HMC832LP6GETR datasheet(17/49 Pages) AD

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HMC832

Data Sheet

VCO SUBSYSTEM

Figure 39. PLL and VCO Subsystems

The HMC832 contains a VCO subsystem that can be

configured to operate in:

•

Fundamental frequency (fo) mode (1500 MHz to

3000 MHz).

•

Divide by N mode, where N = 2, 4, 6, 8 … 58, 60, 62

(25 MHz to 1500 MHz).

All modes are VCO register programmable, as shown in

Figure 39. One loop filter design can be used for the entire

frequency of operation of the HMC832.

VCO Calibration

VCO Autocalibration

The HMC832 uses a step tuned type VCO. A simplified step

tuned VCO is shown in Figure 41. A step tuned VCO is a VCO

with a digitally selectable capacitor bank allowing the nominal

center frequency of the VCO to be adjusted or stepped by

switching in and out of the VCO tank capacitors. Note that

more than one capacitor can be switched in at a time.

A step tuned VCO allows the user to center the VCO on the

required output frequency while keeping the varactor tuning

voltage optimized near the mid voltage tuning point of the

HMC832 charge pump. This enables the PLL charge pump to

tune the VCO over the full range of operation with both a low

tuning voltage and a low tuning sensitivity (kVCO).

The VCO switches are normally controlled automatically by the

HMC832 using the autocalibration feature. The autocalibration

feature is implemented in the internal state machine. It manages

the selection of the VCO subband (capacitor selection) when a

new frequency is programmed. The VCO switches may also be

controlled directly via Register 0x05 for testing or for other

special purpose operations. Other control bits specific to the

VCO are also sent via Register 0x05.

To use a step tuned VCO in a closed loop, the VCO must be

calibrated such that the HMC832 knows which switch position

on the VCO is optimum for the desired output frequency. The

HMC832 supports autocalibration of the step tuned VCO. The

autocalibration fixes the VCO tuning voltage at the optimum

midpoint of the charge pump output, then measures the free

running VCO frequency while searching for the setting which

results in the free running output frequency that is closest to the

desired phase-locked frequency. This procedure results in a

phase-locked oscillator that locks over a narrow voltage range

on the varactor. A typical tuning curve for a step tuned VCO is

shown in Figure 40. Note that the tuning voltage stays in a

narrow range over a wide range of output frequencies.

SPI

LD_SDO

VCO_REG0x01[0]

VCO_REG0x01[3]

÷1, ÷2, ÷4, ÷6, ... ÷62

VCO_REG0x01[2]

VCO_REG0x01[1], EN

VCO_REG0x00[8:1]

VCO_REG0x00[0]

LOOP

FILTER

VCO

VCO CAL

VOLTAGE

VCO_REG0x07[3:0]

VCO_REG0x02[5:0]

VCO

CONTROL

VSPI

VTUNE

RF_N

RF_P

VDD

MASTER ENABLE

VCO SUBSYSTEM

VCO_REG0x03[1:0]

VCO_REG0x03[3]

VCO_REG0x03[2]

VCO_REG0x01[5]

VCO SUBSYSTEM

PERFORMANCE

TUNING

CONTROL

MODULATOR

DIVIDER

PHASE

FREQUENCY

DETECTOR

CHARGE

PUMP

DIVIDER

XREFP

CAL

Rev. A | Page 16 of 48

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