Williams Almost digital communication systems require some form clock

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Williams Almost digital communication systems require some form clock

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Application Note October 1985 Circuit Techniques Clock Sources
Williams Almost digital communication systems require some form clock source. Generating accurate stable clock signals often difficult design problem. Quartz crystals basis most clock sources. combination high stability time temperature, wide available frequency range make crystals priceperformance bargain. Unfortunately, relatively little information appeared circuitry crystals engineers often view crystal circuitry black art, best left skilled practitioners (see box, "About Quartz Crystals"). fact, highest performance crystal clock circuitry does demand variety complex considerations subtle implementation techniques. Most applications, however, don't require this level attention relatively easy serve. Figure shows five forms simple crystal clocks. Types through commonly referred gate oscillators. Although these types popular, they often associated with temperamental operation, spurious modes outright failure oscillate. primary reason this inability reliably identify analog characteristics gates used gain elements. uncommon circuits this type gates from different manufacturers produce markedly different circuit operation. other cases, circuit works, influenced status other gates same package. Other circuits seem prefer certain gate locations within package. consideration these difficulties, gate oscillators generally best possible choice production design; nevertheless, they offer discrete component count, used variety situations, bear mention. Figure shows CMOS Schmitt trigger biased into linear region. capacitor adds phase shift circuit oscillates crystal resonant frequency. Figure shows similar version higher frequencies. gate gives inverting gain, with capacitors providing additional phase shift produce oscillation. Figure gate used allow 10MHz operating frequency. input resistance elements does allow high value, single resistor biasing method. R-C-R network shown replacement this function. Figure version using gates. Such circuits particularly vulnerable spurious operation attractive from component count standpoint. linearly biased gates provide degrees phase shift with feedback path coming through crystal. capacitor simply blocks gain path. Figure shows circuit based discrete components. Contrasted against other circuits, provides good example design flexibility certainty available with components specified linear domain. This circuit will oscillate over wide range crystal frequencies, typically 2MHz 20MHz. 2.2k resistors diodes compose pseudo current source which supplies base drive. 25°C base current 1.2V 1VBE 18µA saturate transistor, which would stop oscillator, requires near zero. collector current necessary this
IC(sat) (delete sat)
with 18µA base drive beta
required. 18µA
beta spread 2N3904's 210. transistor should saturate.even supply voltages below similar fashion, effects temperature also determined. temperature over 25°C 70°C -2.2mV/°C -99mV.
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Application Note
100kHz 1MHz 68pF 0.25µF
74C14 43pF
6.8M 4049
68pF
74LS04
10MHz 68pF
68pF 68pF 68pF
(1a)
74LS04 1200pF 74LS04
(1b)
0.1µF
(1c)
5MHz
2.2k
20MHz
2N3904 22pF
CRYSTALS PARALLEL RESONANT AT-CUT TYPES
100pF
(1d) (1e) Figure Typical Gate Oscillators Preferred Discrete Unit
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compliance voltage current source will move: -2.2mV/°C 45°C -198mV. Hence, first order compensation occurs: -198mV 99mV 99mV total shift. This remaining 99mV over temperature causes shift base current:
18µA 0.5V 70°C current 15µA 18µA 15µA 25°C current
Because free running frequency circuit close crystal's resonance, crystal "steals" energy from forcing crystal's frequency. crystal activity readily apparent Trace Figure which LT®1011's input. Trace LT1011's output. circuits this type, important ensure that enough current available quickly start crystal resonating while simultaneously maintaining time constant appropriate frequency. Typically, free running frequency should above crystal resonance with resistor feedback value calculated allow about 100µA into capacitor-crystal network. This type circuit recommended above hundred because comparator delays.
