Nuts and Volts - April 2017

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 waveforms.

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 ±0.5 Hz.

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 will occur.