RF2514PCBA datasheet(8/14 Pages) RFMD

8 / 14 page

11-46

RF2514

Rev A5 040115

that any inductance in the traces connecting the induc-

tor(s) to the VCO pins will contribute to the overall res-

onator inductance and should be subtracted from the

calculated value of L.

A parameter of the VCO that is necessary for calculat-

ing the loop filter values is the VCO sensitivity, KVCO

(sometimes referred to as VCO gain). To determine the

VCO sensitivity, first connect the control voltage input

point (LOOP FLT pin) to ground and note the fre-

quency. (The frequency can be observed at the output

if the LD FLT pin is connected to VCC.) Then connect

the same point to the supply and again note the fre-

quency. The difference between these two frequencies

divided by the supply voltage is the VCO sensitivity

expressed in Hz/V. There is little that the designer can

do to increase the VCO sensitivity since it is largely

determined by the tuning capacitance of the on-chip

varactors. While increasing the inductor value will

increase the tuning sensitivity, it will also lower the cen-

ter frequency of the VCO's tuning range. A very small

capacitance (1pF or less) may be added across the

VCO pins, which will have the effect of lowering the

VCO center frequency and decreasing VCO sensitivity,

but this is likely to be neither necessary nor desirable in

most applications.

Should adequate centering of the VCO range be

unachievable with standard inductor values, two

options are available for proper centering. First, a two-

inductor resonator may be used with one inductor

being one standard value higher than the other. Sec-

ond, the tuning range of the VCO may be extended at

the upper limit of the control voltage by increasing the

VCO bias resistor(s). This allows the internal varactor

diodes to be slightly forward biased, further increasing

the resonator capacitance and thereby extending the

lower frequency operation. Care should be taken not to

reduce the VCO bias so much that the circuit ceases

operation at the minimum required supply voltage.

External to the part, the designer needs to implement a

loop filter to complete the PLL. The loop filter converts

the output of the charge pump into a voltage that is

used to control the VCO. Internally, the VCO is con-

nected to the charge pump output through a 4k

Ω resis-

tor. The loop filter is then connected in parallel with this

point at pin 12 (LOOP FLT). This limits the loop filter

topology to a second order filter usually consisting of a

shunt capacitor and a shunt series RC, as shown in the

following schematic.

The transfer function is

where the time constants are defined as

and

The frequency at which unity gain occurs is given by

This is defined as the loop bandwidth.

Once the desired phase margin (PM) and loop band-

width (

ωLBW) are chosen, it is possible to calculate the

time constants. These are found using the equations

and

The phase detector gain, KPD, is calculated by dividing

the charge pump current by 2

π. For the RF2514, the

charge pump current is 100

μA.

With these known, it is then possible to determine the

values of the filter components.

VCO

Charge Pump

Loop Filter

()

⋅

)

⋅

(

⋅⋅

-------------------------------------------

⋅

-------------------⎠

⎞

⎝

⎛

⋅

LBW

⋅

-------------------

()

sec

()

tan

–

LBW

--------------------------------------------------

LBW

⋅

------------------------

-----

VCO

⋅

LBW

⋅

-----------------------------

LBW

⋅

()

LBW

⋅

()

----------------------------------------

⋅⋅

-----1

–

⎝⎠

⎛⎞

⋅

------

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