[Table of Contents]
People old and young
enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World was published
from May 1959 through December 1971.
As time permits, I will be glad to scan articles for you. All copyrights (if any) are hereby
article from the May 1967 edition of Electronics World shows you just
how long the Frequency
company has been around! In fact, here is an excerpt
from their website: "Frequency Electronics, Inc. was founded in 1962
and is now a well-established public company in Long Island, New York,
and trades on the NASDAQ Stock Exchange under the symbol FEIM." Mr.
Irwin Math, who wrote the article, has surely retired by now. The discussion
on short- and long-term stability is as relevant today as it was 45
See all the available
Selecting Frequency and Time Standards
By Irwin Math / Project Engineer, Frequency Electronics, Inc.
How precision oscillators and clocks are rated and exactly what
they are capable of. A discussion of stability and other important standard
Highly precise frequency and time standards
are becoming more and more widely used in today's sophisticated scientific
world. The missile and space age demands, for example, are far in excess
of the requirements of only a few years ago. In fact, the stability
of the research laboratory's frequency standard of 10 years ago is now
taken for granted in the oscillators of many types of modern industrial
As a result, all too often the engineer or technician
will specify a complex, ultra-stable frequency standard when a much
less stable one would have been sufficient for his application. Similarly,
he can often mistakenly interpret a lower order oscillator as easily
as one of much higher quality. For these reasons, the engineer and technician
as well as anyone with even a casual interest in the measurement field
should have a good
of just how precision oscillators and clocks
are rated and exactly what they are capable of doing.
Long-Term and Short-Term Stability
In the "good
old days", it was enough to ask for a 1-MHz oscillator with a stability
of ±0.001%. Anything better than 0.1% was considered rather good. But
today things have changed. All but the simplest of oscillators that
are used as standards are rated in parts per 10". For example, a 1-MHz
oscillator with a stability of ± 1 Hz is rated at 1 MHz ±1 part in 106
or 1 part per million. Similarly, a good frequency standard may have
a stability of ±1 part in 1010
The general expression for
frequency stability is S=ΔF/F where F is the nominal frequency and ΔF
is the allowable variation. Therefore, the actual frequency variation
of our frequency standard (considering its output is 1 MHz nominal)
is ±0.0001 Hz or ±1 x 10-10
of the nominal frequency.
Fig. 1. Measuring long-term stability
by means of peak readings.
However, this information is not enough. While it does indicate
stability, it does not specify over what period of time this stability
must be measured, The complete stability of a frequency-standard oscillator
must be given in two parts: the long term and the short term.
The long-term stability of an oscillator is basically the average
change in frequency over a long fixed period of time as compared to
some absolute reference. Usually, long-term stability is measured in
one of two ways. The first of these, as indicated in Fig. 1, depends
on readings of frequency taken continuously for several days. The maximum
peak-to-peak deviation over any 24-hour period is measured and the stability
is then defined as ± one-half this maximum. For example, since both
oscillators in Fig. 1 have a peak-to-peak variation of 1 part in 108
per day, their long-term stability would be specified as ±5 parts per
per day. Notice, though, that oscillator #2 has a much
longer average rate of frequency change than does oscillator # 1. Therefore,
while they both are ±5 parts in 109
per day oscillators,
oscillator #2 is obviously superior.
Fig. 2. Measuring long-term stability
by average readings.
The second method, and by far the more accurate one, is based
on the average change of frequency over a given time interval. As shown
in Fig. 2, the stability of oscillators #1 and #2 is now determined
by the slope of their average frequencies. Now it is easy to see that
oscillator #2 is substantially better than oscillator #1.
Fig. 3. Automatic recording of an
oscillator with a short-term
stability of plus or minus 5 parts in 109
To further clarify the measurement and eliminate any remaining
doubt, a few oscillator manufacturers incorporate both the average long-term
frequency change and the day-to-day change in their long-term stability
specification. For example, a 1-MHz oscillator such as the one shown
in Fig. 4 with a rated long-term stability of ±1 part in 1010
is one whose maximum daily and average frequency will change by no more
than ±1 cycle per 10 billion cycles per day or 0.0001 cycle per day.
