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Crystal Controlled Oscillators have been one of the most widely used oscillators during the last eighty years and remain as important today, although the technical requirements have become more demanding.
The main application for this type of oscillator is to stabilize frequencies in transmitters and receivers. Receiver sensitivities have improved as crystal technology advanced, allowing narrow channels and thereby improving sensitivity. EMF engineers have 30 years experience building crystal oscillators. This hands-on experience is invaluable when designing the circuit and when selecting the proper crystal. Please contact our engineers to discuss your specific requirements. |
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Application Information
General
Crystal oscillators designed and manufactured by EMF Systems are fixed frequency and voltage controlled. Both types use either a fundamental oscillator alone or as a driver for a transistor or step-diode multiplier.
FIXED FREQUENCY
Crystal oscillators have exceptional close-in phase noise and are often used as references for Synthesizers and Phase Locked oscillators. In these cases the phase noise becomes even more critical since the multiplication to higher frequencies increases the phase noise by approximately N2 where N is the multiplier factor. This can be converted to db by 20 log N and added to the noise of the fundamental.
VOLTAGE CONTROLLED
Many of the considerations discussed for VCOs apply equally well to VCXOs: tuning speed, linearity, noise, post tuning drift, etc. Linearity is measured by drawing a "best" straight line through the actual tuning curve as shown in Figure 1 and obtaining the frequency error of the worst point not falling on the line and dividing it by the total deviation bandwidth, DF. Multiplying by 100 gives the linearity error in percent.
Crystal oscillators are extremely stable devices compared to free running oscillators and for this reason, it is sometimes assumed they can operate under all possible conditions without changing frequency. This is not true. Crystals, and therefore crystal oscillators, are not absolute devices and frequency differences and changes occur. There may be an initial error in frequency due to a calibration tolerance. Frequency also changes with temperature, supply voltage, load impedance, and when an oscillator is vibrated. Some of these changes are minor and can be ignored in many applications. Temperature is the major cause of error is crystal oscillators. Ovens and TCXOs (temperature compensated crystal oscillators) are used to improve stability.
Fundamental oscillators are used to approximately 200 MHz; transistors or step-diode multipliers to 1.0 GHz. Models with multipliers have signals at multiples of the fundamental driver frequency called sub-harmonic signals; internal filters are used to suppress these to 40db in standard units and 60db in optional models.
Crystal oscillators can be electronically tuned over small bandwidths using varactors, although some frequency stability must be sacrificed. For example, typical stability for a VCXO with a ±.01% frequency swing is ±.002% for a 0ºC to +50ºC. If the frequency swing is increased to ±.05%, the stability would be approximately ±.005% over the same temperature range.
Figure 1
EMF Systems 300 Series:
- Series 300 Crystal Oscillators
- Series 390 Surface Mount Crystal Oscillators
- Model 332029: Multiple-Output VCXO Crystal Oscillator
- Model 332035: Voltage Controlled Crystal Oscillator
EMF Systems, Inc.
1900 West College Ave.
State College, PA 16801
A Division of Spectrum Microwave, Inc.
PHONE 814-272-2700 FAX 814-272-2796 EMAIL Sales@SpectrumMicrowave.com
