鈥?/div>
鈮?mW
drive level
鈥?Q = 110,000
These are suggested specifications only and may differ
depending on the specific application. However, crystal se-
ries resistance is important and should be carefully consid-
ered. As a rule of thumb, series resistance decreases as
crystal package size increases (for example, HC6 vs. HC49)
and crystal frequency increases.
Designs using the MICRF001 use crystal values from
2.2959MHz to 3.3674MHz. For customers who cannot meet
the series resistance target and Q values within these fre-
quency values, an external discrete oscillator is suggested.
This oscillator (see Figure 2) provides much higher gain than
the circuit within the MICRF001 and will oscillate with crystals
having series resistance up to 400鈩? This oscillator is quite
small and inexpensive, requiring no critical components. The
oscillator output would simply be connected to the REFOSC
pin of the MICRF001 instead of connecting a crystal directly.
V
CC
Figure 1. Internal Oscillator
Reference Oscillator
The on-board oscillator, together with an externally con-
nected resonator or clock signal, establishes the operating
frequency of the Micrel receiver. When using a resonator
(ceramic resonator or crystal), its series resistance should be
minimized to ensure oscillation. In cases where the resonator
series resistance is too great, the oscillator may oscillate at a
diminished peak-to-peak level, or may fail to oscillate entirely.
Micrel recommends that series resistances for ceramic reso-
nators and crystals do not exceed 50鈩?and 100鈩?respec-
tively.
XTAL
R1
C1
R2
C2
R3
Q1
2N3904
C3
Signal Output
(to MICRF001 REFOSC pin)
V
SS
Figure 2. External Oscillator
External Crystal Oscillator Example Circuit
Choose R1, R2 large (several 100k鈩?
Choose C1, C2 small (10pF to 50pF)
Choose R3 for 50碌A(chǔ) to 200碌A(chǔ) transistor bias current
C3 is dc blocking cap, value not critical
QwikRadio is a trademark of Micrel, Inc. The QwikRadio ICs were developed under a partnership agreement with AIT of Orlando, Florida
Micrel, Inc. 鈥?1849 Fortune Drive 鈥?San Jose, CA 95131 鈥?USA 鈥?tel + 1 (408) 944-0800 鈥?fax + 1 (408) 944-0970 鈥?http://www.micrel.com
July 1999
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Application Hint 32
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2
