Equipement Set-up to measure source impedance of components As shown in the 1961 catalog

One of the most time consuming parts of the process was the 803A VHF Bridge balance using a 417A VHF receiver tuned to the frequency of interest. The first improvement was to substitute these two elements of the test set into a single instrument, using a completely new concept of vector measurement. The HP 8405A Vector Voltmeter, introduced in 1966, is the bottom-right instrument shown in the picture below. It is a two-channel broadband millivoltmeter, measuring simultaneously amplitude and phase from 1 to 1000 MHz with a sensitivity of 100 microvolts full scale, and a phase resolution of 0.1°.

Thanks to the early 60s sampling technology evolution and to the HP hot-carrier diodes "in-house" development, the concept of Vector Voltmeter initiated with the 8405A evolved into a number of instruments up to the Vector Network Analyzer (V.N.A), which is still today one of the Agilent Technologies Company's leading products.

Just after the 8405A, in 1967, two new Vector Voltmeters were introduced, the HP 4800A for audio and IF frequency, and the HP 4815A dedicated to the RF-VHF spectrum. They both included the signal source necessary for the measurements in their full frequency range, thus reducing the test set shown in the above picture to a single, standalone instrument.

Cover of Application Note 77-1, introducing the "S" parameters Courtesy of the Hewlett Packard Company

Introducing the "S" Parameters

The other very important evolution brought by the Vector Voltmeter, stood in the measurement concept itself. Evaluation of an unknown impedance is now made possible relative to a standardized reference value.

Unlike the "Y" or "H" parameters, previously used to characterize two-port networks, the "S" or "Scattering" parameters were determined with input and output ports terminated in the characteristic impedance of the transmission line (Zo), a much more practical condition to obtain in RF and microwave work.

Scattering parameters quickly became the standard for characterizing linear active devices such as transistors at RF and microwave frequencies.

Furthermore, since the "S" parameters can be measured with a 50 Ohm load and are in units of reflection coefficient, they can be plotted directly on the SMITH CHART.

Early HP 8410 Network Analyzer Configuration

1967 - HP 8410A
Direct Smith Chart Display

The last steps to directly draw the curve of an impedance variation as a function of a frequency change on a Smith Chart required two more improvements. The first was a swept signal source which automaticaly varied the signal applied to the D.U.T. between the two limits of the spectrum of interest. The second was a Vector Voltmeter with added circuitry to directly display its magnitude and phase information on a CRT.

This description, impossible to make shorter, does not attempt to reduce the performance of the 8410A Network Analyzer which we are talking about. Without question the 8410A is one of the most important contributions made by HP to the evolution of microwave technology which came next. The HP 8410 would stay the standard of the industry during nearly 20 years. It would evolve from the original configuration shown in the picture above to a fully automated version, the 8409B (see chapter below). Eighteen years later, in 1985, the performance of microprocessor technology would be integrated into its successor the HP 8510A to begin another twenty-year leadership. Today, Agilent Technologies V.N.A. and P.N.A. product line still use the basic concepts that were developed for the first 8410. They have 50 years old proven performing hearts commanded by an up-to-date powerful processor brain :-)

Back to the original 8410A after this minor lapse into poetry. This instrument system, shown in the picture above being used to make a swept-impedance measurement , is based on the use of two wideband samplers to convert the input frequencies to a constant IF frequency. RF-to-IF conversion takes place entirely in the 8411A harmonic frequency converter, (see picture below) which converts input frequencies over a range of 0.11 to 12.4 GHz to 20-MHz IF signals. The phase and amplitude of the two RF input signals are maintained in the IF signals. The 8410A network analyzer mainframe provides the phase-lock circuitry to maintain the 20-MHz IF frequency while frequency is being swept, takes the ratio of the reference and test channels by use of identical AGC amplifiers, and then converts down to a second IF of 278 kHz. It also has a precision 0- to 69-dB IF attenuator with 10- and 1-dB steps for accurate IF substitution measurements of gain or attenuation.
The 8413A plug-in for the 8410A mainframe compares the amplitudes of the two IF signals and provides a meter readout of their ratio directly in dB with 0.1-dB resolution. It also compares phase in degrees over a 360° unambiguous range with 0.2° resolution on the meter. Phase difference is presented on the same meter when the appropriate function button is pressed. Separate calibrated outputs (dB/mV and deg/mV) provide direct-coupled signals for simultaneous display of phase and amplitude versus frequency on an oscilloscope or X-Y recorder. In addition, higher resolution can be obtained through the use of sensitive oscilloscopes, X-Y recorders, or other voltage-indicating devices. The 8414A polar display plug-in is used in place of the 8413A phase/gain indicator to display amplitude and phase components in polar coordinates on a cathode-ray tube. This is especially convenient for reflection coefficient measurements. CRT overlays are provided for Smith Chart readout of normalized impedance or admittance. Outputs proportional to the horizontal and vertical deflections are available for X-Y recorder Smith Chart plots for highest resolution.

(From top to Bottom) HP 8410 Network Analyzer with 8414 Polar Display Model 8743A - 2 to 12.4 GHz Reflection Transmission Test Set Model 8745A - .1 to 2 GHz S-Parameter Test Set HP 8690A Sweep Oscillator

1969 - 8745A and 8743A Test Sets

The next evolution to the 8410A took the form of dedicated Test Sets which greatly improved the ease of use and speed of measurement of the network analyzer.

