LTC6948 datasheet(29/38 Pages) AD

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LTC6948

6948fa

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applicaTions inForMaTion

R and N Divider and Numerator Programming

Program registers Reg06 to Reg0A with the previously

determinedRandNdividerandnumeratorvalues.Because

the AUTORST and AUTOCAL bits were previously set to

1, CAL and RSTFN do not need to be set:

Reg06 = h10

Reg07 = h4C

Reg08 = h37

Reg09 = h6C

Reg0A = h90

Reference Input Settings and Output Divider

Programming

From Table 1, FILT = 0 for a 100MHz reference frequency.

Next, convert 7dBm into VP–P. For a CW tone, use the

following equation with R = 50:

VP-P ≅ R •10(dBm –21)/20

(15)

This gives VP-P = 1.41V, and, according to Table 2, set

BST = 1.

Now program Reg0B, assuming maximum RF± output

power(RFO[1:0]=3accordingtoTable14)andOD[2:0]=2:

Reg0B = h9A

Lock Detect and Charge Pump Current Programming

Next,determinethelockindicatorwindowfromfPFD.From

Table 3 we see that LKWIN[1:0] = 0 with a tLWW of 5ns for

CPLE = 1 and fVCO = 3.843GHz. The LTC6948 will consider

the loop locked as long as the phase coincidence at the

PFD is within 90°, as calculated below.

phase = 360° • tLWW • fPFD = 360 • 5n • 50M

= 90°

Choosing the correct COUNTS value depends upon the

OSR.SmallerratiosdictatelargerCOUNTSvalues,although

application requirements will vary. A COUNTS value of 32

will work for the OSR of 333. From Table 5, LKCT[1:0] = 1

for 32 counts.

Using Table 6 with the previously selected ICP of 5.6mA

gives CP[3:0] = 7. This gives enough information to pro-

gram Reg0C:

Reg0C = h0D

Charge Pump Function Programming

This example uses the additional voltage clamp features

to allow the monitoring of fault conditions by setting

CPCHI = 1 and CPCLO = 1. If something occurs and the

system can no longer lock to its intended frequency, the

charge pump output will move toward either GND or

VCP+, thereby setting either the TLO or THI status flags,

respectively. Disable all the other charge pump functions

(CPMID, CPINV, CPRST, CPUP, and CPDN), allowing the

loop to lock:

Reg0D = hC0

The loop should now lock. Now un-mute the output by

setting OMUTE = 0 (assumes the MUTE pin is high).

Reg02 = h04

LTC6948 Datasheet(PDF) 29 Page - Analog Devices

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