MICRF102YM TR【PDF 7/12 页】IC TRANSMITTER ASK UHF 8-SOIC

December 2006

7

MICRF102

Micrel

Applications Information

Design Process

The MICRF102 transmitter design process is as follows:

1) Set the transmit frequency by providing the cor-

rect reference oscillator frequency.

2) Ensure antenna resonance at the transmit fre-

quency by:

L

ANT

= 0.2 ?Length ?ln(Length/d - 1.6) ?10

-9

?k

Where:

Length is the total antenna length in mm.

d is the trace width in mm.

k is a frequency correction factor.

L

ANT

is the approximate antenna inductance in

henries.

Note 1. The total inductance, however, will be a little greater

than the L

ANT

calculated due to parasitics. A 2nH should be

added to the calculated value. The L

ANT

formula is an ap-

proximated way to calculate the inductance of the antenna.

The inductance value will vary however, depending on PCB

material, thickness, ground plane, etc. The most precise way

to measure is to use a RF network analyzer.

3) Calculate the total capacitance using the follow-

ing equation.

C

f L

T

ANT

=

?nbsp ?nbsp ?/DIV>

(

)

1

4

2

??/DIV>

Where:

C

T

total capacitance in farads.

?= 3.1416.

f = carrier frequency in hertz.

L

ANT

inductance of the antenna in henries.

4) Calculate the parallel and series capacitors,

which will resonate the antenna.

4.1) Ideally for the MICRF102 the series and paral-

lel capacitors should have the same value or as

close as possible.

4.2) Start with a parallel capacitor value and plug in

the following equation.

C

S

T

VAR

P

=

+

(

)

?/DIV>

1

Where:

C

VAR

is the center varactor capacitance (5pF for the

MICRF102) in farads.

C

P

is the parallel capacitor in farads.

C

S

is the series capacitor in farads.

Repeat this calculation until C

S

and C

P

are very close and

they can be found as regular 5% commercial values.

Note 2. Ideally, the antenna size should not be larger than

the one shown in Figure 7. The bigger the antenna area,

the higher the loaded Q in the antenna circuit will be. This

will make it more dif cult to match the parallel and series

capacitors. Another point to take into consideration is the

total AC rms current going through the internal varactor in

the MICRF102. This current should not exceed 16mA rms.

The parallel capacitor will absorb part of this current if the

antenna dimensions are appropriate and not exaggerated

larger than the one shown here.

Note 3. A strong indication that the right capacitor values

have been selected is the mean current with a 1kHz signal

in the ASK pin. Refer to the Electrical Characteristics for

the current values.

Note 4. For much smaller antennas, place a blocking capaci-

tor for the series capacitance (around 100pF to 220pF) and

use the following formula for the parallel capacitance C

T

=

C

P

+ C

VAR

. The blocking capacitor is needed to ensure that

no dc current ows from one antenna pin to the other.

5) Set PC pin to the desired transmit power.

Reference Oscillator Selection

An external reference oscillator is required to set the transmit

frequency. The transmit frequency will be 32 times the refer-

ence oscillator frequency.

f

TX

REFOSC

= ?/DIV>

32

Crystals or a signal generator can be used. Correct reference

oscillator selection is critical to ensure operation. Crystals

must be selected with an ESR of 20?or less. If a signal

generator is used, the input amplitude must be greater than

200 mV

PP

and less than 500 mV

PP

.

Antenna Considerations

The MICRF102 is designed speci cally to drive a loop antenna.

It has a differential output designed to drive an inductive load.

The output stage of the MICRF102 includes a varactor that

is automatically tuned to the inductance of the antenna to

ensure resonance at the transmit frequency.

A high-Q loop antenna should be accurately designed to set

the center frequency of the resonant circuit at the desired

transmit frequency. Any deviation from the desired frequency

will reduce the transmitted power. The loop itself is an induc-

tive element. The inductance of a typical PCB-trace antenna

is determined by the size of the loop, the width of the antenna

traces, PCB thickness and location of the ground plane.

The tolerance of the inductance is set by the manufacturing

tolerances and will vary depending upon how the PCB is

manufactured.

The MICRF102 features automatic tuning. The MICRF102

automatically tunes itself to the antenna, eliminating the need

for manual tuning in production. It also dynamically adapts

to changes in impedance in operation and compensates for

the hand-effect.

Automatic Antenna Tuning

The output stage of the MICRF102 consists of a variable

capacitor (varactor) with a nominal value of 5.0pF tunable

over a range of 3pF to 7pF. The MICRF102 monitors the

phase of the signal on the output of the power ampli er and

automatically tunes the resonant circuit by setting the varactor

value at the correct capacitance to achieve resonance.

In the simplest implementation, the inductance of the loop

antenna should be chosen such that the nominal value is

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