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AN1983 Crystal Oscillators and Frequency Multipliers Using the NE602 and NE5212

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AN1983 Crystal Oscillators and Frequency Multipliers Using the NE602 and NE5212 INTEGRATED CIRCUITS AN1983 Crystal oscillators and frequency multipliers using the NE602 and NE5212 1991 Dec Philips Semiconductors Philips Semiconductors Application note Crystal oscillators and frequency multipliers AN1983 using the NE602 and NE5212 INTRODUCTION than their LC counterparts. Figure 1 shows the equivalent circuit of This paper shows how to use the NE602 and NE5212 in VHF and a quartz crystal. UHF LO generators. The NE602 is a very low power active mixer, yet 7th overtone crystal oscillations are relatively easy to build. Optimization of such circuits is shown in the first part of this paper. Although designed as a fiber optic amplifier, the NE5212 can be used as a feedback oscillator with good results. The second part of L this paper shows how the NE602 can be used as a frequency dou- bler. Combinations of the two techniques can give the RF designer CO a wide variety of low power VHF/UHF signal sources from overtone C oscillators and/or frequency multiplier chains. R OSCILLATORS One of the most esoteric topics in RF theory is oscillators. Oscilla- tors are fundamental elements in both receiver and transmitter cir- cuitry. Often they are the limiting elements to a receiver’s SL01148 performance. Much work has and is being conducted to build better Figure 1. Equivalent Circuit of a Crystal oscillators and the oscillator’s more sophisticated cousin, the syn- The exact equivalent circuit constants in Figure 1 will be affected by thesizer. several mechanical considerations, of which physical size of the At first glance oscillators would appear to be rather simple. A stage crystal is perhaps the most important. The larger the crystal, the with a gain of more than one is configured with a feedback circuit. lower the resonant frequency. The “cut” of the crystal will effect Oscillation occurs at the frequency where the feedback gain is temperature, frequency range, and other parameters. Figure 2 greatest. The feedback circuit is usually resonant, configured either shows the relationship of impedance to frequency in the vicinity of from LC or crystal components and providing a narrow frequency resonance. Several important points can be drawn from this chart. band of feedback gain. Difficulty in oscillator design is curious in There are two resonant frequencies, a series and a parallel reso- view of the fact that amplifiers often oscillate undesirably, and it nance. These are shown by the impedance minimum and maximum seems it is often more difficult to build a gain stage that does not respectively. oscillate than one that does. In fact, oscillators are easy to build. The difficulty lies in building good oscillators. In turn, a “good” oscil- lator can be defined as meeting the designer’s criteria. Predictability and repeatability are two objectives to any engineering design. These features are especially elusive to electronic oscillator design. Oscillators are found in many, if not most analog and digital electron- Z ic devices. Oscillators found in radio receivers are of primary impor- tance to our discussion. The local oscillator or “LO” in a receiver is critical for proper receiver performance. The LO is usually formed from an LC circuit, crystal, or phase locked loop (PLL) synthesizer which usually derives its clock from a crystal. ω SL01149 The use of LC oscillator VFOs “variable frequency oscillator” has Figure 2. been eclipsed to a large extent since the introduction of digital syn- An important point relating to oscillator design is that large phase thesizers. The VFO is still used in inexpensive consumer radios and shifts can be noticed over relatively small changes in frequency for some high performance radios. The VFO can be less expensive crystals. This is not surprising when the very high circuit Q is con- than a synthesizer since it does not require a memory or micropro- sidered. This very large df/dw term allows the crystal to compen- cessor circuit for control. Carefully designed VFOs can also exhibit sate for undesired phase shifts in the feedback circuit without better phase noise response than PLL synthesizers, a critical con- “pulling” the operating frequency significantly. The circuit in Figure 5 sideration for high dynamic range receiver designs. Synthesizers shows a simple non-inverting feedback oscillator. The phase shift are preferred in higher-end consumer radios and other channelized across a series resonant circuit is zero. However, the phase shift of systems which demand some type of digital frequency control. Digi- the transfer function of the amplifier will probably be slightly offset tal control allows automatic scanning, frequency hopping, and other from zero. Therefore, the exact frequency of oscillation will be slight- functions not practical with an analog VFO. ly offset from the exact resonance of the crystal with the crystal Crystal oscillator circuits often require demanding tolerances. This “locking in” the exact phase/frequency combination. This