Voltage-controlled oscillators (VCOs) are usually designed to operate in an ideal 50-Ω environment. These oscillators often have to drive a much different load, however. When the VCO output encounters a load with 12-dB return loss for all possible phase angles, it generally suffers frequency variations that must then be measured. When this measurement is performed manually, it is quite time consuming. In a two-page white paper titled, "Line Stretchers Ease VCO Load-Pull Testing," Mini-Circuits (Brooklyn, NY) presents a way to perform these tests quickly and automatically.
At the heart of this brief but detailed application note is an electronic line stretcher. To explain its significance, the note first examines the conventional method of performing load-pull tests on a VCO. Because of the shortcomings of conventional electronic phase shifters and mechanical line stretchers, this approach can be quite tedious while consuming several hours of an engineer's time.
As an alternative, the company's engineers developed a line of electronic line stretchers with full 360-deg. phase shifts across wide frequency ranges. For example, the first three models boast an octave or more of frequency range over a total range of 110 to 1300 MHz. The three-port devices have a nominal return loss of 10 to 12 dB. In an example provided in the application note, the output frequency is shown as a function of the electronic-line-stretcher phase shift. The peak-to-peak difference in the displayed frequency curves provides the load pull. This application note succeeds in presenting a new and automated alternative for VCO load-pull measurements.
Mini-Circuits, P.O. Box 350166, Brooklyn, NY 11235-0003; (718) 934-4500, FAX: (718) 332-4661, Internet: www.minicircuits.com