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參數(shù)資料
| 型號： | 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 |
| 文件頁數(shù)： | 8/24頁 |
| 文件大?。?/td> | 327K |
| 代理商： | TRF2056PWR |
TRF2056
LOW VOLTAGE 1.2GHz FRACTIONALN/INTEGERN SYNTHESIZER
SLWS111– NOVEMBER 2000
16
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
main divider – fractional-N sidebands and compensation
Programming a fractional-N channel means the main divider and prescaler divide by N or N + 1 as dictated by
the operation of the fractional accumulator. Because the main divider operation is integer in nature and the
desired VCO frequency is not, the output of the main phase detector is modulated with a resultant fractional-N
phase ripple that produces sideband energy if left uncompensated. This phase ripple is proportional and
synchronized to the contents of the fractional accumulator that is used to control fractional-N sideband
compensation. Only channels that require a nonzero value of NF have the fractional-N sideband energy. The
fractional-N sidebands, which appear at offset frequencies from the VCO fundamental tone, are multiples of
NF/FMOD. Figure 7 shows the fractional-N phase detector ripple for a 3/8 fractional channel.
240 kHz
03
6
1
4
7
2
5
0
Main Phase
Detector Reference
Main Phase
Detector VCO
Feedback
Main Phase
Detector Fractional-N
Ripple
Fractional
Accumulator State
Figure 7. Fractional-N Phase Detector Ripple for 3/8 Channel
The TRF2056 device has internal circuitry that provides a means to compensate for the phase detector
fractional-N phase ripple, thereby significantly reducing the magnitude of the fractional-N sidebands. Because
the current waveform output of the main PLL proportional charge pumps is modulated with the phase detector
fractional-N phase ripple, a fractional-N compensation charge-pump output is summed with the main PLL
proportional charge pump.
Figure 8 shows the fractional-N ripple magnitude on the main PHP charge-pump output. The magnitude is
essentially constant, and the pulse width is modulated with the contents of the fractional accumulator. The area
under the main PHP charge-pump curve represents the amount of charge delivered to the loop filter network.
In order to minimize fractional-N sidebands in the VCO spectrum, the compensation current waveform is
generated to have equal and opposite sign magnitude area to the main PHP charge pump.
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