phenome- is the case when they are used for second IF conversion as in the na exists for all crystal oscillator configurations. NE602 circuits. Another case is where an oscillator with very low If the frequency of operation must be more exactly specified than phase noise is desired for high performance systems. The NE602 this “pull” a “trimming” capacitor or inductor can be placed in the uses a Colpitts type oscillator configuration. An excellent feedback circuit to change the feedback phase relationship and force the type oscillator can be built using the NE5212. These two oscillator crystal frequency to move slightly. Alternatively very careful design types will be presented here in detail. and experiments must be performed to assure consistent PROPERTIES OF CRYSTALS performance. Often, if the crystal oscillator is to be operated over a Crystal resonators operate on a mechanical vibration principle called wide temperature range the temperature coefficients become the the “piezoelectric” effect. This is in contrast to the LC circuit configu- largest term in frequency tolerance calculations. These coefficients ration. Quartz crystals can exhibit circuit Qs up to 1000 times higher are provided by the crystal manufacturers. 1991 Dec 2 Philips Semiconductors Application note Crystal oscillators and frequency multipliers using the AN1983 NE602 and NE5212 Typical values for a 2MHz crystal relating to Figure 1 might be design is the series resistance R. Figure 3 plots a typical L=250mH, C=.04pF, R=125 ohms, and C1=4pF. Notice the high relationship between frequency and maximum R. The value of R is C1/C and L/C ratios. Of paramount importance in crystal oscillator critical when designing the loading circuit for the crystal. 200k 100k 10k CRYTAL LOADING RESISTANCE 1k 100 10 1kHz 10kHz 100kHz 1MHz 10MHz 20MHz FUNDAMENTAL FREQUENCY SL01150 Figure 3. Crystals are usually specified by their series or parallel resonant the passive oscillator components (inductors and capacitors) is frequencies, or both. An oscillator will nearly always operate near its relatively easy. Accurate prediction of the active device and crystal series resonance, the parallel resonance being more of a performance is extremely difficult. Practical design considerations convenience of measurement. The terms “series” and “parallel” are possible, particularly if we limit the number of configurations. resonant operation are often used but simply refer to a low or high loading impedance across the crystal’s terminals respectively. The “loading” impedance can be measured by removing the crystal and “look into” the circuit at the operating frequency. L 1 L 3 L 5 L N The thickness or mass of the crystal will largely determine its CO resonant frequency. However, the upper maximum resonant C1 C3 C5 CN frequency is about 20MHz. Above this frequency the crystal is too thin, and can crack easily. Piezoelectric effects allow for “overtone” R1 R3 R5 RN operation at odd order harmonic terms. Practice allows for operation of overtone crystals up to about 200MHz. Any crystal will “try” to SL01151 oscillate at its fundamental resonance. If the circuit does not allow a favorable phase response for oscillation at the fundamental Figure 4. Overtone and Fundamental Equivalent Circuit frequency, overtone oscillation is possible. An overtone oscillator DESIGNING CRYSTAL OSCILLATORS circuit must be configured to allow proper phase relationships to Two oscillator configurations will be treated in this section. Figure 5 occur at the desired overtone frequency and improper phase shows a model of a feedback oscillator. The amplifier is relationships at all lower order frequencies. non-inverting and should have a gain of greater than 1. The crystal Figure 4 shows the equivalent circuit for a crystal, including the will show a zero degree phase shift at series resonance, and overtone elements. The overtone and fundamental frequencies are consequently the circuit will oscillate very close to this frequency. never exactly harmonically related. Therefore, a crystal to be used in Figure 6 shows a very simple practical circuit. This circuit is actually an overtone application must be specified for that exact overtone an outstanding crystal oscillator. The output into a 50Ω load is frequency. The exact frequency of operation will be determined by 0dBm. The frequency stability is excellent particularly with small the cumulative phase response of the entire circuit, in overtone as variations in supply voltage. The NE5212 has a differential output well as fundamental oscillators. Predicting the phase response of 1991 Dec 3 Philips Semiconductors Application note Crystal oscillators and frequency multipliers using the AN1983 NE602 and NE5212 thus the inverting output acts as an effective buffer. Consequently impedance transformers and/or buffers. The Colpitts oscillator is not load fluctuations have little effect on the oscillator performance. very linear. However, for the purpose of understanding its operation, it is valid to think of it as an emitter follower. 0° Total Phase Shift NE602 VCC R LOAD RSOURCE 6 + SL01152 7 Figure 5. SL01155 Figure 8. + – NE5212 –90o SL01153 C 1 o Figure 6.
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