RF2512 データシートの表示（PDF） - RF Micro Devices

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RF2512

UHF TRANSMITTER

RF Micro Devices 

RF2512

11

state where both inputs are high will not occur. The

other states are either charging or discharging the loop

filter. The loop filter integrates the pulses coming from

the charge pump to create a control voltage for the

voltage controlled oscillator.

The voltage controlled oscillator (VCO) is a tuned

differential amplifier with the bases and collectors

cross coupled to provide positive feedback and a 360°

phase shift. The tuned circuit is located in the collec-

tors. It is comprised an external varactor, a capacitor

and external inductors. The designer selects the induc-

tors for the desired frequency of operation. These

inductors also provide DC bias for the VCO. The output

of the VCO is buffered and applied to the prescaler cir-

cuit, where it is divided down and compared to the ref-

erence oscillator frequency.

The PLL and VCO circuitry can be enabled by setting

applying a "high" logic level to pin 4 (PLL ENABL).

Conversely, the PLL and VCO circuitry will be turned

off if the level is tied "low".

The transmit amplifier is a two stage amplifier con-

sisting of a driver and an open collector final stage. It is

capable of providing 12dBm of output power into a

50Ω load while operating from a 3.6V power supply.

The output power is adjustable by the setting of pin 7

(LVL ADJ). This analog input allows the designer a

12dB range of output power. As the LVL ADJ voltage is

reduced, the output power and current consumption

are reduced. LVL ADJ must be low when the transmit-

ter is disabled.

Additionally, the transmitter circuitry can be disabled

entirely by applying a "low" logic level to pin 11 (TX

ENABL). During transmission, this pin should be tied

"high". This pin controls all circuitry except for the PLL

circuitry.

During transmission the transmitter is enabled and the

impedance of the output pin, pin 8 (TX OUT), is low.

When the transmitter is not enabled, the impedance

becomes high.

The RF2512 contains onboard band gap reference

voltage circuitry which provides a stable DC bias over

varying temperature and supply voltages.

Designing with the RF2512

The reference oscillator is built around the onboard

transistor at pins 1, 2 and 3. The intended topology is

of a Colpitts oscillator. The Colpitts oscillator is quite

common and requires few external components, mak-

11-16

ing it ideal for low cost solutions. The topology of this

type of oscillator is as seen in the following figure.

VCC

C2

X1

C1

This type of oscillator is a parallel resonant circuit for a

fundamental mode crystal. The transistor amplifier is

an emitter follower and the voltage gain is developed

by the tapped capacitor impedance transformer. The

series combination of C1 and C2 act in parallel with the

input capacitance of the transistor to capacitively load

the crystal.

The nominal capacitor values can be calculated with

the following equations.

C1

=

6---0-----⋅---C----l--o---a--d-

freqMHz

and

C2 =

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

------1------- – --1----

Cload C1

The load capacitance is usually 32pF. The variable freq

is the oscillator frequency in MHz. The frequency can

be adjusted by either changing C2 or by placing a vari-

able capacitor in series with the crystal. As an exam-

ple, assume a desired frequency of 14MHz and a load

capacitance of 32pF. C1=137.1pF and C2=41.7pF.

These capacitor values provide a starting point. The

drive level of the oscillator should be checked by look-

ing at the signal at pin 2 (OSC E). It has been found

that the level at this pin should generally be around

500mVPP or less. This will reduce the reference spur

levels and reduce noise from distortion. If this level is

higher than 500mVPP then decrease the value of C1.

The values of these capacitors are usually tweaked

during design to meet performance goals, such as

minimizing the start-up time.

Additionally, by placing a variable capacitor in series

with the crystal, one is able to adjust the frequency.

This will also alter the drive level, so it should be

checked again.

An important part of the overall design is the voltage

controlled oscillator. The VCO is configured as a dif-

ferential amplifier. The VCO is tuned via the external

inductors, capacitor, and varactor. The varactor capaci-

Rev B9 010509

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