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
05349-008
R2 = 6.2k
Ω
C3 = 10pFC1 = 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
F
CHANNEL
= F
REF
× (N + F)
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
F
DEV
= 58 × (9.8304 MHz)/2
14
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
