GUEST CONTRIBUTION BY PROF. ULRICH L. ROHDE

Since 1933, Rohde & Schwarz has been known for precision measurement technology, where frequency and time are crucial physical quantities that have always been of paramount importance. Utilizing a standard frequency, for instance, enables a highly accurate determination of time. 1000 oscillations per second, or as named after physicist Heinrich Hertz, 1000 Hz, translates to a reciprocal of 1 millisecond.

The task of developing such a time standard was important to my father. When the first oscillators based on piezoelectricity were invented in 1938, the idea of using them as frequency standards became apparent. The initial stable quartz oscillators operated at 100 kHz and had what we now call a CT-cut. In modern quartz oscillators, we find the stress-compensated cut, known as SC. The actual oscillator comprised a 100 kHz section and a robust 1 kHz tuning fork. The signal, multiplied a hundredfold (harmonics of the distorted oscillation) from this fork, was then compared and phase-locked to the 100 kHz signal. An output from the 100 kHz stage was then directly fed into the clockwork.

On July 7, 1938, my father applied for the German Reich Patent 691 848 'for a frequency standard with a piezoelectrically controlled oscillator,' which was granted on May 7, 1940.

**Two examples of how crucial high time accuracy is are as follows:**

After the invention of the cesium atomic resonance frequency/time standard, Einstein's statement regarding time (Theory of Relativity) was put to the test. Two identical atomic clocks were used—one remained stationary while the other was taken on a journey aboard an airplane. Upon its return, the time readings of both clocks were compared, revealing a noticeable difference. This difference was explained as follows:

*That's tied to Einstein's Theory of Relativity, which states that time passes more slowly for objects in motion compared to those at rest. Similarly, the closer one is to a massive object like Earth, the slower time passes. Hence, for individuals in space, time actually passes around 1.0000000007 times faster than it does for someone on Earth.*

Another example is the precise determination of the Earth's acceleration due to gravity, 'g,' from the measured falling velocity, 'v.' The falling velocity is determined as follows:

**v = 0.5 x g x t²**

From the extremely precise measurement of velocity using an electronic stopwatch, the value of Earth's acceleration can be calculated:

**g = 9.81 m/s²**

In both cases, an extremely accurate time standard is required.

The image above depicts the setup, which, compared to other frequency standards, was, in the broadest sense, portable. Besides its high frequency accuracy, this was the advantage of the R&S system.

The intriguing aspect of this frequency standard is its utilization of a feedback loop, enabling the combination of oscillators with excellent short-term stability (for high signal quality) and those with superior long-term stability (for precise timekeeping). Back then, constructing quartzes that weren't yet in a high vacuum and had low susceptibility to microphonics wasn't feasible, so the short-term stability was guaranteed by the aforementioned tuning fork.

In modern frequency and time standards, the reference roles have switched. The XSRM frequency/time standard, first introduced by R&S around 1970, employed a low-noise 5 MHz quartz oscillator utilizing the long-term stability of rubidium gas resonance while relying on quartz for short-term stability and thereby signal quality.

Today, for instance, highly accurate signals stabilized by cesium and/or hydrogen masers are used in GPS satellites.

*U. L. Rohde, 'Mathematical Analysis and Design of an Ultra Stable Low Noise 100 MHz Crystal Oscillator with Differential Limiter and Its Possibilities in Frequency Standards,' 32nd Annual Symposium on Frequency Control, Atlantic City, NJ, USA, 1978, pp. 409-425, doi: 10.1109/FREQ. 1978.200269.*

First publication in the Rohde & Schwarz magazine 'Inside,' November 2023.

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