LTC6945_15 datasheet(21/28 Pages) LINER

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LTC6945

6945fa

For more information www.linear.com/LTC6945

applicaTions inForMaTion

Using Table 5 with the previously selected ICP of 11.2mA,

gives CP[3:0] = 11. This is enough information to program

Reg09:

Reg09 = hDB

To enable the lock indicator, write Reg07:

Reg07 = h01

Charge Pump Function Programming

The DC1649 includes an LT1678I op amp in the loop filter.

This allows the circuit to reach the voltage range speci-

fied for the VCO’s tuning input. However, it also adds an

inversion in the loop transfer function. Compensate for

this inversion by setting CPINV = 1.

This example does not use the additional voltage clamp

features to allow fault condition monitoring. The loop

feedback provided by the op amp will force the charge

pump output to be equal to the op amp positive input

pin’s voltage. Disable the charge pump voltage clamps

by setting CPCHI = 0 and CPCLO = 0. Disable all the other

chargepumpfunctions(CPMID,CPRST,CPUPandCPDN)

to allow the loop to lock:

Reg0A = h10

The loop should now lock. Now unmute the output by

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

Reg02 = h08

REFERENCE SOURCE CONSIDERATIONS

A high quality signal must be applied to the REF± inputs as

they provide the frequency reference to the entire PLL. As

mentioned previously, to achieve the part’s in-band phase

noise performance, apply a CW signal of at least 6dBm

into 50Ω, or a square wave of at least 0.5VP-P with slew

rate of at least 40V/µs.

The LTC6945 may be driven single-ended to CMOS levels

(greater than 2.7VP-P). Apply the reference signal directly

without a DC-blocking capacitor at REF+, and bypass REF–

to GND with a 47pF capacitor. The BST bit must also be

set to “0”, according to guidelines given in Table 2.

TheLTC6945achievesanin-bandnormalizedphasenoise

floor of –226dBc/Hz (typical). To calculate its equivalent

input phase noise floor LM(IN), use Equation 10:

LM(IN) = –226 + 10 • log10(fREF)

(10)

For example, using a 10MHz reference frequency gives

an input phase noise floor of –156dBc/Hz. The reference

frequency source’s phase noise must be approximately

3dB better than this to prevent limiting the overall system

performance.

IN-BAND OUTPUT PHASE NOISE

Thein-bandphasenoiseproducedatfRFmaybecalculated

by using Equation 11.

LM(OUT) = –226+10 •log10 fPFD

(

)

+20 • log10

fRF

fPFD

⎛

⎝

⎜

⎞

⎠

⎟

LM(OUT) = –226+10 •log10 fPFD

(

)

+20 • log10

⎛

⎝

⎜

⎞

⎠

⎟

(11)

As seen for a given PFD frequency fPFD, the output in-band

phase noise increases at a 20dB-per-decade rate with the

N divider count. So, for a given output frequency fRF, fPFD

should be as large as possible (or N should be as small as

possible) while still satisfying the application’s frequency

step size requirements.

OUTPUT PHASE NOISE DUE TO 1/f NOISE

In-band phase noise at very low offset frequencies may

be influenced by the LTC6945’s 1/f noise, depending upon

fPFD. Use the normalized in-band 1/f noise of –274dBc/Hz

with Equation 12 to approximate the output 1/f phase

noise at a given frequency offset fOFFSET:

LM(OUT –1/f) (fOFFSET) = –274 + 20 • log10(fRF)

(12)

– 10 • log10(fOFFSET)

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