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Common Sense Oscillator Techniques

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Common Sense Oscillator Techniques COMMON SENSE OSCILLATOR TECHNIQUES INTRODUCTION. ASICs and microprocessor clocking such as a flip-flop or a Uni-Junction elements market size is estimated at 100 Transistor (UJT). Waveforms are typically million units per month. Often these exponential in nature and rich in harmonics. requirements were satisfied by designing The frequency of oscillation is determined mother-boards with thru-holes and the by the time constant, supply voltage, and purchase of “full size” or “half size” logic threshold trigger level. In general these clocks. Driven by PCMCIA and disk drive oscillators are not very frequency stable but designers, updated oscillator cells are now function well for strobes, turning indicators included in most ASICs. These oscillator or flashing lights. cells can be used to provide time base clock outputs without the necessity of a packaged Harmonic oscillators conform to clock. This technique permits PCB real Barkhausen’s criteria, requiring both phase estate reduction of 60% which in turn has and gain criteria be satisfied to provide the added advantage of reducing EMI. sustained oscillator. Clocking costs are reduced directly and additional cost reductions indirectly when Σ Gain around a close loop ≥ 1 one considers space costs. Σ Phase shifts around a closed loop = n * It is the fundamental purpose of this paper 360 deg where n is an integer value to indicate to the novice ways to utilize the oscillator cells in ASICs to provide clocking Harmonic oscillators utilize both inductive requirements. This paper will discuss the and capacitive elements or their mechanical effects of output impedance, the external and sonic equivalents, such as the elements components, and the methods for selecting represented in the equivalent circuit of a same. quartz crystal. Harmonic oscillators are characterized by sinusoidal waveforms and FUNDAMENTALS. Oscillators may be excellent frequency stability. The frequency categorized as either of the relaxation or is determined by the inductors and harmonic type. Relaxation oscillators capacitors and the propagation delay in the consist of a single energy storage element active circuitry. This paper will be limited to (capacitor or inductor) and a resistor. The the generation of harmonic oscillations C-R or L-R time constant is usually used to using the new oscillator cells. trigger a device sensitive to a voltage level 10400 N.W. 19th Street Miami, Florida 33172 Tel: 305-593-6033 Fax: 305-594-3973 1 RELAXATION HARMONIC AT Fundamental Strip. Low cost miniature OSCILLATOR OSCILLATOR fundamental AT strip crystals are commonly Flip-Flop L/C used from 3.5 to 40 MHz. Crystal resonator (Inductor/ Capacitor) design specified that the plate’s thickness (in mils) = 66.4 / Frequency (in MHz). Strip Uni-Junction Crystal Transistor crystals are troubled with flexure and face Ceramic Resonator shear coupling that must be accounted for in the width to thickness design ratio. In addition, the upper and lower frequency CRYSTAL CHOICES. Oscillator design for limits are restricted by process design and clocking must be simple, low cost and difficulties. At the low end of the frequency reliable. The basic AT and BT cuts invented spectrum, the thickness is almost 20 mils in 1934 [1] are still the cuts of choice. AT and requires special processing such as cuts are used in both fundamental and uni-directional contouring and edge tilting. overtone modes. BT cuts are used in the At 40 MHz, the AT crystal is only 1.66 mils fundamental mode only. thick and must be handled with vacuum picks. AT Cut. AT cut resonator’s frequency vs. temperature characteristics follow 3rd order AT Third Overtone Strip. In designing curvature with an inflection point at +25C. overtone strip crystals, coupling problems Standard frequency vs. temperature restrict the upper frequency limits to about specifications are ±0.005% (50 ppm) or 55 MHz. In addition to having the flexure ±0.01% (100 ppm) from -20C to +70C. and face shear coupling modes common in Stability limits are ±0.001% (10 ppm), from - fundamentals, there are also couplings to 20C to +70C. the harmonic of both Z thickness shear and X thickness shear modes. Dworksy[2] BT Cut. BT cut resonator’s frequency vs. states, “Clearly overtone design of strip temperature characteristics follow a resonators is much more complicated than parabolic curve with the maximum fundamental mode device design, and frequency near room temperature. tighter tolerance requirements for Consequently, for the BT cut, the frequency repeatable device performance are to be decreases at both higher and lower expected.” temperatures. A crystal that is calibrated to nominal frequency at +25C will be -100 ppm BT Fundamental Strip. Low cost at +70C and will be changing at -4 ppm per miniature fundamental BT strip crystal degree C. resonators are 1.521 times thicker than AT crystals. The thicker resonator reduces the See figure 1 for a graphical comparison of cost of processing so it follows that many both cuts. strip crystal units in the 30 to 45 MHz frequency band are fundamental BT plates. 