ADF7012 datasheet(18/28 Pages) AD

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ADF7012

Rev. 0 | Page 18 of 28

Setting Tuning Sensitivity Value

The tuning sensitivity or kV is usually denoted in MHz/V and is

required for the loop filter design. It refers to the amount that a

change of a volt in the voltage applied to VCOIN pin, changes

the output frequency. Typical data for the ADF7012 over a

frequency range is shown.

FREQUENCY (MHz)

120

100

200

400

300

600

500

800

900

1000

700

1100

Figure 37. kV vs. VCO Frequency

Charge-Pump Current

The charge-pump current allows the loop filter bandwidth to be

changed using the registers. The loop bandwidth reduces as the

charge pump current is reduced and vice versa.

Selecting Loop Filter Bandwidth

Data Rate

The loop filter bandwidth should usually be at two to three

times the data rate. This ensures that the PLL has ample time

to jump between the mark and space frequencies.

ACP

In the case where the ACP specifications are difficult to meet,

the loop filter bandwidth can be reduced further to reduce the

phase noise at the adjacent channel. The filter rolls off at 20 dB

per decade.

Spurious Levels

In the case where the output power is quite high, a reduced loop

filter bandwidth reduces the spurious levels even further, and

provides additional margin on the specification.

The following sections provide examples of loop filter designs

for typical applications in specific frequencies.

PA MATCHING

The ADF7012 exhibits optimum performance in terms of

transmit power and current consumption only if the RF output

port is properly matched to the antenna impedance.

ZOPT_PA depends primarily on the required output power,

and the frequency range. Selecting the optimum ZOPT_PA

helps to minimize the current consumption. This data sheet

contains a number of matching networks for common

frequency bands. Under certain conditions it is recommended

to obtain a suitable ZOPT_PA value by means of a load-pull

measurement.

ANTENNA

LPF

RFOUT

ZOPT_PA

DVDD

Figure 38. ADF7012 with Harmonic Filter

The impedance matching values provided in the next section

are for 50 Ω environments. An additional matching network

may be required after the harmonic filter to match to the

antenna impedance. This can be incorporated into the filter

design itself in order to reduce external components.

TRANSMIT PROTOCOL AND CODING

CONSIDERATIONS

PREAMBLE

SYNC

WORD

FIELD

DATA FIELD

CRC

Figure 39. Typical Format of a Transmit Protocol

A dc-free preamble pattern such as 10101010… is recom-

mended for FSK/ASK/OOK demodulation. Preamble patterns

with longer run-length constraints such as 11001100…. can also

be used. However, this can result in a longer synchronization

time of the received bit stream in the chosen receiver.

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