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Application Hint 32
Micrel
Application Hint 32
Receiver Crystal Selection
Introduction
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: * connect a ceramic resonator, * connect a crystal, or * drive this pin with an external timing signal. 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 microprocessor 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 sinusoidal). An externally applied signal should be ac-coupled and resistively-divided down, or otherwise limited, to approximately 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.
Active Bias 200k REFOSC 30pF 30pF VSSBB VSSBB 250 30A VDDBB
Typical Crystal Specification 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: * Parallel-resonant mode with 20pF load capacitance * Crystal series resistance 100 * 50ppm initial tolerance * 100ppm from -40C to +85C * 10ppm/year aging * 1mW drive level * Q = 110,000 These are suggested specifications only and may differ depending on the specific application. However, crystal series resistance is important and should be carefully considered. 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 frequency 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. 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.
VCC
Figure 1. Internal Oscillator Reference Oscillator The on-board oscillator, together with an externally connected 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 resonators and crystals do not exceed 50 and 100 respectively.
XTAL
R1 C1 R2 C2 R3 Q1 2N3904 C3 Signal Output (to MICRF001 REFOSC pin)
VSS
Figure 2. External Oscillator External Crystal Oscillator Example Circuit Choose R1, R2 large (several 100k) Choose C1, C2 small (10pF to 50pF) Choose R3 for 50A to 200A 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
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Application Hint 32
Application Hint 32
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
MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131
TEL
USA
+ 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. (c) 1999 Micrel Incorporated
Application Hint 32
2
July 1999

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