Many of the earliest measurement functions, testing for amplitude, noise, and phase at high frequencies, are still performed, but now benefit from embedded computers and software.
Test equipment in recent years has taken advantage of embedded microprocessors, digital signal processing (DSP), and software to provide unprecedented accuracy and ease of use for RF/microwave engineers. But, in locating back over the years to commemorate Microwaves & RF's 50 years of service to this industry, such was not always the case. Companies have changed drastically over those five decades, as have the test and measurement instruments they offer. In this first of a series of historical looks backward at microwave products and technologies, the topic is test equipment and these images may spark memories for those in this industry for a while.
This industry's historians are well aware of how some of the names have changed. In test and measurement, a number of the pioneers are still in operation, some with the same name, such as Tektronix, and some who have branched off from their parent company, such as Agilent Technologies has done from its beginnings as Hewlett-Packard Company. Other early innovators, such as Wiltron Company, have been acquired (by Anritsu), while many early equipment developers, including Pacific Measurements, Marconi Instruments, Systron Donner, and The Singer Manufacturing Company, are no more. A more comprehensive lineage of this industry's companies will be included in the special Microwaves & RF 50th Anniversary issue in November.
In 1965, Hewlett-Packard Co. was offering a line of sweep generators with frequency coverage from 1 to 40 GHz, including the model 691B with at least 60 mW output power from 1 to 2 GHz and model 697A with at least 5 mW output power from 26.5 to 40.0 GHz. These sweepers featured PIN diode leveling and modulation, and delivered swept concurrent voltages to drive an x-y recorder or oscilloscope.
Around that same time, the DeMournay-Bonardi Division of Datapulse, Inc. offered their model 3000 Precision Insertion Loss Measurement System (PILMS) for measuring signal attenuation with 0.001 dB accuracy (Fig. 1). It provided a dynamic range of 20 dB and its frequency range could be extended to 90 GHz with external mixers. The PILMS incorporated dual-channel, audio-bridge techniques, a ratio transformer, and low-noise cable. It essentially compares audio-modulated RF signals from an external signal generator to return demodulated signals from a device or system under test. The following year, Microlab/FXR of Livingston, NJ introduced a VSWR meter that offered a total measurement range of 70 dB that could be switched in 5-dB steps. It operated over 20-, 40-, and 400-Hz bandwidths with a noise figure of 5 dB. It measured reflection coefficients and relative attenuation.
Also in 1965, Kay Electric Company of Pine Brook, NJ provided noise-figure measurements on their model 792-AFE automatic noise-figure meter. Working with a noise source, the basic model provided noise measurements from 10 to 900 MHz but could be extended to 26.5 GHz with external source. The instrument showed noise measurements on a large 3-in. analog meter (Fig. 2). For generating signals, the Boonton Division of Hewlett-Packard Co. introduced their model 3200A oscillator with accuracy of 2 percent from 10 to 500 MHz. The tunable oscillator delivered more than 25 mW output power. Meanwhile, Tektronix introduced a plug-in module designed to modify the firm's model 530, 540, and 550 oscilloscopes to sampling operation. With vertical sensitivity of 2 to 200 mV/div, the module worked with internal triggering and featured a risetime of 0.35 ns.
A major development in 1967 was the introduction of a "digital" power meter from Pacific Measurements of Palo Alto, CA. Specified for frequencies from 10 Mc to 12.4 Gc, they employed a crystal diode detector to measure power from -40 to +10 dBm with 0.1 dB resolution. In reality, the power meter (Fig. 3) used a digital readout to show measured power levels with accuracy of 5 percent. There was little digital about the measurement channel, which included the diode detector, an input amplifier, compensation amplifier, calibrator (which provided precise 30-MHz signals at levels of 10 W and 1 mW), and output summing amplifier. As an option, it could provide binary-codeddecimal (BCD) digital output signals, and the meter could be triggered by an external computer to perform as many as 1000 readings per second.
That same year, Hewlett-Packard changed the way that microwave engineers would characterize their components, with the introduction of the HP 8540 series vector network analyzers (VNAs). The HP 8542B (Fig. 4), for example, provided a frequency range of 110 MHz to 12.4 GHz and allowed accurate measurements of amplitude and phase. The measurement system, which sold for about $250,000, included a 16-b Hewlett- Packard computer. The first system was sold to Microwave Associates in Burlington, MA, which would later become known as M/A-COM.
In terms of spectrum analysis, one of the leaders at the time was Polarad Electronic Instruments of Long Island City, NY with their model 2400 spectrum analyzer (Fig. 5). It used the display screen of an oscilloscope to offer a calibrated 70- dB dynamic range from 10 MHz to 12.4 GHz. The spectrum analyzer performed frequency sweeps from 10 kHz to 4 GHz and achieved -105 dBm sensitivity. Other suppliers of spectrum analyzers at the time included Ailtech (Farmingdale, NY), Hewlett-Packard Co. (with their model HP 8566A covering signal levels from -134 to +30 dBm and frequencies from 100 Hz to 22 GHz), and Tektronix (with their model 7L18 spectrum analyzer capable of covering 1.5 to 60.0 GHz).
With time, refinements improved many of the basic instrument functions. In 1980, Micro-Tel Corp. of Baltimore, MD offered their model 1290 receiver for automatic insertion-loss measurements from 10 MHz to 18 GHz. It was among the many measurement instruments that made use of external computers by means of the IEEE-488 General Purpose Interface Bus (GPIB). During that same time period, Rohde & Schwarz of Munich, Germany offered their SWOB 5, essentially a scalar network analyzer (SNA) with 76-dB dynamic range from 100 kHz to 1000 MHz. Companies such as Wiltron and Hewlett- Packard also offered SNAs, while Pacific Measurements, with their model 1038- N10, had a 76-dB dynamic range from -60 to +16 dBm at microwave frequencies. It showed plots of amplitude versus frequency with a unique light-emittingdiode (LED) display and could be automated via external GPIB computer.
At that time companies such as John Fluke Manufacturing Company were selling synthesized signal generators with phase-locked stability. Fluke's model 6070A, for example, spanned 200 kHz to 520 MHz while the model 6071A extended that frequency range to 1040 MHz. It was a company associated with electronics for aircraft systems, the Electron Dynamics Division of Hughes Aircraft Co. of Torrance, CA, that would pave the way for practical millimeter-wave test signals with their 4785xH series of frequency synthesizers (Fig. 6). Capable of generating phase-locked signals from 26.5 to 110.0 GHz, these microprocessor-controlled test signal sources could generate at least 1 mW output power through the highest frequencies.