HMC832 Datasheet, PDF (19/49 Pages) Analog Devices – Cellular infrastructure

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,
Register 0x02.
tXTAL is the period of the external reference (crystal) oscillator.
The VCO autocalibration counter, on average, expects to
register N counts, rounded down (floor) to the nearest integer,
for every PD cycle.
XREF
t_PD
REG0x02
÷R
÷ 2ⁿ
REGA[14:13]
m = [0, 2, 4, 5]
÷ 2^m
REGA[2:0]
n = [0, 1, 2, 3, 5, 6, 7, 8]
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 × 2^m
(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/2^{n + 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)) × 2^m)
(8)
For guaranteed hold of lock, across temperature extremes, the
resolution should be better than 1/8^th the frequency step caused
by a VCO subband switch change. Better resolution settings
show no improvement.
CALIBRATION WINDOW
START
t_MMT = RT_XTAL × 2ⁿ
STOP
50MHz MAX FOR
FSM + VSPI CLOCKS
FSM
VCO
V
CTR
Figure 42. VCO Calibration
Table 7. Autocalibration Example with fXTAL = 50 MHz, R = 1, m = 0
Control Value Register 0x0A[2:0]
n
2ⁿ
tMMT (µs)
0
01
0.02
1
12
0.04
2
24
0.08
3
38
0.16
4
5 32
0.64
Rev. A | Page 18 of 48
tCAL (µs)
4.92
5.04
5.28
5.76
8.64
fERR Maximum
±25 MHz
±12.5 MHz
±6.25 MHz
±3.125 MHz
±781 kHz