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STW81100 bảng dữ liệu(PDF) 11 Page - STMicroelectronics |
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STW81100 bảng dữ liệu(HTML) 11 Page - STMicroelectronics |
11 / 29 page 11/29 STW81100 6 General Description The block diagram of Figure 2 shows the different blocks, which have been integrated to achieve an inte- ger-N PLL frequency synthesizer. The STW81100 consists of 2 internal low-noise VCOs with buffer blocks, a divider by 2, a divider by 4, a low-noise PFD (Phase Frequency Detector), a precise charge pump, a 9-bit programmable reference di- vider, two programmable counters and a dual-modulus prescaler. The A-counter (6 bits) and B counter (9 bits) counters, in conjunction with the dual modulus prescaler P/ P+1 (64/65), implement an N integer divider, where N = B*P +A. The division ratio of both reference and VCO dividers is controlled through an I2C bus interface. All devices operate with a power supply of 3.3 V and can be powered down when not in use. 7 Circuit Description 7.1 Reference input stage The reference input stage is shown in Figure 12. The resistor network feeds a DC bias at the Fref input while the inverter used as the frequency reference buffer is AC coupled. Figure 12. Reference Frequency Input Buffer 7.2 Reference Divider The 9-bit programmable reference counter allows the input reference frequency to be divided to produce the input clock to the PFD. The division ratio is programmed through the I2C bus interface. 7.3 Prescaler The dual-modulus prescaler 64/65 takes the CML clock from the VCO buffer and divides it down to a man- ageable frequency for the CMOS A and B counters. It is based on a synchronous 4/5 core which division ratio depends on the state of the modulus input. 7.4 A and B Counters The A (6 bits) and B (9 bits) counters, in conjunction with the dual modulus prescaler make it possible to generate output frequencies which are spaced only by the reference frequency divided by the reference division ratio. Thus, the division ratio and the VCO output frequency are given by these formulas: N = B x P + A INV BUF VDD Fref Power Down INV BUF VDD Fref Power Down F VC O BP A + ⋅ () F ref ⋅ R ------------------------------------------ = |
Số phần tương tự - STW81100_1 |
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Mô tả tương tự - STW81100_1 |
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