MICRF221 datasheet(14/26 Pages) MICREL

14 / 26 page

Micrel Inc.

MICRF221

October 2008

M9999-100108

(408) 955-1690

RF Ocsillator Calculation

Crystal Y1 is the reference clock for all the device’s

internal circuits. Internally, the device has a Pierce

Oscillator

configuration,

requiring

the

external

capacitors, C9 and C10, to adjust the crystal center

frequency. The exact values for these capacitors

depend

the

printed

circuit

board’s

stray

capacitance.

For example, with a top ground plane or longer traces,

the value of these capacitors will be less than 10pF

since there will be more stray capacitance. If a

different layout from the one presented here is used,

the capacitor values are optimized by getting the best

sensitivity of the device. Crystal characteristics of

10pF load capacitance, 30ppm, ESR < 200Ω, -40ºC to

+105ºC

temperature

range

are

desired.

Table 4 shows the crystal frequencies and two of

Micrel’s

approved

crystal

manufactures

(www.hib.com.br or www.abracon.com).

Crystal frequency is calculated using:

REFOSC = RF Carrier/(64+(1.1/12))

The local oscillator is a low side injection type, so for

the 915.0MHz carrier, the local oscillator is calculated

by:

64 × REFOSC = RF Local OSC

64 × 14.27643MHz = 913.69MHz

That is, its frequency is below the RF carrier

frequency and the image frequency is below the LO

frequency. See Figure 8. The product of the incoming

RF signal and local oscillator signal will yield the IF

frequency, which is demodulated by the detector

circuits.

Figure 7. Plotting the Shunt Inductor and Series

Capacitor

Figure 8. Low Side Injection Local Oscillator

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