PE3293 datasheet(13/18 Pages) ETC

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PE3293

Product Specification

PEREGRINE SEMICONDUCTOR CORP.  | http://www.peregrine-semi.com

 Peregrine Semiconductor Corp. 2003

Page 13 of 18

Loop Filter

Second/Third Order Loops

Choosing the optimum loop filter for a design

encompasses many trade offs. The rule of thumb

for choosing the loop filter bandwidth is 10 percent

of the step size. A second order loop (C1 C2 R2 and

C4 C5 R5 in Figure 9 omitting C3 R3 C6 and R6) will

provide the least amount of components and the

fastest lock times. If lock time is an issue, one might

try opening up the loop filter, although if it is too

wide, instability will dominate and worsen lock time.

If lock time is not an issue, a narrower second order

filter will minimize residual FM without requiring

additional components.

Third Order loop filters (C1 C2 R2 C3 R3 and C4 C5 R5

C6 R6 in Figure 9) provide a good compromise

between lock time and residual FM. We have found

using a third order loop with 20 dB of rejection at the

step size will halve the Residual FM as measured

with a similar second order loop, with minimum

effect on lock time.

Loop Filter Bandwidth Design Considerations

As part of the spur compensation circuitry, the

PE329x series PLLs contain capacitors to ground

internal to the charge pump. PLL1 contains a 50 pF

capacitor and PLL2 contains a 100 pF capacitor.

To ensure accurate loop filter calculations, it is

critical that the calculated value of the first shunt

capacitor (C1 & C4 in Figure 11) be at least 100 pF

for PLL1 and 200 pF for PLL2. With this

requirement satisfied, the remaining loop

components can be calculated.

For a stable loop, it is also important that the loop

bandwidth be less than or equal to one tenth of the

step size.

Digital Control Lines

Control Line Noise

We have noticed frequency jitter during programming

when a low impedance, such as a capacitor to

ground, is placed next to any control line pin (clock,

data, and load enable). The use of a 51 k ohm

resistor in series with the control line will eliminate

the problem with no effect to programming time.

Enable Line Voltage

The PE329x series PLLs use a level sensitive load

enable. Therefore the digital controller must provide

an active low to the part at all times except when the

data is to be loaded into the shift register. If the PLL

controller does not hold the voltage low, a high

impedance resistor to ground should be added to the

enable line to ensure stable operation.

5 Volt Operation:

The PE329x series PLLs are not capable of

accepting control voltages greater than 3.3 volts.

Interface to 5 volt controllers requires the addition of

resistor dividers to comply with the 3.3 volt maximum

operation voltage.

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