This shift (about 16%) provides compensation transistor shift with temperature, which moves about from 25°C 70°C. Thus circuit's behavior over temperature quite predictable. resistor, diode tolerances mean that only first order compensations over temperature appropriate. Figure shows another approach. This circuit uses standard RC-comparator multivibrator circuit with crystal connected directly across timing capacitor.
registered trademarks Linear Technology Corporation
85kHz 100pF LT1011
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Figure Crystal Stabilized Relaxation Oscillator
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Application Note
Figures another comparator based approach. Figure LT1016 comparator with negative feedback. resistors common mode level device's positive input. Without crystal, circuit considered very wideband (50GHz GBW) unity gain follower biased 2.5V. With crystal inserted, positive feedback occurs oscillation commences. Figure useful with AT-cut fundamental mode crystals 10MHz. Figure similar, supports oscillation frequencies 25MHz. Above 10MHz,
1MHz 10MHz CRYSTAL
AT-cut crystals operate overtone mode. Because this, oscillation occur multiples desired frequency. damper network rolls gain high frequency, insuring proper operation. preceding circuits will typically provide temperature coefficients 1ppm/°C with long term year) stability 5ppm 10ppm. Higher stability achievable with more attention circuit design control temperature. Figure shows Pierce class circuit with fine frequency trimming provided paralleled fixed
10MHz 25MHz CUT)
1V/DIV
5V/DIV
LT1016 LATCH OUTPUT
820pF
LT1016 LATCH OUTPUT
10µs/DIV
0.068µF
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200pF
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Figure Figure Waveforms
Figure 1MHz 10MHz Crystal Oscillator
Figure 10MHz 25MHz Crystal Oscillator
LT1005 0.1µF 33pF 1000pF 10pF OUTPUT (50) 100pF 2N3904 SELECT TYPICAL 0.1µF 34.8k 34.8k
MAIN SUPPLY
"OSCILLATOR READY" MAIN POWER 2N3904
5.6k
VCONTROL 2N3904 1N914
100k
LT1001
1N914 -15V THERMAL FEEDBACK
8.2µF
5.6k
330pF 3.3k 34.8k
2N6387 DARLINGTON
Oscillator
MAR-6 RESISTOR YELLOW SPRINGS INST. #44014 75°C 35.39k BLILEY #BG61AH-55, 75°C TURNING POINT. 5MHz FREQUENCY 0.01µF
8.2k
Oven Control
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Figure Ovenized Oscillator
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Application Note
variable capacitors. transistor provides 180° phase shift with loop components adding another 180°, resulting oscillation. LT1005 voltage regulator LT1001 used precision temperature servo control crystal temperature. LT1001 extracts differential bridge signal drives Darlington stage power heater, which monitored thermistor. practice, sensor tightly coupled heater. feedback values should optimized thermal characteristics oven. this case, oven constructed aluminum tube stock long wide 1/8" thick. heater windings were distributed around cylinder assembly placed within small insulating Dewar flask. This allows 75°C setpoint (the zero "turnover" temperature crystal specified) control 0.05°C over 70°C. LT1005 regulator sources bridge drive from auxiliary output also keeps system power until crystal's temperature (hence, frequency) stabilized. When power applied negative thermistor high value, causing LT1001 saturate positive. This turns zener-connected biasing Q3's collector current pulls regulator's control low, disabling output. When oven arrives control point, LT1001's output comes saturation servo controls oven point well below Q2's zener value. This turns enabling regulator source power whatever
84.5k* 56.5mV/°C LT1005 6.8k -15V MAIN OUTPUT SYSTEM