10400 N.W. 19th Street Miami, Florida 33172 Tel: 305-593-6033 Fax: 305-594-3973 2 Strip Crystal Ranges. Inverted Mesa Crystals. Inverted Mesa crystals utilize and etching technique to ASIC CELLS OSCILLATOR BASICS. manufacture bi-level resonators. The The most widely used oscillator today is the thicker, lapped portion is used for mounting MOS gate oscillator using an unbuffered support. The thinner, etched portion inverter. Almost all microprocessors, along contains the electrode. Fundamental with many other digital applications, have crystals are available from 60 to 120 MHz this circuit built inside the cell or are driven and third overtones to 350 MHz. A new from an external oscillator using this ASIC product, tab mesa crystals, has recently circuit. been marketed. This product uses less Oscillator cells have several characteristics than the full 360 degree thicker support that are important to the oscillator designer. region and can be packaged in 2- x 6-mm tubular packages. Rin Rin is high -10 Meg ohms HC-49 and HC-45 Fundamental and Third Rout. Rout is dependent on the type of logic Overtone Crystals. Fundamental and 3rd being used. ECL logic has low impedance overtone crystal housed in conventional HC- – in the range of 7 ohms. CMOS logic has 45 and HC-49 holders are also excellent higher output impedance – in the region of choices as clocking elements. These 700 ohms. HCMOS logic is typically crystals are leaded devices that should be between 250 and 500 ohms. An oscillator thru-hole mounted. Third leads can be designer should obtain the output added for more rugged, low profile impedance information from the mounting. Lead forming for SMT manufacturer. If this information is not applications can be done; however, due to available, a simple output impedance test processing and handling difficulties, this is circuit is described in figure 2. presently not a big ticket item. Gain. Gain is derived by biasing an inverter in a linear mode. Biasing is done by 10 MHz 20 MHz 30 MHz 40 MHZ connecting a resistor from the inverter’s Fundamental AT strip resonator output to input. This connection results in high gain and wide bandwidth, but both lack consistency. 30 MHz 40 MHz Fundamental BT strip resonators Phase shift. Phase shift at f0, the frequency of oscillation is approximately 180 degrees through the amplifier from input to output. 40 MHz 50 MHz An additional 180 degrees phase is derived rd 3 O.T. AT strip resonators. from the pi feedback network. To insure reliable start-up and stable operation, the 10400 N.W. 19th Street Miami, Florida 33172 Tel: 305-593-6033 Fax: 305-594-3973 3 circuit must have the ability to shift phase an crystal, phase shifting should be going additional 20 degrees beyond the 180 through 90 degrees, less the effective degree minimum feedback requirement. phase shift from the inverter’s time delay – reference Table 1. Phase shift is modified by the time delay from input to output of the amplifier – Basic Feedback Circuit. A basic Pierce typically 1 to 2 nanoseconds. Since oscillator circuit is shown in Figure 3. frequency is the reciprocal of time, time Components include the phase shifting delay is equated to additional phase shift of elements C1 and C2 and an output the inverter. The external pi circuit must impedance adjusting resistor, Rx. Note Rx is shift 180 degrees less than the delay’s a discrete circuit component and is not equated shift. included in the output impedance of the inverter. If Rx is omitted in oscillator designs Table I shows phase shift for 1 and 2 (and unfortunately many times it is), nanosecond delay at selected frequencies. problems easily arise. The designer should This reduced phase shift information will be always layout his circuit for this component useful in Bode plot analysis. and use a zero ohm resistor (short) if Rx is not required for his feedback configuration. ( ) Feedback Element Selection. For this paper, feedback external to the ASIC cell The designer should make sure there is will be reduced to a “pi” or parallel tank sufficient dynamic range to make up for configuration. It is noteworthy that a “T” component or power supply change. For configuration has similar characteristics but example, a finger in a circuit will disturb is not within the scope of this paper. There phase shift and allow a crystal to start, lock are four (4) possible combinations of on, and continue to oscillate. reactance’s that can occur with a non- loaded tank-circuit. These four cases are To meet Barkhausen’s phase shift criteria, shown in Figure 4. we must shift 180 degrees externally (plus a minimum of 25 additional degrees) in the Figure 4, case 1, has capacitors in both legs feedback loop.
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