Clock Oscillator Application Notes

Frequency Stability: The most common stabilities are 25, 50 and 100
PPM. Overall stability usually includes accuracy at 25˚C, effects due to
changes in operating temperature, input voltage, aging, shock and
vibration. The ± 100 PPM stability has been the most popular as it is
sufficient to run microprocessors. The telecommunications industry has
been moving toward tighter and tighter stabilities. Stabilities beyond
±100 PPM are no longer offered in commercial (0-70˚C) applications, since
standard process controls achieve this stability as a minimum. Requesting
50 PPM is usually a little more expensive. Clock Oscillators requiring
25 PPM can significantly affect the price. For tighter than 25 PPM stability
applications, please consult the factory or consider a TCXO.
The purpose of these application notes is to help customers in specifying
Clock Oscillators. Background information about the type of Oscillators
offered by ECS is included along with some common definitions and
helpful formulas. The ECS Oscillator product line consists of Clock
Oscillators, TCXOs, VCXOs, VCTCXOs and VCOs.
Clock Oscillator: The standard clock oscillator is the most common
type of oscillator used and has applications in virtually every aspect of
the electronics industry. The clock oscillator is used to establish a
reference frequency used for timing purposes. A typical application is the
sequencing of events in a computer.
TCXOs ( Temperature Compensated Crystal Oscillators)
A crystal controlled clock oscillator typically consist of an amplifier and a
feedback network that selects a part of the amplifier output and returns
it to the amplifier input. A simplified block diagram of such a circuit is
shown below in (Fig. 1).
typically consists of tight tolerance quartz crystal, a temperature
compensation network, an oscillator circuit and a variety of buffer
and/or output stages determined by the output requirement. The crystal
has a characteristic of changing frequency when a capacitor is inserted in
series with the crystal unit as shown in (Fig. 2).
Figure 1) Simplified Block Diagram of a Crystal Controlled Clock Oscillator
C (pF)
Figure 2) Load Capacitance Characteristics of Crystal Unit
The basic criteria for oscillation in an oscillator are: 1. The open loop gain
must be greater than the losses around the oscillator loop and 2. The
phase shift around the oscillator loop must be either 0 or 360 degrees.
Utilizing the above characteristics, frequency can be stabilized by
inserting a temperature compensation circuit consisting of thermistors,
resistors and capacitors in the oscillation loop as shown in (Fig. 3). The
temperature compensation network is used to sense the ambient
temperature and “pull” the crystal frequency in a manner which
reduces frequency vs. temperature effect of the quartz crystal.
An oscillator can be used to generate different types of waveforms. The
most common types of waveforms produced by an oscillators are
sinusoidal and square.
The main parameters used in specifying a clock oscillator are listed
Logic TTL, HCMOS: In general, an HCMOS oscillator will drive TTL
circuitry (not vice versa). The industry is moving away from the TTL
logic as IC manufacturers are discontinuing the supply of many common
TTL IC’s. Most ECS clock oscillators are HCMOS/TTL compatible.
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Compensation circuit
VCXOs (Voltage Controlled Crystal Oscillator) are crystal
controlled oscillators in which the output frequency can be adjusted by
varying the external control voltage across a variable capacitor (varactor
diode) within the oscillator circuit. The associated change in frequency
due to the change in control voltage is known as pullability. VCXOs are
used widely in telecommunications, instrumentation and other electronic
equipment where a stable but electrically tunable oscillator is required.
Power supply
Oscillation circuit
The varactor diode is a semiconductor device that is designed to act as a
variable capacitor when a voltage is applied to it. When used in series
with a crystal, as shown in (Fig. 4), changing the control voltage causes
diode capacitance to change. This change in capacitance causes the total
crystal load capacitance to change and subsequently causes a change in
crystal frequency.
Figure 3) Temperature Compensation Circuit
A TCXO is generally required when overall stability needs are greater
than those of a clock oscillator. Also, the long term aging effects of a
TCXO are better than those of most clock oscillators.
Input Voltage: Most TCXOs are designed to operate at 5VDC, 3.3 VDC
or a combination of both.
RF Output: A TCXO can be manufactured with various types of
outputs: sine wave, clipped sine wave, TTL, HCMOS and ECL. Be sure to
specify the desired output type, signal requirements and the load that the
oscillator will be driving.
