Crystal Oscillator Information
XO - Non-Compensated Crystal Oscillators
Non-compensated crystal oscillators employ no correction or control to reduce the frequency variation
over temperature beyond the intrinsic crystal stability. However, 'non-compensated' is a bit of a misnomer
since AT-cut crystals exhibit an S-shaped cubic frequency vs temperature curve which can hold the
frequency variation below a few parts per million for modest temperature excursions and below a few
tens of parts per million for extreme temperature variations. The angle of the crystal cut determine the
temperature curve and the manufacturer selects the most appropriate curve for the design temperature
range.
Typical AT-Cut crystal curves for various cut angles.
Simple XOs are a good choice when the stability requirements are not stringent because they are less
expensive, smaller, and more reliable than TCXOs and OCXOs
TCXOs - Temperature Compensated Crystal Oscillators
Temperature compensated crystal oscillators typically employ a thermistor network to generate a
correction voltage which reduces the frequency variation over temperature. The correction voltage is
usually applied to a varactor diode in the crystal circuit such that the crystal frequency may be varied by a
small amount. TCXO stability can approach 0.1 PPM but several problems must be addressed. A TCXO
that resides at one temperature extreme for an extended period of time may exhibit a frequency shift when
returned to normal room temperature. Usually this hysterisis or "retrace" error is temporary but a
seemingly permanent offset is common. Retrace errors are usually less than about 0.1 PPM but can be
much higher, especially if the mechanical tuning device (trimmer capacitor or potentiometer) is shifting.
This hysterisis makes the manufacture of TCXOs with specifications approaching 0.1 PPM quite difficult.
The high precision crystals found in oven oscillators exhibit less retrace but they are unsuitable for use in
TCXOs because they often exhibit activity dips at temperatures below the designed oven temperature and
SC-cuts and high overtone types cannot be tuned by a sufficient amount to compensate for the frequency
excursion.
Several other considerations that often do not appear on data sheets should be kept in mind:
TCXOs may exhibit temporary frequency drift when the ambient temperature changes if the thermistor
network does not have the same thermal time-constant as the crystal.
Adjusting the mechanical tuning (to compensate for aging) may change the electrical tuning sensitivity in
some designs causing the thermistor network to under-correct or over-correct.
Warm-up may not be as fast as expected if internal heat sources slowly warm the thermistor network and
crystal.
Phase noise may be inferior to oven designs and the noise may change over temperature.
TCXOs are preferred to oven oscillators in low power applications and when a warm-up period is not
acceptable. The only warm-up time is the time required for the components to reach thermal equilibrium
and the total current consumption can be very low - often determined by the output signal power
requirements.
Older TCXO designs employ from one to three thermistors to flatten the crystal temperature frequency
curve. Newer designs employ digital logic or a microprocessor to derive a correction voltage from values
or coefficients stored in memory. Some designers are requesting non-compensated oscillators fully
characterized over temperature so that a system microprocessor can provide the required correction
voltage. These oscillators are designed so that the crystal has a long thermal time constant to reduce shortterm instability and the electrical tuning is trimmed to a specified sensitivity. Each oscillator is given a
"curve" number much the way crystals are graded and the external micro measures the ambient
temperature calculates a correction voltage for the specific curve. The calculation may be an interpolation
between points in a look-up table or a computation using a cubic equation with temperature and the
oscillator's "angle" as inputs.
OCXOs - Oven-Controlled Crystal Oscillators
High performance crystal oscillators employ temperature control circuitry to hold the crystal and critical
circuitry at a precise, constant temperature. The best controllers are proportional, providing a steady
heating current which changes with the ambient temperature to hold the oven at a precise set-point,
usually about 10 degrees above the highest expected ambient temperature. Temperature induced
frequency variations can be greatly reduced by an amount approaching the thermal gain of the oven. The
crystal for the oven is selected to have a "turning-point" at or near the oven temperature further reducing
the sensitivity to temperature. The combination of the high oven gain with operation near turning point
yields temperature stabilities as good as 0.0001 PPM over a temperature range that would cause the
crystal to change by 10 PPM.
Highly polished, high-Q crystals which often have significant activity dips may be designed with no
activity dips near the operating temperature providing better stability and phase noise than crystals
designed for wide temperature ranges. Ovens also allow the use of SC-cut crystals which offer superior
characteristics but which are impractical for ordinary TCXOs because of their steep frequency drop at
cooler temperatures.
Oven oscillators come in all levels of performance and complexity. Low cost oven oscillators provide
better temperature stability than more expensive TCXOs but the power consumption is much higher. At
the other end of the spectrum are high-performance oven oscillators exhibiting superb short-term stability,
very low aging rates and high immunity to temperature and other enviornmental effects. Oven oscillators
insulated with Dewar flasks instead of foam are available for superior temperature performance and lower
power consumption.
Oven oscillators require a few minutes to warm and the power consumption is typically one or two watts
at room temperature. SC-cut crystals stabilize as soon as they reach the operating temperature but AT-cut
crystals exhibit a significant thermal transient effect which can take many minutes to settle. A typical ATcut crystal will drop in frequency well below any point on the static crystal curve due to the sudden
application of oven heat. The frequency will exponentially drift back up to the nominal frequency in most
oscillators. In some designs the oven controller overshoots significantly during initial warm-up and then
cools back down to the final oven temperature. This cooling transient can kick the AT-cut in the other
direction and may actually result in a shorter warm-up time even though the controller takes longer to
settle. Hand tweaked designs can achieve fairly good warm-up times with carefully selected overshoot but
the advent of quick settling SC-cut crystals obsoleted this approach.
