The Lineage Of The Paramp
The comments regarding the parametric amplifier (or "paramp") \["Forty Years Ago," June 26, p. 66\] brought back fond memories indeed. Use of the paramp (with a gallium-arsenide Gunn Effect diode pump) as a low-noise radar receiver front end became very popular.
When teaching a graduate-level course in microwave circuits and components, I always devoted a special session to the paramp, during which the defining equation was derived and the operation of the device was explained. I would add to Steve Scrupski's "formidable challenge to understanding" by pointing out that the theoretical basis of paramp operations requires the use of an arcane expression known as Mathieu's equation, generally used (as is Hill's equation) in the study of wave propagation in periodic structures (in conjunction with Floquet's theorem).
It's interesting to note that the parametric amplifier (realized as a mechanically resonant system) was described by Faraday in 1831 and by Lord Rayleigh in 1883. Thus, its distinguished classical lineage is well established.
Randolph A. Kallas
For Those Of Us Still Using Print
In the June 12 issue \[p. 140\], I discovered information regarding a web-only Idea For Design (IFD). I'm sure that I'm not the only person who reads the magazine away from a computer terminal. I might wish to take my copy out to the picnic tables on sunny days at lunchtime, or read it in bed in the evening, or on an airplane, or in the little room with no telephone or computer. There's something to be said for getting away from the ceaseless drone of the hard drive and the flicker of the fluorescent lights to do a little thoughtful reading. A web-only IFD is useless under those circumstances.
In my view, "features" such as a web-only IFD represent negative improvements to your publication. I take no issue with posting listings on the web—this reduces the chance of typos should I wish to use that particular idea. However, pulling some IFDs in their entirety from the printed page and moving them to the web is an annoyance. I fail to understand what problem has been solved in creating this new class of IFDs accessible only by electronic means. I hope you reconsider this decision and run all IFDs in the print version as well as on the web. Those of us who do occasionally enjoy life away from a computer monitor will thank you.
Analog Devices Inc.
The web-only IFD that your comments address was deemed too large for the 1- to 1.5-page guideline we use for the IFD section. Please take some consolation in the thought that in earlier days, when web-only content wasn't available, you wouldn't have had an opportunity to see this idea at all. Our web site, www.elecdesign.com, allows us to offer readers a larger and more varied array of content than ever before.—Jim Boyd, IFD Editor
Simplicity Is Better
The editorial in the May 29 issue \["Can Computers Be Made As Easy To Use As Appliances?" p. 46\] was right on the mark. I'm a physicist who has been working with computers for many years. I still have trouble accessing certain files and sites that I find on the web. I think more standardization is needed in the industry. Versatility is fine, but simplicity is better. Compatibility is a big concern of mine, especially for storage media. My old PC at work was recently replaced. I didn't realize until a few weeks went by that the new one cannot accept the older 7-in. floppy disks. Now I have to find another old PC to transfer the files to a newer diskette. Too much time is spent with housekeeping procedures.
Crafting A Noncomplex Circuit
"Build A Vector Network Analyzer" \[Ideas For Design, May 29, p. 107\] is a catchy title, but not a particularly useful circuit. First of all, travelling waves (incident and reflected) are an interpretation of the spatial altering voltages or currents in an extended transmission medium. Therefore, they aren't applicable to a discrete circuit, like the one presented by Dunbar. Admittedly, the voltage ratio VREF/VINC happens to formally be equal to the reflection factor of a transmission line with characteristic impedance RO terminated by an impedance ZDUT, namely:
which can be solved for ZDUT. But, a much simpler circuit consists of just a single resistor R1 in series with the unknown impedance Z connected to the signal generator, and the measurement of the voltages V1 (across the series resistor) and VZ (across the device under test), from which:
is most easily obtained. In particular, the magnitude of Z can be determined from the voltage magnitudes without any phase angle measurements. So, an ac voltmeter may be used instead of an oscilloscope that has inherently poorer accuracy. This is in contrast to Dunbar's method, which requires the measurement of two complex voltages and the solution of a complex equation, even if only the magnitude of the unknown impedance is desired. This approach can be extended by the additional measurement of the generator output voltage amplitude, to the classic "3-voltmeter method," that delivers the real and imaginary parts of Z from the three voltages by a simple calculation.
Army Research Lab