Ask Harlan, November 27, 2006

From: Mike O’Driscoll, student

Dear Harlan,
With regards to RF mixers, how would you define the sensitivity of a mixer? Is it possible to measure sensitivity?

Dear Mike,

I have always regarded sensitivity as the signal level needed for a minimum detectable signal above the noise level. It is a function of noise temperature of the nonlinear element, conversion loss and bandwidth. The method of measurement is to increase the input signal level until an output signal appears above noise. There is an excellent discussion of all the sensitivity factors in Microwave Mixers, S. Maas, Artech House Inc., 1993, ISBN#0-89006-605-1.

From: Kamal Jeet, ISRO

Dear Harlan,
Does the change in resistivity of a substrate have any role/affect on the dielectric loss (tan (delta)), for example, building circuits on low and high resistivity substrate?

Dear Kamal,

Yes, the resistivity will affect the dielectric loss. The higher the resistivity the lower the loss.

From: Rahul Lal, engineering student

Dear Harlan,
I need simplified block diagrams of digital microwave radio systems. Could you provide this with a simple description?

Dear Rahul,

I cannot put block diagrams in this column and there is no universal simplified answer; however, there are a number of block diagrams for different types of systems in Advanced Techniques for Digital Receivers, P. Page, Artech House Inc., 2000, ISBN#1-58053-053-2.

From: Eduard Pfaeffli, Ruag Electronics

Dear Harlan,
What is going on with project H.A.A.R.P.? Web site contributions are pretty scary, especially when considering weather-manipulation, which is intended as a mighty weapon without known potential counter-measures. How much energy ERP is it capable to emit to the ionosphere at the moment?

Dear Eduard,

I am not familiar with project H.A.A.R.P.; however, power levels in the many Megawatt range have been available for years and Terawatt levels have been demonstrated for short pulses.

From: Peter Thigpen, University of Connecticut

Dear Harlan,
I am an MBA student at the University of Connecticut, currently taking a course entitled "Managing Emerging Technologies." I have selected (or been assigned) Extremely High Frequency (EHF) Radio Waves as a term paper topic. The paper is designed to be a brief (eight pages or less), non-technical overview. For instance, the goal might be to enlighten my company about the capabilities and possible applications of EHFs in a commercial environment. I was wondering if you can point me in the direction of available publications I can use as references.

Dear Peter,

EHF frequencies (30 to 300 GHz) are being used for high resolution mapping, remote sensing, satellite-to-satellite communications, covert battlefield radios, police speed radars, radio astronomy, wireless LANs and a number of other applications. The August 2005 and July 2006 issues of Microwave Journal have papers on some of these applications. You can access them on our web site at www.mwjournal.com Click on archives.

From: Gholamhossein Heidari Roodaki, Tehran University

Dear Harlan,
Could you please send me the following articles: "100:1 Bandwidth Balun Transformer," IEEE Proceeding 1965, J.W. Duncan; "Transmission Line Conversion," Marchand. I cannot find them.

Dear Gholamhossein,

I do not have access to old IEEE papers. You will have to contact the IEEE directly. However, the Marchand paper has been reprinted in Classic Works in RF Engineering, Walker, et al., Artech House, 2006, ISBN#1-58053-056-7.

From: Kamal Singh, Semiconductor Laboratory

Dear Harlan,
How does the leakage current affect the performance of RF circuits in a low and high resistivity silicon substrate? Is there any method to change the resistivity of the wafer without any degradation in the RF performance?

Dear Kamal,

Leakage current will affect both circuit losses and circuit isolation and cross talk. I am not a semiconductor chemist, but I believe that resistivity can be altered by appropriate doping; however, it will certainly affect the RF performance.

From: Suram Verma, SLIET

Dear Harlan,
Going with recent efforts of wireless power transmission, do you think that it will be possible in the near future to transfer power using microwaves just like information transmission everywhere?

Dear Suram,

It is possible to transfer power using microwaves now. There have been a number of examples of power transfer such as power transfer in space to rectennas and even some experimental transfer to helicopters. The pioneer in this work was the late Bill Brown at Raytheon Co. There are many of his papers included in the literature.