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參數(shù)資料
| 型號： | TRF2050PW |
| 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, SO-20 |
| 文件頁數(shù)： | 19/34頁 |
| 文件大?。?/td> | 543K |
| 代理商： | TRF2050PW |
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TRF2050
LOW-VOLTAGE 1.2-GHz FRACTIONAL-N/INTEGER-N SYNTHESIZER
SLWS030E– JUNE 1996 – REVISED OCTOBER 2000
26
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
main divider – fractional-N sidebands and compensation (continued)
0
3
6
1
47
25
0
Pulse-Width
Modulation
Pulse-Amplitude
Modulation
Fractional
Accumulator State
Main PHP Charge Pump
Fractional-N Ripple
Magnitude
Compensation Charge
Pump Fractional-N
Ripple Magnitude
mA
A
Figure 23. Main PHP and Compensation Charge Pump Fractional-N Waveforms for 3/8 Channel
The compensation waveform is pulse-amplitude modulated with the contents of the fractional accumulator. The
main PHP pulse magnitude is much larger than the compensation pulse magnitude but the compensation pulse
has a much longer duration than that of the main PHP pulse. The compensation pulse is optimally centered
about the main PHP charge pump pulse in order to avoid additional sideband energy due to the phase offset
between the main and compensation pulses.
The following example illustrates a method for determining correct values for RN, RF, and CN for minimal
fractional-N sidebands based on VCO frequency and reference frequency.
Assumptions:
The main VCO is locked on channel
953-MHz
±10-MHz main VCO operation, 942.99–962.91 MHz
19.44-MHz reference frequency
240-kHz phase detector reference frequency
500-
A peak main PHP current
D Determine the fundamental fractional-N pulse width portion of the main PHP charge-pump output waveform
for the lower, upper, and mean frequencies.
Frac
PW–LWR +
1
f
PD
–
N
f
VCO
+
1
240 kHz
–
3929
942.99 MHz
+ 132.557 ps,
Frac
PW–UPR +
1
f
PD
–
N
f
VCO
+
1
240 kHz
–
4012
962.91 MHz
+ 129.815 ps,
Frac
PW–MEAN +
Frac
PW
*LWR ) FracPW–UPR
2
+
132.557 ps
) 129.815 ps
2
+ 131.186 ps.
The mean-unit pulse width of the fractional-N portion of the main PHP charge-pump output waveform over
the VCO frequencies of interest is 131.186
ps. This fundamental pulse width is modulated by the contents
of the fractional accumulator. For the 3/8 fractional-N channel example, the pulse width varies as shown
in Table 5.
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