AN12 - Circuit Techniques for Clock Sources

1 Application Note 12AN12-1an12faOctober 1985 Circuit Techniques for Clock SourcesJim WilliamsL, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Almost all digital or communication systems require some form of Clock source. Generating accurate and stable Clock signals is often a difficult design crystals are the basis for most Clock Sources . The combination of high Q, stability vs time and temperature, and wide available frequency range make crystals a price-performance bargain.

2 Unfortunately, relatively little information has appeared on circuitry for crystals and engineers often view crystal circuitry as a black art, best left to a few skilled practitioners (see box, About Quartz Crystals ).In fact, the highest performance crystal Clock circuitry does demand a variety of complex considerations and subtle implementation Techniques . Most applications, however, don t require this level of attention and are relatively easy to serve. Figure 1 shows five (5) forms of simple crystal clocks. Types 1a through 1d are commonly referred to as gate oscillators.

3 Although these types are popular, they are often associated with temperamental operation, spurious modes or outright failure to oscillate. The pri-mary reason for this is the inability to reliably identify the analog characteristics of the gates used as gain elements. It is not uncommon in circuits of this type for gates from different manufacturers to produce markedly different Circuit operation. In other cases, the Circuit works, but is influenced by the status of other gates in the same pack-age. Other circuits seem to prefer certain gate locations within the package.

4 In consideration of these difficulties, gate oscillators are generally not the best possible choice in a production design; nevertheless, they offer low discrete component count, are used in a variety of situations, and bear mention. Figure 1a shows a CMOS Schmitt trigger biased into its linear region. The capacitor adds phase shift and the Circuit oscillates at the crystal resonant frequency. Figure 1b shows a similar version for higher frequencies. The gate gives inverting gain, with the capacitors providing additional phase shift to produce oscillation.

5 In Figure 1c, a TTL gate is used to allow the 10 MHz operating frequency. The low input resistance of TTL elements does not allow the high value, single resistor biasing method. The R-C-R network shown is a replacement for this function. Figure 1d is a version using two gates. Such circuits are particularly vulnerable to spurious operation but are attractive from a component count standpoint. The two linearly biased gates provide 360 degrees of phase shift with the feedback path coming through the crystal. The capacitor simply blocks DC in the gain path.

6 Figure 1e shows a Circuit based on discrete components. Contrasted against the other cir-cuits, it provides a good example of the design flexibility and certainty available with components specified in the linear domain. This Circuit will oscillate over a wide range of crystal frequencies, typically 2 MHz to and 33k resistors and the diodes compose a pseudo current source which supplies base 25 C the base current is: 1 VBE33k=18 ATo saturate the transistor, which would stop the oscilla-tor, requires VCE to go to near zero.

7 The collector current necessary to do this is: IC(sat)=5V1k=5mAwith 18 A of base drive a beta of: 5mA18 A=278 is requiredAt 1mA the DC beta spread of 2N3904 s is 70 to transistor should not at supply volt-ages below similar fashion, the effects of temperature may also be vs temperature over 25 C 70 C is: C 45 = Note F20 MHzOUT74LS0474LS041200pF5 MHzOUT(1a)(1b)(1c)(1e)(1d)ALL CRYSTALS PARALLEL RESONANT AT-CUT TYPESAN-12 F015 VFigure 1. Typical Gate Oscillators and the Preferred Discrete UnitFigure 2.

8 Crystal Stabilized Relaxation OscillatorThe compliance voltage of the current source will move: 2 C 45 C = , a first order compensation occurs: 198mV 99mV = 99mV total remaining 99mV over temperature causes a shift in base current: 25 C current= A70 C current= A18 A 15 A=3 AThis 3 A shift (about 16%) provides a compensation for transistor hFE shift with temperature, which moves about 20% from 25 C to 70 C. Thus the Circuit s behavior over temperature is quite predictable. The resistor, diode and VBE tolerances mean that only first order compensations for VBE and hFE over temperature are 2 shows another approach.

9 This Circuit uses a standard RC-comparator multivibrator Circuit with the crystal connected directly across the timing capacitor. Because the free running frequency of the Circuit is close to the crystal s resonance, the crystal steals energy from the RC, forcing it to run at the crystal s frequency. The crystal activity is readily apparent in Trace A of Figure 3, which is the LT 1011 s input. Trace B is the LT1011 s output. In circuits of this type, it is important to ensure that enough current is available to quickly start the crystal resonating while simultaneously maintaining an RC time constant of appropriate frequency.

10 Typically, the free run-ning frequency should be set 5% to 10% above crystal resonance with a resistor feedback value calculated to allow about 100 A into the capacitor-crystal network. This type of Circuit is not recommended for use above a few hundred kHz because of comparator delays. +100pF85kHz50kLT10110UT1k5V5V10k10k10kAN -12 F02 Application Note 12AN12-3an12faFigures 4a and 4b use another comparator based approach. In Figure 4a, the LT1016 comparator is set up with DC negative feedback. The 2k resistors set the common mode level at the device s positive input.