The First Commercial Application of Frequency Synthesis

The first HP 5100A Synthesizer was introduced in the 1963 catalog. It was a DC to 50 MHz signal source providing a highly stable and pure signal derived from an internal 1 MHz quartz oscillator or from an external frequency standard. The digital frequency selection was made in increments as fine as 0.01 Hertz by front panel pushbutton or by remote switch closure.

The basic principle that conducts the 5100 development was the following: "The sources that provide the highest frequency stability are single frequency sources, commonly called "Frequency Standards." Having these high quality standards to hand, it is natural to look for some method to translate their stability to other desired frequencies. This translation when the operation is something more than a single arithmetic operation, is commonly know as frequency synthesis. Hence, a variable frequency synthesizer is an instrument that translates the frequency stability of a single frequency standard, to any one of many other possible frequencies, usually over a broad spectrum."

The 5100A was of the "Direct Synthesis" type which simply performs a series of arithmetic operations on the signal from the frequency standard to achieve the desired output frequency. The direct synthesis approach has the pronounced advantages of permitting fine resolution and fast switching (lower than 1 millisecond for any frequency change), as well as an extremely clean output signal. On the same principle, an UHF version of the same instrument, the 5105A (shown in the photo above) would be introduced in 1967 to cover the 0.1 to 500 MHz spectrum with a 0.1 Hertz resolution.

More information about the Model 5100 Series Synthesizer can be found in the NEWS chapter of this web site. A complete article dedicated to this instrument was added on March 30, 2007, just after a fully functional unit was found on the French second (perhaps third or fourth :) hand market and added to the HP Memory collection. More recently, in 2014, a Model 5103A was also added to the collection. Related HP Journal articles and Application Note N° 96 are also available from that link.

HP's First Large Team Project

Important comments about the 5100A Synthesizer project, are given by Bill Hewlett in the introduction of his book: "Inventions of Opportunity" edited in 1983:

"The development of the synthesizer was of additional interest in that it marked a change in HP's product-development procedure. Most prior developments were basically a one-person, or at most a small-team, short-term effort. In contrast, development of the frequency synthesizer was distinctly a team effort, requiring almost 40-man-years of engineering spread over a period of about three years—a major commitment to a new technology. This effort resulted in an instrument that provided almost unheard of resolution, and had an added advantage in that it was electrically programmable—an important feature in light of the rapidly developing field of automated testing."

Complexity of the 5100A synthesizer described by Bill Hewlett, is clearly illustrated by looking inside the instrument.

Barney Oliver Viewpoint About Synthesizers Future Evolution,
From the Hewlett Packard Journal, May 1964 edition

"About twelve years ago the digital frequency counter made its commercial debut. It revolutionized the art of frequency measurement; and a tedious, complex process of multiple heterodyning was reduced to a simple act of plugging in the signal and reading the answer. The tremendous time saving more than justified the cost of the counter and these instruments have become ubiquitous. They are found today in applications undreamed of a decade ago.
With the advent of the digital frequency synthesizer I believe we face an analogous situation. Here we have a single instrument that can generate any frequency over a range far greater than most oscillators or signal generators — a frequency known to quartz crystal accuracy of a part in 10E10 or better. This one instrument can do the work of a whole battery of general and special-purpose generators and do it better and more conveniently. The initial model has been engineered to the most exacting specifications and construction to meet the need for a high-performance instrument. Later lower-priced models will provide high performance over a reduced frequency range.
It will be very interesting to watch the applications develop for frequency synthesizers. Almost certainly they will become as common as counters. Almost certainly there are applications in your own work where a synthesizer would save time and money and do a better job."

Another Leap Forward in Production Testing

Thanks to the frequency synthesizer, testing frequency sensitive devices could now be done automatically, rapidly, with great precision and with a spectrally pure stimulus. As an example of one of many applications made possible by a synthesizer's remote programming capability, the plot below shows the response of a single side band crystal filter. Less than one minute was required for the complete plot.

First Synthesized "Signal Generator"

Before year 1973 there had been two distinct types of signal sources in the market place, each fulfilling a specific range of applications. One of these is the familiar signal generator (e.g. HP 606, 608, 612, etc.), and the other the synthesizer (e.g. HP 5100, 5105, etc.). In many cases both are necessary even in the same test set-up. Hewlett-Packard's 8660A/B Synthesized Signal Generator family is a new solution to this hybrid application. It combines the stability, programmability and frequency resolution of a synthesizer with the modulation and voltage calibration of a high-quality signal generator. This compact, all solid-state system has the versatility of plug-in modulation and RF sections and features directly computer-compatible programming of frequency, voltage level, and modulation.

To accomplish the synthesized aspect of this generator's performance, the indirect synthesis approach has been used. There are seven L-C tuned oscillators, each phase locked for stability and accuracy. A crystal oscillator at 10 MHz is the ultimate source, and all internal signals are referenced to it. Picture below outlines the 8660 family. There are two mainframes, each fulfilling separate requirements. The 8660A with a minimum of front panel controls is suited for systems applications; the 8660B with many new front panel features is a powerful bench instrument. Besides these different mainframes and plug-ins, there are other options available to narrow the choice of instrument further toward a specific application.

Close-Up View of the Technology Evolution

The two photos shown below are the top and bottom inside view of the 8672A Microwave Synthesizer . Comparing them to the 5100A inside views shown at the top of this page gives an immediate feeling of what the technology evolution was during the golden years of the 60s and 70s . Less than 15 years separate the concepts of these two instruments.