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.
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):
If fLO ≤ 1125MHz:
Program bit D24 (Reg 06) to 1 (divide-by-4 mode)
fVCO = fLO x 4
D24 = 0 (divide-by-2 mode)
fVCO = fLO x 2
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).
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)|
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.