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15 page 
���������������������������������������������������������������� Maxim Integrated Products 15 MAX2870 23.5MHz to 6000MHz Fractional/ Integer-N Synthesizer/VCO Voltage-Controlled Oscillator The fundamental VCO frequency of the device guarantees gap-free coverage from 3.0GHz to 6.0GHz using four individual VCO core blocks with 16 sub-bands within each block. Connect the output of the loop filter to the TUNE input. The TUNE input is used to control the VCO. Tune ADC A 3-bit ADC is used to read the VCO tuning voltage. The ADC value can be read back by bits 22:20 in register 6. The ADC uses the ranges shown in Table 3. Note that the digital or analog lock detect might still be valid when the tuning voltage is out of the compliance range. VCO Autoselect (VAS) State Machine An internal VCO autoselect state machine is initiated when register 0 is programmed to automatically select the correct VCO if bit VAS_SHDN = 0 (register 3, bit 25). If VAS_SHDN = 1, then the VCO can be manually selected by bits VCO. The state machine clock, fBS, must be set to 50kHz. This is set by the BS bits. The formula for setting BS is: BS = fREF/50kHz where fREF is the reference frequency. The BS (register 4, bits 19:12) value should be rounded to the nearest integer. If the calculated BS is higher than 1023, then set BS = 1023. If fREF is lower than 50kHz, then set BS = 1. The time needed to select the correct VCO is 10 x fBS. The RETUNE (register 3, bit 24) bit is used to enable or disable the VAS auto-retune function. Should the 3-bit TUNE ADC detect that the VCO control voltage (VTUNE) has drifted into the 000 or 111 state, the VAS will initiate an auto-retune if RETUNE = 1. If RETUNE = 0, then this function is disabled. Phase Shift Mode After achieving lock, the phase of the RF output can be changed in increments of P/M x 360N. The absolute phase cannot be determined, but it can be changed relative to the current phase. To change the phase, do the following: 1) Achieve lock at the desired frequency. 2) Set the increment of phase relative to the current phase by setting P = M x {desired_phase_change}/360N. 3) Enable the phase change by setting CDM = 10. 4) Reset CDM = 00. Low-Spur Mode The device offers three modes for the sigma-delta modu- lator. Low-noise mode offers lower in-band noise at the expense of spurs. The spurs can be reduced by setting SDN = 10 (register 2, bits 30:29) or SDN = 11 for different modes of dithering. The user can determine which mode works best for their application. Table 3. ADC VCO Status ADC VCO STATUS 000 Out-of-lock, VTUNE < 0.5V 001 In-lock, 0.5V < VTUNE < 0.7V 010 In-lock, 0.7V < VTUNE < 1.3V 011 Not used 100 Not used 101 In-lock, 1.3V < VTUNE < 2.1V 110 In-lock, 2.1V < VTUNE < 2.5V 111 Out-of-lock, VTUNE > 2.5V |
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