Keywords: Faster Lock Time, Tuner Related Parts

APPLICATION NOTE 6448

# Tuner Configuration for Faster Lock Time

Abstract: 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.