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Keysight Technologies
Crystal Oscillator Testing
White Paper
Introduction
A policy concerning the testing of the reference oscillators contained within many instruments
such as counters, synthesizers and spectrum analyzers has been defined and adopted in Keysight
Technologies service centers worldwide.
Quartz crystal oscillators are used extensively in measurement instruments to provide an accurate refer-
ence frequency or time base against which measured signals may be compared.
There are two characteristics of such devices which users may wish to know:
1. The absolute oscillator frequency once it has warmed up
2. The stability of the oscillator frequency over time.
Our standard policy is to test the first parameter (and to adjust where necessary) but not the second. This
article is designed to help explain the rationale for this decision.
High-stability ovenized oscillators have their stability specified in terms of a drift rate per day. This means
that within a 24-hour period the frequency will not change by more than the permitted tolerance over
that period. This is in contrast to lower stability oscillators whose aging rate is stated over a month or a
year. The frequency may well change in the short term more than the specified amount, but the long-term
trend will comply with the specification -- see Figure 1.
2
1.8
1.6
Upper aging rate limit
1.4
Parts per million
Relative offset
1.2
1 Trend
0.8
0.6
0.4
0.2 Lower aging rate limit
0
1 4 7 10 13 16 19 22 25 28
Days
Figure 1. A representation of the frequency stability of an uncompensated or temperature compensated quartz
crystal oscillator. Note how the aging rate would far exceed, or be far less than, specification if measurements
were taken over a shorter period than the specified one month for this oscillator. The apparent aging rate would
also vary considerably depending when the measurements were made.
Experience
Our company manufactured precision crystal oscillators for many years and
during that time extensive analysis of their performance was made. In fact,
hundreds of thousands of high-stability oscillators were sold giving a significant
population from which to base our strategy for testing the devices.
The analysis determined that it is not generally necessary to test the drift
rate of high-stability oscillators. In fact, it has been estimated that there is a
99.995% probability that a Keysight 10811-series oscillator that performs to its
other specifications will also pass the aging rate specification. It is also a fact
that an aging-rate check is seldom part of the recommended calibration proce-
dure for any instruments fitted with precision crystal oscillators, whoever the
manufacturer.
Test Procedure
For confidence and repeatability, the oscillator is allowed to warm up according
to the following minimum times:
1. Room temperature and temperature compensated crystals: 1 hour
2. Medium performance oven: 12 hours
3. High performance oven: 48 hours
Once the instrument has warmed up, its absolute frequency is measured. This is
compared to test limits that have been established for each oscillator type and,
if necessary, the frequency is optimized to nominal.
Formula for calculating
Relative frequency offset
(Measured frequency