MICRF218 datasheet(15/24 Pages) MICREL

15 / 24 page

Micrel

MICRF218

September 2007

M9999-090607

(408) 944-0800

Application Information

Figure 7. QR218HE1 Application Example, 433.92 MHz, Narrow Band

The MICRF218 can be fully tested by using one of

many evaluation boards designed at Micrel for this

device. As simple demonstrator, the QR218HE1

(Figure 7) offers a good start for most applications. It

has a helical PCB antenna with its matching network,

a bandpass-filter front-end as a pre-selector filter,

matching network and the minimum components

required to make the device work, which are a crystal,

Cagc, and Cth capacitors.

The matching network of the helical PCB antenna (C9

and L3) can be removed and a whip antenna (ANT2)

or a RF connector (J2) can be used instead. Figure 7

shows the entire schematic of it for 433.92MHz. Other

frequencies can be used.

Matching network values

for other frequencies are listed in the tables below.

Capacitor C9 and inductor L3 are the passive

elements for the helical PCB matching network. Tight

tolerance is recommended for these devices, like 2%

for the inductor and 0.1pF for the capacitor. PCB

variations may require different component values and

optimization. Table 2 shows the matching elements

for

the

device

frequency

range.

For

additional

information look for Small PCB Antennas for Micrel RF

Products application note.

Freq (MHz)

C9 (pF)

L3(nH)

315.0

1.2

390.0

1.2

418.0

1.2

433.92

1.5

Table 2. Matching Values for the Helical PCB Antenna

If whip antenna is used, remove C9 and place the

whip antenna in the hole provided in the PCB. Also,

RF signal can be injected there (add RF connector).

L1 and C8 form the pass-band-filter front-end. Its

purpose is to attenuate undesired outside band noise

which

reduces

the

receiver

performance.

calculated by the parallel resonance equation:

C8)

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