For example, suppose the oscillator and the line were the same voltage, but 180 degrees out of phase. The line would be at voltage +V, while the oscillator would be at –V. The load would see a
transient voltage of 2V as the relay switches between
sources. (For a line voltage of 115 Vrms, V can be
anywhere from 0 to 160 volts, depending on when in the
cycle the switching occurs. A 320 volt transient is
possible). This stresses both the relay and the load,
possibly to the point of doing damage.
If the 60 Hz line and oscillator output were
synchronized and of the same magnitude, the switching
would occur between two identical voltages with no
transient at all. The conclusion is that if you want to switch
a load between sources, they should be synchronized and
of equal magnitude. This article shows how to achieve
synchronization (identical frequency and phase) of the
The oscillator frequency is determined by a quartz
crystal, and the line frequency is controlled by the utility
company. In some locations, this may vary by as much as
±0.5 Hz over the course of a day. The question is: How
can we synchronize the crystal oscillator to the line?
Almost by definition we cannot change the frequency of
the crystal, yet it must be adjusted to the line frequency
which may be anywhere from 59. 5 Hz to 60. 5 Hz.
It is possible to “pull” a crystal off of its nominal
frequency by adding reactive components into the
resonant circuit. This technique might be able to pull the
crystal a few kHz off nominal.
Assuming the 15. 36 MHz crystal can be pulled ± 5
kHz, that represents about ±325 PPM (or ±0.02 Hz)
which is not enough to cover a potential discrepancy of
An effective technique used here is to distort the
waveform so it fits in the allotted cycle time of the 60 Hz
line. For example, consider the waveforms in Figure 1. The
yellow trace is the (simulated) line signal running at 61.74
Hz, simulating a very high line frequency. The blue trace is
the 60.00 Hz output of the crystal controlled oscillator,
synchronized to the fast running line.
The synchronization circuit restarts the 60 Hz (blue)
waveform at (nominally) the zero crossover of the 61.74
Hz waveform and effectively chops off a small segment of
the oscillator’s 60 Hz waveform. This forces the 60 Hz
oscillator waveform into the same time period as the
61.74 Hz line signal, at the expense of some distortion.
Figure 1 shows the restart of the 60 Hz (blue)
Customers sometimes require that our crystal controlled digitally
synthesized 60 Hz oscillators be synchronized to the 60 Hz line.
Synchronization is essential when you need to switch a load
between two independent sources, such as the 60 Hz line and a
60 Hz oscillator (with a power amplifier). A simple relay can be
used to switch between them. However, the sources must be
synchronized. Otherwise, large potentially damaging transients
Synchronizing a 60 Hz
Oscillator to the Line
40 April 2017