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HMC832

Data Sheet

Autocalibration Using Register 0x05

Autocalibration transfers switch control data to the VCO

subsystem via Register 0x05. The address of the VCO subsystem

in Register 0x05 is not altered by the autocalibration routine.

The address and ID of the VCO subsystem in Register 0x05

must be set to the correct value before autocalibration is

executed. For more information see the VCO Serial Port

Interface (VSPI) section.

Automatic Relock on Lock Detect Failure

It is possible by setting Register 0x07[13] to have the VCO

subsystem automatically rerun the calibration routine and

relock itself if lock detect indicates an unlocked condition for

any reason. With this option the system attempts to relock only

once.

VCO Autocalibration on Frequency Change

Assuming Register 0x0A[11] = 0, the VCO calibration starts

automatically whenever a frequency change is requested. If it is

desired to rerun the autocalibration routine for any reason at

the same frequency, rewrite the frequency change with the same

value and the autocalibration routine executes again without

changing the final frequency.

VCO Autocalibration Time and Accuracy

The VCO frequency is counted for tMMT, the period of a single

autocalibration measurement cycle.

tMMT = tXTAL × R × 2n

(1)

where:

n is set by Register 0x0A[2:0] and results in measurement

periods which are multiples of the PD period, tXTALR.

R is the reference path division ratio currently in use,

tXTAL is the period of the external reference (crystal) oscillator.

The VCO autocalibration counter, on average, expects to

for every PD cycle.

N is the ratio of the target VCO frequency, fVCO, to the

frequency of the PD, fPD, where N can be any rational number

supported by the N divider.

N is set by the integer (NINT = Register 0x03) and fractional

(NFRAC = Register 0x04) register contents by Equation 2.

N = NINT + NFRAC/224

(2)

The autocalibration state machine and the data transfers to the

internal VCO subsystem SPI (VSPI) run at the rate of the FSM

clock, tFSM, where the FSM clock frequency cannot be greater

than 50 MHz.

tFSM = tXTAL × 2m

(3)

where m is 0, 2, 4, or 5 as determined by Register 0x0A[14:13].

The expected number of VCO counts, V, is given by

V = floor (N × 2n)

(4)

The nominal VCO frequency measured, fVCOM, is given by

fVCOM = V × fXTAL/(2n × R)

(5)

where the worst case measurement error, fERR , is

fERR ≈ ±fPD/2n + 1

(6)

A 5-bit step tuned VCO, for example, nominally requires five

measurements for calibration or in the worst case, six

measurements, and hence, seven VSPI data transfers of 20 clock

cycles each. The measurement has a programmable number of

wait states, k, of 128 FSM cycles defined by Register 0x0A[7:6] =

k. Total calibration time, worst case, is given by

tCAL = k128 tFSM + 6tPD 2n + 7 × 20 tFSM

(7)

or equivalently

tCAL = tXTAL (6R × 2n + (140+(k × 128)) × 2m)

(8)

For guaranteed hold of lock, across temperature extremes, the

resolution should be better than 1/8th the frequency step caused

by a VCO subband switch change. Better resolution settings

show no improvement.

Figure 42. VCO Calibration

Table 7. Autocalibration Example with fXTAL = 50 MHz, R = 1, m = 0

Control Value Register 0x0A[2:0]

tMMT (µs)

tCAL (µs)

fERR Maximum

0.02

4.92

±25 MHz

0.04

5.04

±12.5 MHz

0.08

5.28

±6.25 MHz

0.16

5.76

±3.125 MHz

0.64

8.64

±781 kHz

XREF

CALIBRATION WINDOW

REG0x02

START

VCO

CTR

FSM

50MHz MAX FOR

FSM + VSPI CLOCKS

REGA[14:13]

m = [0, 2, 4, 5]

REGA[2:0]

n = [0, 1, 2, 3, 5, 6, 7, 8]

STOP

tMMT = RTXTAL × 2n

tPD

÷ 2n

÷ 2m

÷ R

Rev. A | Page 18 of 48

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