參數(shù)資料
型號(hào)： TRF2056PWR
廠商： TEXAS INSTRUMENTS INC
元件分類： PLL合成/DDS/VCOs
英文描述： PLL FREQUENCY SYNTHESIZER, 1200 MHz, PDSO20
封裝： PLASTIC, SOP-20
文件頁(yè)數(shù)： 11/24頁(yè)
文件大?。?/td> 327K
代理商： TRF2056PWR
TRF2056
LOW VOLTAGE 1.2GHz FRACTIONALN/INTEGERN SYNTHESIZER
SLWS111– NOVEMBER 2000
19
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION1234
5.
Determine the values of CN and RN for the main PHP charge-pump peak current of 500
A. Assume that
a midrange value of CN equals 128.
RN(k
W) + 18.75
CN
256
1
I(mA)
–0.75
+ 18.75
128
256
1
0.5 mA
–0.75
+ 18 kW
6.
The values of the fundamental compensation pulse magnitude calculated in step 3 and the compensation
pulse width calculated in step 2 are fixed. However, because the VCO can tune over a significant range of
frequencies, the pulse width of the fractional-N portion of the main PHP charge-pump waveform varies;
thus, the area of the same waveform varies. In order to maintain equal areas under the fractional-N portion
of the main PHP charge-pump and compensation waveforms, CN must vary with the VCO frequency. As
the VCO frequency increases, the fractional-N portion of the main PHP charge-pump waveform pulse width
decreases proportionally, thereby decreasing the area under the same waveform. Therefore, CN is
adjusted to equalize the main PHP and compensation waveform areas, as follows:
FracPW-LWR = 132.557 ps for fVCO = 942.99 MHz
FracPW-UPR = 129.815 ps for fVCO = 962.91 MHz
The fundamental area of the fractional-N portion of the main PHP charge-pump waveform (step 1) is
calculated as 0.065593 ps-A. If you calculate the fundamental area of the fractional-N portion of the main
PHP charge-pump waveform using the actual pulse widths above in place of the average pulse width
calculated in step 1, the fractional-N main PHP areas are obtained as follows:
FracArea-LWR = (132.557 ps) (0.500 mA) = 0.066279 (ps-A)
FracArea-UPR = (129.815 ps) (0.500 mA) = 0.064908 (ps-A)
The actual areas under the fractional-N portion of the main PHP waveform require slight modification in the
charge-pump current. The variation of CN required for area equalization is determined using a simple ratio
form:
CN
LWR +
Frac
Area–AVG
CN
AVG +
0.065593
0.066279
128
+ 126
CN
UPR +
Frac
Area
*AVG
Frac
Area
*UPR
CN
AVG +
0.065593
0.064908
128
+ 130
Frac
Area
*LPR
Therefore, for this example, CN can vary from 126 to 130 over the VCO frequency range of 942.99 MHz
to 962.91 MHz for optimum fractional-N sideband suppression. Due to component and circuit tolerances,
additional deviations in CN may be appropriate.
auxiliary divider
The input signal on AUXIN is amplified by a single-ended, ac-coupled input buffer/amplifier that has sufficient
sensitivity (200 mVpp at 100 MHz) for direct connection to a typical VCO. The 12-bit (NA) auxiliary divider
incorporates a divide by 1 (PA = 1) or divide by 4 (PA = 0) prescaler. The total division ratio can be expressed
as:
NTotal = 4 x NA
where PA = 0
NTotal = NA,
where PA = 1 and NA = 4 to 4095.
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