MICRF221 datasheet(12/26 Pages) MICREL

12 / 26 page

Micrel Inc.

MICRF221

October 2008

M9999-100108

(408) 955-1690

Application Information

Figure 5 – QR221BPF Application Example, 915.0 MHz

The MICRF221 receiver can be fully tested by using

one of the many evaluation boards designed at Micrel

for this device. As an entry level, the QR221BPF

(Figure 5) offers a good start for most applications. It

has a connector for a whip antenna (ANT1), a band-

pass filter front end (L1 & C8) as a pre-selector filter,

and a matching network (C3 & L2). It also includes the

minimum components required to make the device

work, which are: a crystal, CAGC, and CTH capacitors.

An RF connector (J2) can be used instead of the whip

antenna when tests require an RF signal generator.

Figure 5 shows the entire schematic for 915.0MHz.

Other frequencies can be used. The values needed

for the various components are listed in the tables

below.

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

purpose is to attenuate undesired outside band noise

which reduces the receiver performance. It is

calculated by the parallel resonance equation

f = 1/(2×PI×(SQRT L1×C8))

Table 2 shows the most used frequency values.

Freq (MHz)

C8 (pF)

L1(nH)

868.35

2.7

915.0

2.7

916.5

2.7

Table 2. Band-Pass-Filter Front-End Values

There is no need for the band-pass filter front end in

applications where it is proven that the outside band

noise does not cause a problem. The MICRF221

receiver incorporates image reject mixers that improve

the selectivity and rejection of outside band noise

significantly.

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