MICRF219A datasheet(17/23 Pages) MICREL

17 / 23 page

Micrel, Inc.

MICRF219A

July 2011

M9999-071811-A

RadioTech@micrel.com

Application Information

Initial Startup

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

MICRF219A. The SHDN pin needs to have 50kΩ

resistor to GND and a coupling capacitor to VDD as

shown in the evaluation board schematic to ensure that

the part starts up in shutdown mode first. Then the micro

controller can bring the SHDN pin voltage down to turn

the part on.

Length of Preamble

When using MICRF219A in auto-polling mode, the

preamble of the corresponding transmitter should be

long enough to guarantee that the MICRF219A 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 x 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. MICRF219A 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.

or (408) 944-0800

Figure 13. Preamble Length Too Short

Figure 14. Sufficient Preamble Length

Antenna and RF Port Connections

The evaluation board offers two options of injecting the

RF input signal: through a PCB antenna or through a

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.

Frequency (MHz)

Z Device (Ω)

315

− j290

390

14 – j230

418

17 – j216

433.92

12 – j209

Table 6. Input Impedance for the Most Used frequencies

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