

PRE-FILTER DATA

(0 TO 1 TRANSITION)

DISCRETIZED

FILTER OUTPUT

16 (MAX)

1 (PA OFF)

Figure 32. Varying PA Output for GOOK (Index Counter = 16).

As is the case with GFSK, GOOK requires the bit stream

applied at TxDATA to be synchronized with the sampling clock,

TxCLK (see the GFSK Modulation section).

FREQUENCY (MHz)

–10

–20

–30

–40

–50

–60

–70

–80

909.43

910.43

OOK

GOOK

910.93

Figure 33. GOOK vs. OOK Frequency Spectra

(Narrow-Band Measurement)

FREQUENCY (MHz)

–10

–20

–30

–40

–50

–60

–70

–90

–80

885.43

910.43

935.93

GOOK

OOK

Figure 34. GOOK vs. OOK Frequency Spectra

(Wideband Measurement)

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