MICRF220AYQS TR【PDF 14/18 页】RCVR ASK/OOK 300-450MHZ 16QSOP

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Micrel, Inc.

Supply Voltage Ramping

When supply voltage is initially applied, it should rise

monotonically from 0V to 3.3V to ensure proper startup

of the crystal oscillator and the PLL. It should not have

multiple bounces across 2.6V, which is the threshold of

the undervoltage lockout (UVLO) circuit inside

MICRF220.

Antenna and RF Port Connections

Figure 8 shows the schematic of the MICRF220

Evaluation Board. Figures 9 thru 11 depict PCB images.

This evaluation board is a good starting point for the

prototyping of most applications. The evaluation board

offers two options of injecting the RF input signal:

through a PCB antenna or through a 50 Ω 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 6 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 6 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.

MICRF220

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 MICRF220. From this

consideration, the tolerance should be ± 50ppm on both

the transmitter and the MICRF220 side. ESR should be

less than 300 ? , 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 MICRF220 crystal oscillator still starts up at

105oC with additional 400 ? series resistance.

The oscillator of the MICRF220 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.

PCB Considerations and Layout

Figures 9 thru 11 illustrate the MICRF220 Evaluation

Board layout. The Gerber files provided are

downloadable from the Micrel website and contain the

remaining layers needed to fabricate this board. When

copying or making one’s own boards, make the traces

as short as possible. Long traces alter the matching

network and the values suggested are no longer valid.

Suggested matching values may vary due to PCB

variations. A PCB trace 100 mils (2.5mm) long has about

1.1nH inductance. Optimization should always be done

with exhaustive range tests. Make sure the individual

ground connection has a dedicated via rather then

sharing a few of ground points by a single via. Sharing

ground via will increase the ground path inductance.

Ground plane should be solid and with no sudden

interruptions. Avoid using ground plane on top layer next

to the matching elements. It normally adds additional

stray capacitance which changes the matching. Do not

Frequency (MHz)

315

390

418

433.92

Z Device ( ? )

23 ? j290

14 – j230

17 – j216

12 – j209

use Phenolic materials as they are conductive above

200MHz. Typically, FR4 or better materials are

recommended. The RF path should be as straight as

possible to avoid loops and unnecessary turns.

Separate ground and V DD lines from other digital or

switching power circuits (such microcontroller…etc).

Known sources of noise should be laid out as far as

possible from the RF circuits. Avoid unnecessary wide

Table 6. Input Impedance for the Most Used Frequencies

traces which would add more distribution capacitance

(between top trace to bottom GND plane) and alter the

RF parameters.

August 2010

14

M9999-082610-A

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