MICRF102

Micrel

MICRF102

8

September 2002

In the simplest implementation, the inductance of the loop

antenna should be chosen such that the nominal value is

resonant at 5pF, the nominal mid-range value of the MICRF102

output stage varactor.

Using the equation:

L

fC

=

1

4

22

ππ

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 MICRF102 acts to trim the total

capacitance value.

Figure 4.

Supply Bypassing

Correct supply bypassing is essential. A 4.7uF capacitor in

parallel with a 100pF capacitor is recommended.

The MICRF102 is susceptible to supply-line ripple, if supply

regulation is poor or bypassing is inadequate, spurs will be

evident in the transmit spectrum.

PC

VDD

VSS

REFOSC

ASK

MICRF102

L

ASK DATA INPUT

Transformer Output to 50‰

Impedance Transformation

Network

OFF

ON

RP2

(6.8k)

RP1

(100k)

+5V

Crystal

Z1

Z3

Z2

ANTP

ANTM

STBY

To 50‰

Termination of

Spectrum Analyzer

Figure 5. Application Test Circuit For Specification Verification

Transmit Power

The transmit power specified in this datasheet is normalized

to a 50Ohm load. The antenna efficiency will determine the

actual radiated power. Good antenna design will yield trans-

mit power in the range of 67dB

µV/m to 80dBµV/m at 3 meters.

The PC pin on the MICRF102 is used to set the transmit

power. The differential voltage on the output of the PA (power

amplifier) is proportional to the voltage at the PC pin.

With more than 0.35V on the PC pin the output amplifier

becomes current limited. At this point, further increase in the

PC pin voltage will not increase the RF output power in the

antenna pins. Low power consumption is achieved by de-

creasing the voltage in the PC pin, also reducing the RF

output power and maximum range.

Output Blanking

When the device is first powered up or after a momentary loss

of power the output is automatically blanked (disabled). This

feature ensures RF transmission only occurs under con-

trolled conditions when the synthesizer is fully operational,

preventing unintentional transmission at an undesired fre-

quency. Output blanking is key to guaranteeing compliance

with UHF regulations by ensuring transmission only occurs in

the intended frequency band.