MICRF104 データシートの表示（PDF） - Micrel

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MICRF104

1.8V, QwikRadio™ UHF ASK Transmitter

Micrel 

MICRF104

Applications Information

Design Process

The MICRF104 transmitter design process is as follows:

1). Set the transmit frequency by providing the

correct reference oscillator frequency

2). Ensure antenna resonance at the transmit

frequency by:

a. Either, matching antenna inductance to the

center of the tuning range of the internal

varactor.

b. Or, matching capacitance with the antenna

inductance by adding an external capacitor (in

series with, or in parallel with, the internal

varactor)

3). Set PC pin for 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

reference oscillator frequency.

fTX= 32 × fREFOSC

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 Ohms or less. If a signal

generator is used, the input amplitude must be greater than

200 mVP-P and less than 500 mVP-P.

Antenna Considerations

The MICRF104 is designed specifically to drive a loop an-

tenna. It has a differential output designed to drive an induc-

tive load. The output stage of the MICRF104 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

inductive 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 manufac-

turing tolerances and will vary depending how the PCB is

manufactured.

In the simplest implementation a single capacitor in parallel

with the antenna will provide the desired resonant circuit.

C

LANTENNA

Figure 3.

The resonant frequency is determined by the equation:

f=

1

4π2 CL

Micrel

The tolerance in the antenna inductance combined with the

tolerance of the capacitor in parallel with it will result in

significant differences in resonant frequency from one trans-

mitter to another. Many conventional loop antenna transmit-

ters use a variable capacitor for manual tuning of the resonant

circuit in production. Manual tuning increases cost and re-

duces reliability.

A capacitor correctly tuned during manufacture may drift over

time and temperature. A change in capacitance will alter the

resonant frequency and reduce radiated power. In addition,

a hand close to the antenna will alter the resonant properties

of the antenna and de-tune it.

The MICRF104 features automatic tuning. The MICRF104

automatically tunes itself to the antenna, eradicating 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 MICRF104 consists of a variable

capacitor (varactor) with a nominal value of 6.5pF tunable

over a range from 5pF to 8pF. The MICRF104 monitors the

phase of the signal on the output of the power amplifier 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

resonant at 6.5pF, the nominal mid-range value of the

MICRF104 output stage varactor.

Using the equation:

L=

1

4π2f2C

If the inductance of the antenna cannot be set at the nominal

value determined by the above equation, a capacitor can be

added in parallel or series with the antenna. In this case, the

varactor internal to the MICRF104 acts to trim the total

capacitance value.

CVARACTOR

CEXTERNAL

LANTENNA

Figure 4.

Starting with the inductance of the antenna the capacitance

value required to achieve resonance can be calculated.

For example a 315MHz transmitter with a 45.1nH inductance

antenna will require no capacitor in parallel with the antenna,

only the internal varactor that will be tuned to 5.66pF, which

is very close to mid range and can be determined using the

equation:

C=

1

4π2f2L

Where:

f = 315Mhz

MICRF104

8

November 8, 2001

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