MICRF211 datasheet(8/18 Pages) MICREL

8 / 18 page

Micrel

MICRF211

March 2007

M9999-030107

(408) 944-0800

Application Information

Figure 3 – QR211HE1 Application Example, 433.92 MHz

The MICRF211 can be fully tested by using one of

many evaluation boards designed at Micrel for this

device. As an entry level, the QR211HE1 (Figure 3)

offers a good start for most applications. It has a

helical PCB antenna with its matching network, a

band-pass-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. By removing the matching

network of the helical PCB antenna (C9 and L3), a

whip antenna (ANT2) or a RF connector (J2) can be

used instead. Figure 3 shows the entire schematic of

it for 433.92MHz. Other frequencies can be used and

the values needed are in the tables below.

Capacitor C9 and inductor L3 are the passive

elements for the helical PCB matching network. A

tight tolerance is recommended for these devices, like

2% for the inductor and 0.1pF for the capacitor. PCB

variations

may

require

different

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)

390.0

1.2

418.0

1.2

433.92

1.5

Table 2. Matching Values for the Helical PCB Antenna

To use another antenna, like the whip kind, remove

C9 and place the whip antenna in the hole provided in

the PCB. Also, a RF signal can be injected there.

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

the

parallel

resonance

equation

f = 1/(2×PI×(SQRT L1×C8)). Table 3 shows the most

used frequency values.

Freq (MHz)

C8 (pF)

L1(nH)

390.0

6.8

418.0

6.0

433.92

5.6

Table 3. Band-Pass-Filter Front-End Values

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

applications where it is proven that the outside band

noise does not cause a problem. The MICRF211 has

image reject mixers which improve significantly the

selectivity and rejection of outside band noise.

Capacitor C3 and inductor L2 form the L-shape

matching network. The capacitor provides additional

attenuation for low frequency outside band noise and

the inductor provides additional ESD protection for the

antenna pin. Two methods can be used to find these

values, which are matched close to 50

Ω. One method

is done by calculating the values using the equations

below and another by using a Smith chart. The latter

is made easier by using software that plots the values

of the components C8 and L1, like WinSmith by Noble

Publishing.

To calculate the matching values, one needs to know

the input impedance of the device. Table 4 shows the

input impedance of the MICRF211 and suggested

matching values for the most used frequencies. These

suggested values may be different if the layout is not

exactly the same as the one made here.

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