MICRF229 datasheet(17/23 Pages) MICREL

17 / 23 page

Micrel, Inc.

MICRF229

April 15, 2015

Revision 1.0

Application Information

Length of Preamble

When

using

MICRF229

auto-polling

mode,

the

preamble of the corresponding transmitter should be long

enough to guarantee that the MICRF229 becomes fully

awake during the preamble portion of the burst. This way

the entire data portion will be received. A good rule of

thumb to use is:

Preamble Length =

1.2 × Sleep Time + Length of Valid Bits Sequence

The factor of 1.2 is to accommodate sleep time variation

due to process shift.

Figure 13 shows an example of insufficient length

preamble.

MICRF229 starts checking 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 14, the preamble length is sufficient. The chip

wakes up during the preamble and is ready for the data

portion.

Figure 13. Preamble Length

− Too Short

Figure 14. 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 12 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 12 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 (L3 and

C9)

were

empirically

derived

for

best

over-the-air

reception range.

Table 11. Input Impedance for the Most Used Frequencies

Frequency (MHz)

Z Device (

Ω)

418

8.98

− j152

433.92

13.5

− j149

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 MICRF229.

From this

consideration, the tolerance should be

±50ppm on both

the transmitter and the MICRF229 side. The ESR should

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

crystal

should

match

the

range

required

the

application. With the Abracon crystal listed in the Bill of

Materials, a typical MICRF229 crystal oscillator still starts

up at 105

C with additional 400Ω series resistance.

The oscillator of the MICRF229 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.

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