LTC6948IUFD-2 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC6948IUFD-2
Linear Technology
'LTC6948IUFD-2' PDF : 36 Pages View PDF
LTC6948
Operation
for the relationship between R, N, NUM, O, and the fREF,
fPFD, fVCO, and fRF frequencies). The output frequency is
then stable over the LTC6948’s entire temperature range,
regardless of the temperature at which it was calibrated,
until the part is reset due to a power cycle or software
power-on reset (POR).
The output of the B divider is used to clock digital calibra-
tion circuitry as shown in the Block Diagram. The B value,
programmed with bits BD[3:0], and fPFD determine the
calibration frequency fCAL. Calculate the B value using
Equation 1.
B ≥ fPFD
fCAL -M AX
(1)
The maximum calibration frequency fCAL-MAX for each part
option is shown in Table 8.
Table 8. Maximum Calibration Frequency
PART
LTC6948-1
fCAL-MAX (MHz)
1.0
LTC6948-2
1.33
LTC6948-3
1.7
LTC6948-4
1.8
The relationship between bits BD(3:0) and the B value is
shown in Table 9.
Table 9. BD[3:0] Programming
BD[3:0]
0
1
2
3
4
5
6
7
8
9
10
11
12 to 15
B DIVIDE VALUE
8
12
16
24
32
48
64
96
128
192
256
384
Invalid
Once the RD[4:0], ND[9:0], NUM[17:0], and BD[3:0] bits
are written and the reference frequency fREF is present and
stable at the REF± inputs, the VCO must be calibrated by
setting CAL = 1 (the bit is self-clearing). The calibration
cycle takes between 12 and 14 clocks of the B divider
output. The MTCAL bit may be set to mute the RF output
during the calibration.
Note that the fREF frequency and TB and BB voltages must
be stable for proper calibration. Stable bias voltages are
achieved after approximately 3 time constants following
power-up.
Setting AUTOCAL = 1 causes the CAL bit to be set auto-
matically whenever any of serial port registers h06 through
h0A is written. When AUTOCAL is enabled, there is no
need for a separate register write to set the CAL bit. See
Table 10 for a summary of the VCO bits.
Table 10. VCO Bit Descriptions
BIT DESCRIPTION
AUTOCAL Calibrate VCO Whenever Registers h06 to h0A Are Written
CAL Start VCO Calibration (Auto Clears)
MTCAL Mute RF Output During Calibration
VCO Automatic Level Control (ALC)
The VCO uses an internal automatic level control (ALC)
algorithm to maintain an optimal amplitude on the VCO
resonator, and thus optimal phase noise performance. The
user has several ALC configuration and status reporting
options as seen in Table 11.
Table 11. ALC Bit Descriptions
BIT DESCRIPTION
ALCCAL Auto Enable ALC during CAL operation
ALCEN Always Enable ALC (Overrides ALCCAL, ALCMON, and
ALCULOK)
ALCHI ALC too High Flag (Resonator Amplitude too High)
ALCLO ALC too Low Flag (Resonator Amplitude too Low)
ALCMON Enable ALC Monitoring for Status Flags Only; Does Not Enable
Amplitude Control
ALCULOK Auto Enable ALC when PLL Unlocked
6948f
18
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