TCXOs also have a frequency adjustment feature which allow for readjustment of the oscillator to its center frequency to compensate for
aging. This adjustment can be provided in the following ways.
Figure 4) Typical VCXO circuit
1) A mechanical adjustment (internal trimmer) within the oscillator
accessible via hole in the enclosure.
2) An electrical adjustment via a lead in the enclosure for either a
remotely located potentiometer or a voltage. An oscillator using this
technique is called a Temperature Compensated Voltage Controlled
Crystal Oscillator or TCVCXO.
3) A combination of both mechanical and electrical adjustment.
Due to the growing applications of VCXOs in digital data transmissions
phase jitter (short-term stability) has become an important consideration.
Phase jitter provides a precise way to establish when a phase transition
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Definitions: The following definitions will aid you in understanding
oscillator performance and terminology.
Symmetry or Duty Cycle: The symmetry of the output waveform at
the specified level (at 1.4 V for TTL, at 1/2 Vcc for HCMOS, or 1/2
waveform peak level for ECL).
Nominal Frequency: The center or nominal output of a crystal
Rise Time (TR): Waveform rise time from Low to High transition
measured at the specified level (20% to 80% for HCMOS, ECL and 0.4 V
to 2.4 V for TTL).
Frequency Tolerance: The deviation from the nominal frequency in
terms of parts per millions (PPM) at room temperature. (25˚C ±5˚C)
model can be offered.
Fall Time (TF): The waveform fall time from High to Low transition,
measured at the specified level (80% to 20% for the HCMOS, ECL and 2.4
V to 0.4V for TTL).
Frequency Stability: The maximum allowable frequency deviation
compared to the measured frequency at 25˚C over the temperature
window, i.e. 0˚C to +70˚C. The typical stability for clock oscillators is
±0.01% (±100 PPM).
Load/Fan Out: The maximum load that the different families of
oscillators can drive is defined as the output load driving capability. The
load driving capability (fan-out) of each family of oscillators is specified
in terms of the number of gates an oscillator can drive.
Frequency Range: The frequency band that the oscillator type or
Operating Temperature: Temperature range within which output
Jitter (short-term stability): The modulation in phase or frequency of
frequency and other electrical, environmental characteristics meet the
the oscillator output.
HCMOS/TTL Compatible: The oscillator is designed with ACMOS
logic with driving capability of TTL and HCMOS loads while
maintaining minimum logic High of HCMOS.
Aging: The relative frequency change over a certain period of time.
Typically, aging for clock oscillators is ±5 PPM over 1 year maximum.
Storage Temperature: The temperature range within which the unit is
safely stored without damaging or changing the performance of the unit.
Tri-State Enable: When the input is left OPEN or tied to logic “1” the
normal oscillation occurs. When the input is grounded (tied to logic “0”,
the output is in HIGH IMPEDANCE state. The input has an internal
pull-up resistor thus allowing the input to be left open.
Supply Voltage: The maximum voltage which can safely be applied to
the VCC terminal with respect to ground.
Output Logic: The output of an oscillator is designed to meet various
Input Voltage (VIN): The maximum voltage which can be safely
specified logic’s, such as TTL, HCMOS, ECL, Sine, Clipped-Sine (DC cut).
applied to any input terminal of the oscillator.
Harmonic Distortion: The non-linear distortion due to unwanted
Output HIGH Voltage (VOH): The minimum voltage at an output
harmonic spectrum component related with target signal frequency. Each
harmonic component is the ratio of electric power against desired signal
output electric power and is expressed in terms of dbc, i.e. -20 dBc.
Harmonic distortion specification is important especially in sine output
when a clean and less distorted signal is required.
of the oscillator under proper loading.
Output LOW Voltage (VOL): The maximum voltage at an output of
the oscillator under proper loading.
Input HIGH Voltage (VIH): The minimum voltage to guarantee
threshold trigger at the input of the oscillator.
Dual and Multiple Outputs: More than one signal is capable of
being generated from a single oscillator. The signals may be related
(usually a multiple or divisor of the signal produced by a single crystal).
Input LOW Voltage (VIH): The maximum voltage to guarantee
threshold trigger at the input of the oscillator.
Start-Up Time: The start up time of an oscillator is defined as the time
an oscillator takes to reach its specified RF output amplitude.
Supply Current: The current flowing into Vcc terminal with respect to
ground. Typically supply current is measured without load.
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