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Micrel Inc.
MICRF221
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 on 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 < 200 ? , -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
October 2008
14
M9999-100108
(408) 955-1690
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