应用笔记 6448

Tuner Configuration for Faster Lock Time


摘要 : This application note describes the configuration for faster lock time for the MAX2112/MAX2120/MAX2121 direct-conversion satellite tuners.

Overview

The MAX2112/MAX2120/MAX2121 VCO autoselect (VAS) circuits are hardwired for a clock period of 57340/fXTAL. Given MAX2120 and fXTAL = 4MHz, this translates to a VAS clock period of 14.34ms. If the VAS must transition from the lowest to highest frequency VCO, the lock time would be approximately 14.34ms x 23 bands, or 330ms. This can be unacceptably slow for many applications. This number can be cut down to 172ms by simply programming the VCO[4:0] bits of Register 07 to 10011, which seeds the VAS operation from nearly the center of the VCO range.

Configuration for Faster Lock Time

For the fastest possible lock time, the VCO[4:0] bits can be seeded according to the following algorithm, given a desired LO frequency (fLO):

  1. Enable VAS mode by programming the VAS bit to 1 (Reg 07).
  2. Choose VCO divider mode (bit D24 of Reg 06) and calculate fVCO:

    If fLO ≤ 1125MHz:

    Program bit D24 (Reg 06) to 1 (divide-by-4 mode)

    fVCO = fLO x 4

    Else:

    D24 = 0 (divide-by-2 mode)

    fVCO = fLO x 2

  3. Choose VCO seed value (VCO[4:0]) based on lookup table (see Table 1):

    If fVCO < f2_3, then VCO[4:0] = 01010 (VCO2).
    Else, if fVCO < f3_4, then VCO[4:0] = 01011 (VCO3).
    Else, if fVCO < f4_5, then VCO[4:0] = 01100 (VCO4).
    . . .
    Else, if fVCO < f21_22, then VCO[4:0] = 11101 (VCO21).
    Else, if fVCO < f22_23, then VCO[4:0] = 11110 (VCO22).
    Else VCO[4:0] = 11111 (VCO23).

  4. Calculate and load N counter value (and F counter value for MAX2112/MAX2120/MAX2121).
    • For the MAX2112, the F counter LSB word must be loaded last to initiate a new VAS sequence.
    • For the MAX2120/MAX2121, the N counter LSB word must be loaded last to initiate a new VAS sequence.

This algorithm seeds the VCO to within ±2 VCOs and thus guarantees a lock time of < 14.34ms × 2 < 30ms for the MAX2112/MAX2120/MAX2121 using a 4MHz XTAL. The MAX2112/MAX2120/MAX2121’s lock time is faster by the ratio of fXTAL/4MHz.

Table 1. Lookup Table for VCO Seed Values
VCO Transition Frequency (MHz)
  MAX2112 MAX2120/2121
f0_1 = 2135 2101
f1_2 = 2200 2165
f2_3 = 2275 2249
f3_4 = 2355 2313
f4_5 = 2445 2396
f5_6 = 2545 2491
f6_7 = 2660 2598
f7_8 = 2760 2707
f8_9 = 2770 2730
f9_10 = 2865 2816
f10_11 = 2965 2911
f11_12 = 3070 3013
f12_13 = 3190 3126
f13_14 = 3330 3256
f14_15 = 3480 3402
f15_16 = 3640 3572
f16_17 = 3685 3633
f17_18 = 3795 3737
f18_19 = 3915 3848
f19_20 = 4035 3965
f20_21= 4170 4092
f21_22 = 4325 4239
f22_23= 4500 4401

Note: Since the minimum required VCO frequency is 2250MHz (1125MHz x 2), the algorithm starts with "if fVCO < f2_3," which is close to 2250MHz.

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APP 6448:
应用笔记 6448,AN6448, AN 6448, APP6448, Appnote6448, Appnote 6448