LMX2487 National Semiconductor Corporation, LMX2487 Datasheet - Page 26

• Manufacturer Part Number: LMX2487
• Description: 3.0 Ghz - 6.0 Ghz High Performance Delta-sigma Low Power Dual Pllatinum? Frequency Synthesizers With 3.0 Ghz Integer Pll
• Manufacturer: National Semiconductor Corporation

www.national.com
REGISTER
Programming Description
2.1 R0 REGISTER
Note that this register has only one control bit, so the N counter value to be changed with a single write statement to the PLL.
2.1.1 RF_FN[11:0] -- Fractional Numerator for RF PLL
Refer to section 2.6.1 for a more detailed description of this control word.
2.1.2 RF_N[10:0] -- RF N Counter Value
The RF N counter contains an 16/17/20/21 and a 32/33/36/37 prescaler. The N counter value can be calculated as follows:
N = RF_P·RF_C + 4·RF_B + RF_A
RF_C ≥Max{RF_A, RF_B} , for N-2
RF_B counter has an unused bit, where this extra bit is used by the delta-sigma modulator for the purposes of modulation.
Consult the tables below for valid operating ranges for each prescaler.
Operation with the 16/17/20/21 Prescaler (RF_P=0)
Operation with the 32/33/36/37 Prescaler (RF_P=1)
2040-2043
2044-2045
R0
227--230
>
RF_N
49-63
64-79
97-226
>
1023
RF_N
<
2039
1023
<
80
231
...
2045
49
...
97
23
22
0
1
.
0
1
.
21
0
1
.
0
1
.
20
19
RF_N[10:0]
FM
RF_C [5:0]
0
1
.
RF_C [5:0]
2nd Order Modulator: 97-109, 129-145, 161-181, 193-217, 225-226
0
1
-1 ... N+2
.
18
3rd Order Modulator: 99-107, 131-143, 163-179, 195-215
Possible with a second or third order delta-sigma engine.
4th Order Modulator: 103, 135-139, 167-175, 199-211
Possible only with a second order delta-sigma engine.
17
1
1
(Continued)
.
1
1
.
FM
N Values Below 97 are Illegal.
N values greater than 2045 are prohibited.
16
is a necessary condition. This rule is slightly modified in the case where the
N values above 1023 are prohibited.
2nd and 3rd Order Modulator: All
2nd and 3rd Order Modulator: All
0
1
.
N Values Below 49 are Illegal.
1
1
.
15
2nd Order Modulator: 49-61
3rd Order Modulator: 51-59
4th Order Modulator: 64-75
4th Order Modulator: None
Legal Divide Ratios are:
4th Order Modulator: 55
Legal Divide Ratios are:
Legal Divide Ratios are:
Legal Divide Ratios are:
14
DATA[22:0]
26
1
1
.
RF_N [10:0]
1
1
.
RF_N [10:0]
13
12
0
0
0
0
1
.
11
RF_B [2:0]
RF_B [2:0]
10
0
1
.
0
0
.
9
RF_FN[11:0]
8
0
1
.
1
1
.
7
6
5
0
1
1
1
.
.
RF_A [1:0]
RF_A [1:0]
4
3
2
0
1
1
1
.
.
1
C0
0
0