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1、50High Frequency Electronics High Frequency Design OSCILLATOR SPECS A Review of Key Oscillator Specifications and What They Mean By Gary Breed Editorial Director U nderstanding the specifications of oscillators is essential when selecting a commercial unit or evaluating the perfor- mance of your own
2、 design. This tutorial-level article offers a review of oscillator specifica- tions, what they mean, and some of the appli- cations that are most affected by particular specifications. These notes will focus primari- ly on crystal oscillator specifications, although many of the same measurements and
3、 perfor- mance factors apply to VCOs, phase-locked loops and direct digital synthesizers. Stability The first specification is stability.This is as simple as it soundsstability is how well an oscillator stays on the desired frequency. The measurement of stability is the deviation in frequency under
4、measurement conditions. The data is typically presented in parts- per-million (ppm), although percentage is occasionally used. High performance oscilla- tors with stability of less than one ppm will be specified in scientific notation (e.g. 1x10-6is 1 ppm). Measurement conditions include: Temperatur
5、eOscillators are measured according to their intended use, which means the temperature range may be 0 to 50 C for indoor applications, 40 to +70 C for outdoor applications, or some other range. The most common crystals are “AT cut” (a specific orien- tation in the quartz blank), which is easy to man
6、ufacture and has a modest, but pre- dictable temperature variation curve.“SC cut” crystals are harder to make, but have much better intrinsic temperature stability, which lessens the need to provide compensation in the oscillator circuitry. TimeCrystals change slightly with age, so performance speci
7、fications include short- term (e.g. one day) and long term (e.g. one year) stability. Oscillators used in reference applications may have 10-year stability speci- fications as well. Shock and vibrationFrequency variation due to shock and vibration is a key specifica- tion in oscillators that will be
8、 used in harsh environments, including military and space applications.The magnitude and type of stress will vary according to the needs of each appli- cationa DSL modem might get dropped on a concrete floor, while some military electronics must survive being fired from big guns. Applications with t
9、he greatest stability requirements include primary references for instrumentation, data communications net- works and research. Other applications with a significant need for high stability can be clas- sified as “secondary references” used in appli- cations such as GPS receivers and general purpose
10、 instruments. Phase Noise You might wonder why phase noise (time/frequency domain noise) is the specifica- tion for oscillators rather than noise that includes amplitude.The answer is straightfor- ward. First, the frequency of the oscillator is whats important, and second, amplitude noise can be lar
11、gely eliminated with limiter circuit- ry. However, if the oscillator is so poorly designed that it has excessive amplitude noise, some of that energy will be converted to phase noise by the limiter or by other nonlin- Whether designing or buy- ing an oscillator, the key specifications must be unders
12、tood in order to make the right choice, or evaluate a design From November 2003 High Frequency Electronics Copyright 2003 Summit Technical Media, LLC RadioFans.CN 收音机爱 好者资料库 52High Frequency Electronics High Frequency Design OSCILLATOR SPECS ear components in the circuit. Phase noise is specified in
13、 dB referenced to the carri- er amplitude (dBc), versus frequency offset from the car- rier (fc). In a reasonably well-designed oscillator, the noise energy decreases with increasing frequency offset, but the slope of the rolloff in noise has a series of different sec- tions, governed by several dif
14、ferent behaviors. Figure 1 is a simplified sketch that illustrates the var- ious influences on crystal oscillator phase noise. The dif- ferent mechanism are described in many papers on oscil- lator measurement, including 1. Rather than describe each in detail, it is sufficient to note that each is a
15、n effect created by the physical properties of the crystal and the active devices in the oscillator circuit. The figure shows how different phenomena contribute to overall phase noise characteristics of an oscillator, each being dominant at different offset frequencies and amplitude levels. A VCO wi
16、ll have a similar plot, but with significantly higher levels of phase noise, since LC, transmission lines, dielectric resonators etc. have much lower Q than a quartz crystal. In a phase-locked loop, a similar plot (Figure 2) is used to show phase noise performance. The shape of the plot is quite dif
17、ferent, since noise amplitude is greatly reduced outside the loop filter bandwidth. The loop filter deter- mines the phase noise close to fc, but a well-designed VCO is needed for best performance at large offsets. Practical PLLs may have noise or discrete spurious responses from circuitry between t
18、he loop filter and VCO, power supply bounce and ground currents, digital switching, and other circuit-related behaviors. Figure 2 is not dimensioned, since different synthesizer implementations will have dif- ferent characteristics. Direct digital synthesizers (DDS) or numerically con- trolled oscil
19、lators (NCO) have unique phase noise charac- teristics, as well as spurs that are a result of the digital circuitry and digital-to-analog conversion. Rather than try to describe them in this short article, the reader is advised to read further on this subject. Phase noise is the single most importan
20、t specification in some communication systems. A receivers that must operate in the presence of nearby strong signals is one such application. A transmitter modulated with a highly complex waveform is another. In each case, excessive phase noise can mask desired signal content. Additional VCO Specif
21、ications A voltage-controlled oscillator will have its own phase noise plot, but also requires tuning range information.The power supply must provide the maximum tuning voltage, while the frequency vs. tuning voltage curve will influence the design of a synthesizers loop filter. Stability vs. tem- p
22、erature data will tell an engineer how much of the tun- ing range is available to be used in his or her application. Jitter in Digital Oscillators Jitter is related to phase noise, but is described in the time domain rather than the frequency domain because it relates to digital signals. Jitter, usu
23、ally specified in picoseconds, is the maximum time variation from the ideal succession of rise and fall transitions in a square wave. This is an important specification for high-speed digital computing and communications systems which must maintain precise timing. Gigabit/second optical sys- tems ar
24、e a key application requiring low jitter specs. Reference 1. D. B. Sullivan, D. W. Allan, D. A. Howe and F. L. Walls, editors, Characterization of Clocks and Oscillators, NIST Technical Note 1337, National Institute of Science and Technology, Time and Frequency Division, 1990. Frequency Offset from
25、fc (Hz) Random Walk frequency noise Flicker frequency noise White frequency noise Flicker phase noise 60 90 120 150 100101102 103 104 Amplitude (dBc) White phase noise Figure 1 The effects of different physical phenomena on crystal oscillator phase noise. Frequency Offset from fc Close-in VCO noise Loop filter rolloff characteristic VCO noise plus circuit noise VCO residual noise Discrete spurs Relative Amplitude Figure 2 The phase noise characteristics of a typical phase-locked loop. RadioFans.CN 收音机爱 好者资料库
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