MICRF230 datasheet(14/20 Pages) MICREL

14 / 20 page

Micrel, Inc.

MICRF230

Application Information

Length of Preamble

When the MICRF230 returns to operation from shut down

stage, the preamble of the corresponding transmitter

should be long enough to guarantee that the MICRF230

becomes fully awake during the preamble portion of the

burst. This way the entire data portion will be received.

Figure 7 shows an example of insufficient length preamble.

MICRF230 starts demodulating output bits during the data

portion of the burst, so by the time it becomes fully awake

and releases DO, part of the data portion is lost. In Figure

8, the preamble length is sufficient. The chip has enough

preambles to be demodulated with a steady data portion.

Figure 7. Preamble Length

− Too Short

Figure 8. Preamble Length

− Sufficient

Antenna and RF Port Connections

The evaluation board offers two options of injecting the RF

input signal: through a PCB antenna or through a 50

SMA

connector.

The

SMA

connection

allows

for

conductive testing, or an external antenna.

Low-Noise Amplifier Input Matching

Capacitor C3 and inductor L2 form the “L” shape input

matching network to the SMA connector. The capacitor

cancels out the inductive portion of the net impedance

after

the

shunt

inductor,

and

provides

additional

attenuation for low-frequency outside band noise.

The

inductor is chosen to over resonate the net capacitance at

the pin, leaving a net-positive reactance and increasing the

real part of the impedance. It also provides additional ESD

protection for the antenna pin. The input impedance of the

device is listed in Table 5 to aid calculation of matching

values. Note that the net impedance at the pin is easily

affected by component pads parasitic due to the high input

impedance of the device. The numbers in Table 5 does

NOT

include

trace

and

component

pad

parasitic

capacitance, which total about 0.75pF on the evaluation

board.

The matching components to the PCB antenna (L2 and

C2) were empirically derived for best over-the-air reception

range.

Table 5. Input Impedance for the Most Used Frequencies

Frequency (MHz)

Z Device (

Ω)

418

8.98

− j152

433.92

13.5

− j150

Crystal Selection

The crystal resonator provides a reference clock for all the

device internal circuits.

Crystal tolerance needs to be

chosen such that the down-converted signal is always

inside the IF bandwidth of MICRF230.

From this

consideration, the tolerance should be

±50ppm on both the

transmitter and the MICRF230 side. The ESR should be

less than 300Ω, and the temperature range of the crystal

should match the range required by the application. With

the Abracon crystal listed in the Bill of Materials, a typical

MICRF230 crystal oscillator still starts up at 105

°C with

additional 400Ω series resistance.

The oscillator of the MICRF230 is a pierce-type oscillator.

Good care must be taken when laying out the printed

circuit board. Avoid long traces and place the ground plane

on the top layer close to the REFOSC pins RO1 and RO2.

When care is not taken in the layout, and the crystals used

are not verified, the oscillator may not start or takes longer

to start. Time-to-good-data will be longer as well.

April 15, 2015

Revision 2.0

MICRF230 Datasheet(PDF) 14 Page - Micrel Semiconductor

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