RF2512 Просмотр технического описания (PDF) - RF Micro Devices

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RF2512

UHF TRANSMITTER

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tance is set by the loop filter output voltage through a

4kΩ resistor.

RESNTR+ RESNTR-

L

L

LOOP

FLT

4 kΩ

To tune the VCO the designer only needs to calculate

the value of the inductors connected to pins 18 and 20

(RESNTR- and RESNTR+). The inductor value is

determined by the following equation.

L

=

æ

è

-2----⋅--1-π-----⋅---f

ö

ø

2

⋅

-1--

C

⋅

1--

2

In this equation, f is the desired operating frequency

and L is the value of the inductor required. The value C

is the amount of capacitance presented by the varac-

tor, capacitor and parasitics. The factor of one-half is

due to the inductors being in each leg.

The setup of the VCO can be summarized as follows.

First, open the loop. Next, get the VCO to run on the

desired frequency by selecting the proper inductor and

capacitor values. The capacitor value will need to

include the varactor and circuit parasitics. After the

VCO is running at the desired frequency, then set the

VCO sensitivity.

The sensitivity is determined by connecting the control

voltage input point to ground and noting the frequency.

Then connect the same point to the supply and again

note the frequency. The difference between these two

frequencies divided by the supply voltage is the VCO

sensitivity expressed in Hz/V. Increasing the inductor

value while decreasing the capacitor value will

increase the sensitivity. Decreasing the inductor value

while increasing the capacitor value will lower the sen-

sitivity.

When increasing or decreasing component values,

make sure that the center frequency remains constant.

Finally, close the loop.

RF2512

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 4kΩ resis-

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

point at pin 23 (LOOP FLT). This limits the loop filter

topology to a second order filter usually consisting of a

shunt capacitor and a shunt series RC. A passive filter

is most common, as it is a low cost and low noise

design. An additional pole could be used for reducing

the reference spurs, however there is not a way to add

the series resistor. This should not be a reason for con-

cern however.

The schematic of the loop filter is

Charge Pump

VCC

Loop Filter

VCO

C1

R2

C2

The transfer function is

F(s) = R2 ⋅

-----------s----⋅---τ---2----+-----1------------

s ⋅ τ2 ⋅ (s ⋅ τ1 + 1)

11

where the time constants are defined as

τ2 = R2 ⋅ C2

and

τ1 = R2 ⋅

-C-----1---⋅---C-----2-

C1 + C2

The frequency at which unity gain occurs is given by

ωLBW

=

---------1---------

τ1 ⋅ τ2

This is also the loop bandwidth.

If the phase margin (PM) and the loop bandwidth

(ωLBW) are known, it is possible to calculate the time

constants. These are found using the equations

τ1

=

-s--e---c---(---P----M-----)---–-----t--a---n---(---P----M-----)-

ωLBW

and

τ2

=

------------1--------------

ω2LBW ⋅ τ1

Rev B9 010509

11-17

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