system clock associated with. crystal circuit values specified, this clock will drift less than 10-9 over 70°C with time drift part 10-9 week. oven approach removing temperature effects crystal clock frequency most effective wide use. Ovens however, require substantial power warm-up time. some situations, this unacceptable. Another approach offsetting temperature effects measure ambient temperature insert scaled compensation factor into crystal clock's frequency trimming network. This open loop correction technique relies matching clock frequency temperature characteristic, which quite repeatable. Figure shows temperature compensated crystal oscillator (TXCO) which uses first order linear correct temperature. oscillator Colpitts type, with capacitive tapped tank network. LT319A picks output network LT319's input provides signal adaptive trip threshold. LT1005 regulator's auxiliary output buffers supply variations main regulator output control allows system shut down without removing power from oscillator, aiding overall stability. ambient temperature sensed linear thermistor network A1's feedback loop with used scaling offsetting. A2's voltage output expresses
LT1055
10k*
OUTPUT
LT1055
LT1034 1.2V
10.7k* FILM RESISTOR YELLOW SPRINGS INST, THERMISTOR NETWORK #44201
MAIN OUTPUT CONTROL
3.955 100k 3.5MHz 2N2222A MV-209 500k 510pF 100k 100k
LT319A
OUTPUT
0.05µF
510pF
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Figure Temperature Compensated Crystal Oscillator (TXCO)
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Application Note
ambient temperature information required compensate clock. correction implemented biasing varactor diode varactor diode's capacitance varies with reverse bias) which series with crystal. varactor's shift capacitance used pull crystal's frequency complementary fashion circuit's temperature error. thermistor maintained isothermally with circuit, compensation very effective. Figure shows results. -40ppm frequency shift over 70°C corrected within 2ppm. Better compensation achievable including order terms temperature voltage conversion more accurately complement nonlinear frequency drift characteristic. Figure another voltage-varactor tuned circuit configured allow frequency shift instead opposing This voltage controlled crystal oscillator (VXCO)
FREQUENCY DEVIATION (ppm)
clean 20MHz sine wave output (Figure suitable communications applications. curve Figure shows 7kHz shift from 20MHz over tuning range. 25pF trimmer sets 20MHz zero bias frequency. many applications, such phase-locking narrow bandwidth secure communications, nonlinear response irrelevant. Improved linearity will require conditioning tuning voltage varactor network's response. circuits this type important remember that limit pulling frequency crystals which high. Achieving wide dynamic "pull" range without stopping oscillator forcing into abnormal modes difficult. Typical circuits, such this one, offer pull ranges several hundred ppm. Larger shifts (e.g., 2000ppm 3000ppm) possible without losing crystal lock, although clock output frequency stability suffers somewhat.
0.01µF
UNCOMPENSATED TEMPERATURE (°C)
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2N2369
100pF
25pF
20MHz 100k TUNING VOLTAGE
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COMPENSATED
4.7k 220pF
0.01µF
2.7k
330pF
MV-1405
OUTPUT 20.0000MHz 20.0070MHz
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Figure TXCO Drift Performance
Figure Voltage Controlled Crystal Oscillator (VCXO)
7000 6000
FREQUENCY SHIFT (Hz)
5000 4000 3000 2000 1000
100mV/DIV
10ns/DIV
TUNING VOLTAGE
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Figure Figure Output
Figure Figure Tuning Characteristics
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Application Note
Noncrystal Clock Circuits Although crystal based circuits universally applied, they cannot serve clock requirements. example, many systems require reliable 60Hz line synchronous clock. Zero crossing detectors simple voltage level detectors often employed, have poor noise rejection characteristics. achieving good line clock under adverse conditions design circuit which takes advantage narrow bandwidth 60Hz fundamental. Approaches utilizing wide gain bandwidth, even hysteresis applied, invite trouble with noise. Figure shows line synchronous clock which will lose lock under noisy line conditions. basic multivibrator tuned free near 60Hz, AC-line derived synchronizing input forces oscillator lock line. circuit derives noise rejection from integrator characteristics network. Figure shows, noise fast spiking 60Hz input (Trace Figure little effect capacitor's charging characteristics (Trace Figure circuit's output (Trace Figure stable.