Two Instruments have all necessary hardware.

To measure the four scattering parameters of a device using separate components, several changes of cabling, couplers, line length, and terminations would be needed. Connecting, disconnecting, and rearranging components would not only eat up time, but would also reduce the accuracy of the measurements. To eliminate the need for changing components, all of the newly developed components that are needed to measure S-parameters have been grouped into two precision transducer instruments.
Model 8745A S-Parameter Test Set operates over a frequency range of 0.1 to 2 GHz. Its high-frequency counterpart, Model 8743A Reflection-Transmission Test Unit, operates from 2 GHz to 12.4 GHz. Model 8745A S-Parameter Test Set is useful for measurements on microwave hardware, solid-state devices, components, and transistors. The two-port device under test is connected once to the Test Set, and the parameters to be measured and the reference port are selected simply by pushing buttons.

The HP Labs specificaly developed many passive components for these Test Sets. High-directivity directional couplers, highly repeatable coaxial switches, low-loss, low-VSWR line-stretcher, to list just a few. They were each a performance leap forward compared to the accessories available on the microwave measurement market of the time. Many of these components were also manufactured separately and sold as Original Equipment Manufacturer parts for insertion in their own production.

A collection of such accessories is shown below. The basic concept of these components was described in the 1969, January issue of the Hewlett Packard Journal.

HP 8505A Network Analyzer

The HP 8505A,
All-in-One Test System

Introduced in July 1976, the HP 8505A is the first stand-alone, all-in-one programmable Network Analyzer.

Covering the .5 to 1300 MHz frequency range it is a complete measuring system that includes a precision swept-frequency source, a three-input tracking receiver with 100 dB dynamic range, a CRT display, a frequency counter and an auto ranging voltmeter that gives direct digital readout of gain/loss, phase, deviation from linear phase, and group delay.

The July 1976 issue of the Hewlett Packard Journal was fully dedicated to this instrument. Different articles described the many new technologies developed and integrated for the first time in the 8505A. A wide-range, continuously variable line stretcher and a wide-range, high-resolution group delay detector are some of these. Many specific hybrid microwave circuits were also developed to achieve the required gain of performance and reduction of size brought by this instrument. The HP 8505A came standard, with every parameter HP-IB programmable.

HP 8754A, Network Analyzer

The HP 8754A,
Economical Network Analyzer

Introduced in October 1979, the HP 8754A took advantage of many concepts developed for the 8505A in a compact and moderately-priced instrument.

The 8754A has a built-in sweeping source with 4 to 1300 MHz coverage, a two-channel receiver that enables simultaneous swept measurements of magnitude ratio and phase angle as well as measurements of absolute power and reflection coefficient.

The 8754A is not HP-IB programmable in itself but the Application Note 294 titled "Semi-Automatic R.F. Network Measurements Using the HP 8754A Network Analyzer and the HP 9825A Desktop Computer" describes a complete solution to digitalize and process the data measured by the HP 8754A.

HP 8755, Scalar Network Analyzer in a 182T Display

The HP8755A
Frequency Response Test Set

The first instrument which could be named a Scalar Network Analyzer was the HP8755A Frequency Response Test Set introduced in the 1973 catalog and described in the November 1972 issue of the Hewlett Packard Journal.

The 8755A was designed as a plug-in to be inserted in an oscilloscope mainframe of the 180 series. The system covers a frequency range of 15 MHz to 18 GHz with three newly designed Thin-Film Hybrid HP 11664A detectors connected to its three input channels. Each of these channels is made of filters and logarithmic amplifiers to achieve a final -50 dBm sensitivity and 60 dB dynamic measuring range. Absolute power at each detector can be measured or the ratio of two detected signals can be displayed directly in dB over the full dynamic range.

The major innovation brought by the system, resided in the new design of the diode detector. Here is another illustration of the HP advance in microwave technology at that time. Thin-Film Hybrid Technology was the only way to get such a performance on such a broadband frequency range. HP outdistanced its competitors on the instrumentation market at that time so much, that some of them, offered Scalar Analyzers that used the HP 11664A Shottky Barrier Diode Detectors in their catalog.

From a Selection Guide for Automatic Microwave Network Analyzers (August 1981) Courtesy of the Hewlett Packard Company

The HP 8755 Evolution

From the very first HP 8755A introduced in 1972, the 8755 system had a continuous evolution all through the 1970s and the beginning of the 1980s.

Various configurations were proposed in the general catalog each time one or several parts of the system brought an improvement to the overall performance.

In 1975, the 8755A was still the main part of two optional systems, HP 8755L and HP 8755M, which both integrated all the necessary devices to build up a complete Scalar Network Analyzer.

In 1978, an upgraded diode detector raised the upper frequency limit of the 8755B to 26.5 GHz, and the 8755S configuration integrated a new device, the 8750A Storage-Normalizer which improved both the accuracy and the convenience of the scalar measurement system.

During the same period, the HP desktop computers were evolving fast. By the beginning of the 1980s, a new, small and powerful controller, the HP85, associated with a new generation of programmable sweepers, the HP8350A, resulted in the introduction of the first Automatic Scalar Network Analyzer, the HP 8755P, shown in the picture on the left.