The short-term stability of an oscillator is its frequency stability
over periods of time from a few seconds to fractions of a second. This
type of specification is important when the oscillator is used as the
source for a frequency multiplication system, especially into the thousands
of MHz. If, for example, a 1-MHz oscillator with a second-to-second
variation of ±1 part in 106
were multiplied to 1000 MHz,
the variation would now be ±1000 Hz per second.
As in long-term
measurements, there are two general methods. One is similar to long
term in that readings of frequency are taken. The time interval, however,
is much shorter. Fig. 3 is a recording of an oscillator with a short-term
variation of ±5 parts in 109
per second. Notice that in any
1-second interval, the frequency never deviates by ±5 parts in 109
from the nominal. This type of recording is used for intervals as low
as 0.1 second. Smaller intervals require too fast a recorder speed and
are measured by the second method.
This method involves a statistical
approach. The oscillator to be measured is fed to a low-noise mixer
along with a very stable reference frequency.
of the two signals is then fed to a counter which measures the period
(or wavelength) of this signal. A typical setup is shown in Fig. 5.
The difference frequency actually triggers a gate, allowing a very stable
10-MHz signal to drive the counter. Since the period of the difference
signal determines how many cycles of 10 MHz will be fed into the counter,
great accuracy can be achieved. Assuming that the difference signal
is 1 Hz, this means that 10,000,000 cycles will pass into the counter
through the gate for each cycle of input. Therefore, only 1/10,000,000
of a second change in the difference signal will cause the counter to
indicate a change. Since such great resolution is possible by this technique,
extremely short intervals of time can be measured and interpreted as
Fig. 4. A typical high-quality frequency
standard which has a
stability of plus or minus 1 part in 1010.
Usually an oscillator with good short-term stability will not
exhibit a good long-term characteristic and vice versa. As a result,
where both long- and short-term stability must be as good as possible,
the technique shown in Fig. 6 is used. A very good long-term standard
and a very good short-term standard are fed to a phase detector. The
d.c. output of the phase detector is then fed back to a voltage-variable
capacitor in the short-term standard which causes its frequency to "lock"
to the frequency of the long-term unit. This "phase-locking" technique
now produces an output with excellent long- and short-term stabilities.
Fig. 5. Test setup employed for short-term
The stability of an oscillator with changes in ambient operating
temperature is another important factor to consider when specifying
high-stability oscillators. Fig. 7 is a curve of the temperature versus
frequency characteristics of a typical high-quality crystal. It can
be seen that changes in crystal temperature result in significant changes
It is therefore extremely important to isolate
the crystal from the effects of varying temperatures. Vacuum bottles (dewars)
are often employed in the highest stability units and, in most cases,
the crystal itself is in a well-regulated, temperature-controlled oven.
Similarly, changes in operating voltage, line voltages, and
load impedances are all important and should be considered when choosing
oscillators. The effects of these parameters are stated in terms of
the maximum frequency changes they produce in the oscillators' output.
Other Important Criteria
Some of the
other more important criteria which should be considered when selecting
equipment of this nature are:
1. Construction - the equipment
should be able to withstand mechanical shock and also occupy the least
amount of space, especially where it is to be part of a physically complex
2. Power consumption - in portable, airborne, or other
such applications, it is desirable to obtain the lowest power consumption
of the oscillators in order to realize the longest usable life.
3. Measurements should be the result of exhaustive tests on many
units of the type to be used, where possible.
the most important "specification" to many technical personnel is cost.
While mechanical construction and ambient temperature isolation will
determine the cost of a unit to an extent, the primary cost factor is
the ultimate stability of the unit. The greater the stability required
(both long- and short-term) the more elaborate the circuitry required
and better the crystal must be. For example, while a ±1 part in 108
oscillator is available for $200, a ±1 part in 1010
can easily cost $2000.
Fig. 6. A phase-locked oscillator
system is illustrated.
Fig. 7. Typical frequency vs. temperature
characteristic for a high-quality crystal.
Oven is set to "turnover-point"