Micrel, Inc.

MAQRF112

February 7, 2013

11

Revision 1.0

Application Information

The MAQRF112 is ideal for driving a 50

Ω load monopole

or a PCB loop antenna. The following sections discuss

PCB loop antenna and 50

Ω output configurations.

Output Matching Network

Part of the function of the output network is to attenuate

the second and third harmonics. When matching to a

transmit frequency, take care not only to optimize for

maximum output power but to attenuate unwanted

harmonics. Proper matching to a PCB antenna or a 50

Ω

load optimizes current requirements.

Layout Issues

PCB layout is extremely important to achieve optimum

performance and consistent manufacturing results. Be

careful with the orientation of the components to ensure

that they do not couple or decouple the RF signal. PCB

trace length should be short, to minimize parasitic

inductance (1in ~ 20nH). For example, depending on

inductance values, a 0.5in trace can change the

inductance by as much as 10%. To reduce parasitic

inductance, the use of wide traces and a ground plane

under signal traces is recommended. Use vias with low

inductance

values

for

components

requiring

a

connection to ground.

Crystal PCB Layout

Crystal

PCB

board

layout

affects

the

calculated

frequency deviation. It is recommended that the crystal

be located close to the XTLIN and XTLOUT pins to

minimize trace lengths. Trace thickness should be no

greater than 20mil.

Antenna Layout

The antenna trace layout affects directivity. No ground

plane should be under the antenna trace. For consistent

performance, do not place components inside the loop of

the antenna. Gerbers for a suggested layout are

available on the Micrel website at: www.micrel.com.

ASK PCB Loop Antenna Application Circuit

Figure 5 is an example of an ASK circuit configuration

using a PCB loop antenna. Table 2 lists modified values

for

both

315MHz

and

433.92MHz

configurations,

including crystal values for 315MHz and 433.92MHz

operation. Values are dependent on PCB board layout.

Refer to the Micrel website for a reference design and

PCB Gerber files.

Frequency

(MHz)

L1

(nH)

C5

(pF)

L4

(nH)

C7

(pF)

Y1

(MHz)

315

470

10

150

4.7

9.84375

433.92

680

10

82

3.9

13.5600

Table 2. PCB Antenna Matching Network

Figure 5. MAQRF112 ASK Application Circuit with PCB Loop Antenna