APPLICATIONHINT32 [ETC]
Receiver Crystal Selection ; 接收晶体选择\n
| 型号： | APPLICATIONHINT32 |
| 厂家： | 
ETC |
| 描述： | Receiver Crystal Selection
|
| 文件： | 总2页 (文件大小：23K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Application Hint 32
Receiver Crystal Selection
Typical Crystal Specification
Introduction
For customers who prefer to use crystals for the resonating
element (such as in fixed-mode operation), Micrel suggests
the following crystal specification as a general starting point:
All timing and tuning operations are derived from the internal
Colpitts reference oscillator. Timing and tuning is controlled
through the REFOSC pin in one of three ways:
• Parallel-resonant mode with 20pF load capacitance
• Crystal series resistance ≤100Ω
• ±50ppm initial tolerance
• connect a ceramic resonator,
• connect a crystal, or
• drive this pin with an external timing signal.
• ±100ppm from –40°C to +85°C
• ≤10ppm/year aging
The third approach is attractive for lowering system cost
further if an accurate reference signal exists elsewhere in the
system; for example, a reference clock from a crystal- or
ceramic-resonator-controlled microprocessor. This method
also provides some noise immunity by synchronizing the
microprocessor clock with the Micrel receiver. The micropro-
cessor clock, whether or not used as the receiver clock,
should not have fast transitions (for example, sawtooth
waveshapes are more likely to have problems than sinusoi-
dal). An externally applied signal should be ac-coupled and
resistively-divided down, or otherwise limited, to approxi-
mately 0.5Vpp. The specific reference frequency required is
related to the system transmit frequency and to the operating
mode of the receiver as set by the SWEN pin.
• ≥1mW drive level
• 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
thecircuitwithintheMICRF001andwilloscillatewithcrystals
having series resistance up to 400Ω. 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.
VDDBB
Active
Bias
200k
REFOSC
250Ω
30pF
30pF
30µA
VCC
VSSBB
VSSBB
R1
Figure 1. Internal Oscillator
Q1
2N3904
C1
XTAL
C3
Signal Output
(to MICRF001 REFOSC pin)
Reference Oscillator
R2
C2
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
minimizedtoensureoscillation. Incaseswheretheresonator
series resistance is too great, the oscillator may oscillate at a
diminishedpeak-to-peaklevel,ormayfailtooscillateentirely.
Micrel recommends that series resistances for ceramic reso-
nators and crystals do not exceed 50Ω and 100Ω respec-
tively.
R3
VSS
Figure 2. External Oscillator
External Crystal Oscillator Example Circuit
Choose R1, R2 large (several 100kΩ)
Choose C1, C2 small (10pF to 50pF)
Choose R3 for 50µA to 200µA 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. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
July 1999
1
Application Hint 32
Application Hint 32
Micrel
The MICRF011/RF022/RF002/RF003 second generation
receivers have reference oscillator values of approximately 2
times that of the MICRF001. Higher frequency values are
easier to manufacture and operate with the Micrel receiver.
MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or
other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.
© 1999 Micrel Incorporated
Application Hint 32
2
July 1999
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