Early network analyzers

 In 1950, HP produced the 805A, a coaxial bridge operating at VHF (30MHz~300MHz), which for the first time provided a visual readout of both impedance magnitude and phase angle.

HP 803A VHF Bridge

The 803A uses the Byrne-bridge principle to control two detectors at the same time, one for sampling the electric field and the other for sampling the magnetic field. Each detector is connected to the ground through a resistor. The voltage is applied to both ends of the slotted-line through the resistor. The impedance amplitude is determined by the position of the two detectors. When the voltage through the resistor is adjusted to be equal, there is a point in the slotted-line where the voltage cancels each other. The position of the slotted line at this cancellation point is the impedance phase angle, which is detected by a movable detector.

The 803A VHF Bridge requires an external RF signal source and a sensitive VHF receiver to measure impedance magnitude and phase angle in the range of 55 to 500 MC. Impedance is read directly in ohms and the phase angle is displayed directly in 100 MC. It is easy to calculate the phase angle at other frequencies. The impedance range is 2 to 2000 ohms.

Required auxiliary equipment:

Operation of the 803A requires an external signal source and external detector.

Signal source: The RF signal source must have AM function and output at least 1mW. HP VHF signal generator can meet the requirements. The frequency range of 608B is 10MC~400MC, and the frequency range of 612A is 450MC~500MC.

Receiver: The detector used with the bridge as a receiver must have a sensitivity of at least 90dB below the minimum input signal (1mW). The 417A VHF detector is suitable for this purpose.

The first network analyzer. From left to right: 608B VHF signal source, 803A VHF bridge, 417A VHF receiver

Receiver meter, an instrument designed by Boonton Radio

Another interesting instrument that makes the list of early network analyzers is the 250 Rx meter. This is important for two reasons: From a technical perspective, it was another early way to calculate the real and imaginary parts of a component. From a business perspective, the 250 Rx meter was not an original HP design, but one of the earliest examples of HP buying other companies in the late 1950s as the company grew. HP bought the Boonton Radio Company (BRC) in 1959, a company well-respected for designing a number of excellent RF instruments, of which the 250A Rx meter was one. HP sold many of BRC's instruments, and even had several (202J, 202H, 207H, 250A) on sale by the time HP's own models came out in the late 1960s. (The 250B was a redesigned and upgraded instrument from HP in 1968.)

Left, 250A, right 250B

Before the "S" parameter

Ten years after the transistor was born, an interesting accessory was produced for use with the 250A Rx meter, one of the very early tools for characterizing transistor parameters. The accessory included an adapter for biasing filters for external power supplies and 3 separate plug-in boards for measuring Yoe, Yib, and Yie.

Yoe = Output Enable, common emitter configuration with input circuit shorted

Yib = Common base configuration with input enabled and output circuit shorted

Yie = Common emitter configuration with input enabled and output circuit shorted

This solution was sufficient for transistor measurements until the late 1950s. The performance and usable frequency range of transistors during this period was far below the upper frequency limit of 250MHz of the Rx table. Above 100MHz, the Y parameters become difficult to measure because it is difficult to obtain a good short circuit, and opens and short circuits often cause the device to oscillate.

The increasing performance of transistors in the early 1960s led to the industry using fine differential parameters, "S" parameters or scattering parameters, which were easily measured above 100MHz. With the introduction of the 1GHz vector voltmeter 8405A by HP in 1966, the "S" parameters became the industry standard.