1972 , Volume , Issue Nov-1972

It makes swept insertion and return loss measurements simultaneously, overa wide frequency range. TO THE MICROWAVE ENGINEER, an instru ment that measures insertion and return loss simultaneously, over a broad band, would be as useful as is the voltmeter to other electronic special ists, provided it is made practical in several im portant ways. • Its measurements must really refer to the micro wave measuring port, not just to the output of some RF converter. • Stability and repeatability of the measurements should be so good the meter can properly be called driftless. • Broad frequency range is necessary, but it must be accompanied all the way by adequate accu racy and dynamic range, yet drive requirements should be low. • Compactness is desirable, and price must be in line. The effort to achieve all those desiderata re sulted in a new instrument (Fig. 1) that makes sev eral contributions. The block diagram (Fig. 2) shows how elements of the new Model 8755 Frequency-Response Measuring System work together. The swept RF output of the microwave source is simply on-off amplitude-modulated at a 30-kHz rate by the Modulator, before it is applied through the direc tional detector to the device to be tested. Three identical Schottky-barrier diode detectors sense the incident, reflected, and transmitted RF waves at the network ports and convert each into a 30-kHz squarewave by envelope detection. The amplitude of the squarewave will be shown to be a measure of the microwave signal at the various ports. It is this low-frequency signal that is processed to dis play insertion loss or return loss in dB/div on the CRT of the oscilloscope into which the 8755A Ana lyzer is plugged. The 'scope may be any of the HP 180 series. Preliminary work showed that only a biased Schottky-barrier diode could yield enough signal-to-noise ratio, at †" 50 dBm, across the bandwidth needed in the planned swept displays. Desirable as this high sensitivity is, the biased diode scheme Cover: Simultaneous swept-1requency display of insertion and return losses speeds up adjustments for optimum performance in a tested device, one of the measurement conveniences provided by HP's new "volt meter for the microwave en gineer," described in the article beginning on this page. Working in any 180-series Oscilloscope, this new plug-in presents two amplitude-re sponse characteristics of unknowns over a fre quency range that extends up to 18 GHz.