ADF7901

Rev. A| Page 9 of 12

CIRCUIT DESCRIPTION

Table 4.

Frequency (MHz)

FSK3

FSK2

FSK1

OOK_SEL

369.5

0

0

0

0

371.1

0

0

1

0

375.3

0

1

0

0

376.9

0

1

0

0

384.0

Don’t

care

Don’t

care

Don’t

care

1

388.3

1

0

0

0

391.5

1

0

1

0

394.3

1

1

0

0

395.9

1

1

1

0

LOOP FILTER

The loop filter integrates the current pulses from the charge

pump to form a voltage that tunes the output of the VCO to the

desired frequency. It also attenuates spurious levels generated by

the PLL. The recommended loop filter design for this circuit is

297 kHz. This is based on the trade-off between attenuation of

beat note spurs and the need to minimize chirp when the PA is

turned on.

CHARGE

PUMP OUT

VCO

R2 = 6.2k

Ω

C3 = 10pF

C1 = 33pF

C2 = 390pF

R1 = 3k

Ω

Figure 7.

Improved spurious performance in FSK mode can be achieved

by using a narrower loop bandwidth. For a data rate of 20 kbps,

a loop bandwidth of roughly 50 kHz would be suitable. The

following components give a loop bandwidth of 51.1 kHz:

C1 = 680 pF

C2 = 15 nF

C3 = 180 pF

R1 = 510 Ω

R2 = 6.2 kΩ

ADIsimPLL is a free software tool offered by Analog Devices

for assistance in designing with ADI’s frequency synthesizers

and ISM band transmitters. To select the correct loop filter

components for use with the ADF7901, open a project for the

ADF7012 device. Then, enter the desired output carrier

frequency and loop bandwidth, and use the 870 μA charge

pump current setting.

ADIsimPLL can be downloaded from www.analog.com.

CHANNEL FREQUENCIES

The nine channel frequencies listed in Table 4 are obtainable

from a single 9.8304 MHz crystal reference by changing the

value of the N and F numbers in the fractional PLL, using

control lines FSK1, FSK2, and FSK3. The channel frequency is

given by

However, the VCO is tuned to operate over a frequency range

of 344 MHz to 401 MHz (typically). Therefore, any channel

frequency within this range can be obtained if the required

reference frequency is used. The N and F numbers for each

channel are listed in Table 5, together with the corresponding

channel frequencies for 9.8304 MHz and, for example purposes,

frequencies for 10 MHz. With the 10 MHz reference, the two

largest N settings give channel frequencies above the maximum

VCO output frequency and are therefore invalid.

Frequency deviation is also dependent on reference frequency.

The relationship is given by

Therefore, the frequency deviation is 34.8 kHz when the

9.8304 MHz reference is used and 35.4 kHz when the 10 MHz

reference is used.

Table 5.

Channel Frequency (MHz)

N

F

9.8304 MHz Ref

10 MHz Ref

37

2406/4096

369.5

375.9

37

3073/4096

371.1

377.5

38

727/4096

375.3

381.8

38

1374/4096

376.9

383.4

39

256/4096

384.0

390.6

39

2048/4096

388.3

395

39

3381/4096

391.5

398.3

40

452/4096

394.3

N/A

40

1118/4096

395.9

N/A