60Hz INPUT SYNC
Figure another synchronous clock circuit. this instance, circuit output locks higher frequency than synchronizing input. Circuit operation time domain equivalent reset stabilized amplifier. LT1055 associated components form stable oscillator. LM329 diode bridge compensating diodes provide stable bipolar charging source located amplifier's negative input. synchronizing pulse (Trace Figure level shifted LT1011 comparator drive FET. When synchronizing pulse appears, turns grounding capacitor (Trace Figure 14). This interrupts normal oscillator action, only small fraction cycle. When sync pulse falls, capacitor's charge cycle, which been reset starts again. This resetting action forces frequency charging synchronous stabilized sync pulse. only evidence this operation output occasional, slightly enlarged pulse width (Trace Figure 14), which caused synchronizing interval. sync adjust potentiometer
0.15µF
10V/DIV
LT1055 60Hz OUTPUT
5V/DIV
5V/DIV
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5ms/DIV
Figure Synchronized Oscillator
SYNC
Figure Figure 11's Waveforms
7.5k*
0.05µF
2N4392 SYNC 330k 3.3k LT1011 LT1055
OUTPUT LM329 1N4148
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Figure Reset Stabilized Oscillator
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Application Note
should trimmed sync pulse appears when capacitor near This minimizes output waveform width deviation allows maximum protection against losing lock drift over time temperature. maximum practical output frequency sync frequency ratio about Pure oscillators final form clock circuit. Although this class circuit cannot achieve stability synchronized crystal based approach, offers simplicity, economy direct frequency output. such they used baud rate generators other frequency applications. designing stable oscillator make output frequency insensitive drift many circuit elements possible. Figure shows clock circuit which depends primarily elements stability. other components contribute very order error terms, even substantial shifts. addition, components have been chosen opposing temperature coefficients, further aiding stability. circuit standard comparator-multivibrator with parallel CMOS inverters interposed between comparator output feedback resistors. This replaces relatively large unstable bipolar saturation losses LT1011 output with superior characteristics MOS. only switching losses rails resistive, they tend cancel. paralleling inverters further reduces errors insignificant levels. With this arrangement, charge discharge time constant capacitor almost totally immune from supply temperature shifts. units need precision types, because shifts them will cancel. addition, effect comparator's input errors also negated because symmetrical nature oscillator. This leaves only network significant error term. nominal-120ppm/°C temperature coefficient polystyrene capacitor partially offset opposing positive temperature coefficient designed into specified resistor. practice, only first order compensation achievable because uncertainty capacitor's exact test circuit, 70°C temperature excursion showed 15ppm/°C with power supply rejection factor less than 20ppm/V. contrast, clock constructed from popular timer, using compensating network, showed 95ppm/°C 1050ppm/V supply shift. Because comparator propagation delays, circuits this type less stable above 5kHz 10kHz operating frequency.
74C04s OUTPUT
5V/DIV 2V/DIV
0.015µF
4.7k LT1011
50V/DIV 200µs/DIV
Figure Figure13's Waveforms
*TRW TYPE MTR-5/+120ppm/°C 0.015µF POLYSTYRENE- 120ppm/°C ±30ppm WESCO TYPE 32-P
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Figure Stable Oscillator
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Application Note
ABOUT QUARTZ CRYSTALS frequency stability repeatability quartz crystals represent nature's best bargains circuit designer. equivalent circuit crystal looks like series-parallel combination elements.
factors affecting resonator performance include method lead attachment, package sealing method internal environment (e.g., vacuum, partial pressure, etc.). Some circuit considerations when using crystals include: Load Capacitance-The reactance crystal must present circuit. Some circuits crystal parallel resonant mode (e.g., crystal looks inductive). Other circuits specified series resonant crystal appears resistive. this mode, circuit's load capacitance, including parasitics, must specified. typical number around 30pF. Resistance-The impedance crystal presents when resonating. Drive Level-How much power dissipated crystal still maintain specifications. 10mW typical. Excessive levels fracture crystal. Temperature Coefficient/Turning Point-The tempco crystal usually specified near "turning point." This temperature which crystal tempco zero. Typically tempco will below 1ppm/°C over operating range turning point around 75°C, although different cuts considerably alter these numbers. Frequency Tolerance-The deviation from ideal frequency when used under specified circuit conditions defined temperature. Tolerances vary from 50ppm less than 1ppm.
Typical Values: 500µH 0.01pF 50,000 static capacitance produced contact wires, crystal electrodes crystal holder. term called motional arm. mechanical mass. includes electrical losses crystal reactive component quartz. Different angles from mother crystal produce different electrical characteristics individual crystals. Cuts optimized temperature coefficient, frequency range other parameters. basic "AT" used most crystals 1MHz 150MHz range good compromise between temperature coefficient, frequency range, ease manufacture other considerations. Other
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT/TP 1101 1.5K PRINTED
www.linear.com
LINEAR TECHNOLOGY CORPORATION 1985

Williams Almost digital communication systems require some form clock

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