VCXOs - Voltage-Controlled Crystal Oscillators
Voltage controlled crystal oscillators typically employ a varactor diode to vary the frequency of
oscillation by application of a tuning voltage. Tuning ranges vary from a fraction of a ppm to hundreds of
ppm. Design considerations include tuning linearity, tuning bandwidth, tuning range, output level stability
over tuning, post-tuning drift, phase noise, and stability.
Linearity is usually specified as a percentage of the total tuning deviation from a straight line fit to the
tuning curve. A 10% linearity specification would allow a deviation of 100 Hz away from the best straight
line fit for an oscillator with a 1 kHz tuning range. The potential problem is that this deviation can occur
rapidly at one end of the range as shown below. The slope is significantly lower at the top of the curve
despite the fact that the oscillator meets a fairly tight linearity specification. For PLL systems where the
tuning slope impacts the loop stability, it may be appropriate to specify the minimum and maximum
tuning slope at all points on the curve in addition to the percent linearity.
Tuning bandwidth is primarily set by the circuitry connecting the tuning voltage to the varactor. A high
tuning bandwidth is usually easy to accomplish but beware of substantial phase shift caused by sudden
roll-off. Try to specify a bandwidth about three to ten times higher than required by the PLL. Alternately,
specify the roll-off characteristic for an additional decade.
The tuning range should be kept as low as possible to reduce the oscillator's susceptibility to noise on the
tuning line. Remember to make allowances for temperature drift and long-term aging combined.
It is common for wide-pull VCXOs to change output level over tuning by a few dB. Make sure to specify
the output level stability if it is important.
Post-tuning drift is relatively minor in VCXOs but it is not zero. A small amount of drift may be seen after
tuning as the crystal and varactor reach a new thermal equilibrium.
Phase noise and stability are usually degraded by the circuit modifications necessary to achieve large
tuning ranges and the tuning voltage must be low noise.
Calculating the effects of tuning port noise on oscillator phase noise:
The spectral density, Sy(f), of the phase noise of an oscillator due to noise on the input is simply
calculated by squaring the product of the tuning sensitivity and the voltage noise density. For example,
consider a 10 MHz oscillator with a tuning sensitivity of 0.5 ppm and a white noise voltage on the tuning
line of 100 nV/root-Hz:
Sy(f) = (0.5E-6 x 100E-9)^2 = 25E-28
The phase noise spectral density is found by multiplying the frequency spectral density, Sy(f), by the
carrier frequency squared and dividing by the offset frequency squared. In the example, the phase noise at
100 Hz offset would be calculated as follows:
S(100) = 10MHz^2 x 25E-28 / 100^2 = 25E-18 = -166 dBc
Where S(100) is the phase noise spectral density at 100 Hz offset.
Note that the square of the offset frequency is in the denominator so the phase noise due to white tuning
noise rolls off at 20dB per decade.
Other Crystal Oscillator Types
Microprocessor Compensated
In addition to the obvious use of a microprocessor to "calculate" a frequency correction voltage to apply
to an electrical tuning input, microprocessors may employ syntheses to achieve frequency accuracy. SCcut crystals offer many stability advantages over AT-cuts but they have a steep frequency drop at colder
temperatures, dropping in frequency far more than they can be tuned, making them unsuitable for
ordinary TCXOs. However, a microprocessor programmed with the crystal's offset frequency over
temperature may be used to synthesize the correct frequency. The synthesizer may be a crude "pulse
swallower" or a modern numerically-controlled oscillator (also called DDS - direct digital synthesis)
depending upon the phase noise and resolution requirements. In addition to correcting for the crystal's
temperature drift, the DDS can generate an arbitrary output frequency on demand. The correction
function need not be in the same package as the oscillator if the system includes a computer-controlled
synthesizer. The oscillator may simply supply crystal temperature data and correction coefficients or a
correction look-up table.
-------------------------------------------------------------------------------Disciplined Oscillators
A microprocessor outfitted with a non-volatile time accumulation register can be programmed to apply
aging correction to a precision ovenized SC-cut crystal oscillator achieving stabilities approaching
Rubidium standards but with superior short-term stability and phase noise. These systems usually require
the application of an external standard for an extended period of time so that the microprocessor can
"learn" the oscillator's aging rate. When a standard is not available for extended periods, the correction
rate may be manually adjusted as part of a regular calibration procedure.
When Rubidium accuracy is needed but the power consumption must be low, a "sleeping" Rubidium
system is appropriate. These systems combine a low power oven oscillator with a Rubidium standard that
is normally off. Once or twice per day power is applied to the Rubidium standard just long enough for it
to stabilize so that the crystal oscillator may be corrected. The total energy required per day is far below
the energy required to keep the Rubidium standard operating.
Oscillators may also be disciplined with transmitted timing signals including Loran-C, GPS, WWVB,
Omega, cellular, CDMA and similar systems providing precision time and frequency information either
as a primary or secondary function. Shortwave band transmissions including those from WWV are also
useful but sky-wave variations limit the accuracy.
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Multi-Crystal Oscillators
Oscillators with more than one crystal address special stability or frequency agility requirements. A VHF
crystal may be phase-locked to a low frequency crystal to achieve low VHF phase noise with the close-in
noise and low aging of the low frequency crystal. Electronically switched crystals can provide multiple
frequency outputs with phase noise performance not available from synthesizers