ref
min
f
BW
80.75
220
=
´
[ ]
ref
2
f
C8
839
6 pF
BW
æ
ö
=
-
ç
÷
è
ø
[ ]
ref
BW
R3
21823
k
f
æ
ö
=
W
ç
÷
è
ø
[ ]
ref
2
f
C6
80.75
nF
BW
æ
ö
=
ç
÷
è
ø
[ ]
ref
BW
R2
7126
k
f
æ
ö
=
W
ç
÷
è
ø
[ ]
ref
2
f
C7
3037
7 pF
BW
æ
ö
=
-
ç
÷
è
ø
VCO
ref
FREQ
0.5
DITHER
f
f
3
8192
+
´
æ
ö
=
´
+
ç
÷
è
ø
SWRS046H – NOVEMBER 2006 – REVISED MARCH 2015
The VCO frequency is given by
(24)
The VCO frequency is divided by 2 and by 4 to generate frequencies in the two bands, respectively.
The VCO sensitivity (sometimes referred to as VCO gain) varies over frequency and operating conditions.
Typically the VCO sensitivity varies between 12 and 36 MHz/V. For calculations the geometrical mean at
21 MHz/V can be used. The PLL calibration (explained below) measures the actual VCO sensitivity and
adjusts the charge pump current accordingly to achieve correct PLL loop gain and bandwidth (higher
charge pump current when VCO sensitivity is lower).
through
can be used for calculating PLL loop filter component values, see
, for a desired PLL loop bandwidth, BW.
(25)
(26)
(27)
(28)
(29)
Define a minimum PLL loop bandwidth as shown in
(30)
If BW
min
> Baud rate/3 then set BW = BW
min
and if BW
min
< Baud rate/3 then set BW = Baud rate/3 in
through
.
There are two special cases when using the recommended 14.7456 MHz crystal:
1. If the data rate is 4.8 kBaud or below and the channel spacing is 12.5 kHz the following loop filter
components are recommended:
C6 = 220 nF
C7 = 8200 pF
C8 = 2200 pF
R2 = 1.5 k
Ω
R3 = 4.7 k
Ω
2. If the data rate is 4.8 kBaud or below and the channel spacing is different from 12.5 kHz the following
loop filter components are recommended:
C6 = 100 nF
C7 = 3900 pF
C8 = 1000 pF
R2 = 2.2 k
Ω
R3 = 6.8 k
Ω
46
Detailed Description
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