Ask Harlan, January 22, 2007

From: Alessandro Chianetta, STM

Dear Harlan,
Could you explain in easy words what dual carrier modulation (DCM) consists of?

Dear Alessandro,

I am not sure how easy these words are, since it is not an easy concept. DCM is used in OFDM-UWB (Orthogonal Frequency Division Multiplexing) (Ultra-wideband) systems. It allows mapping the same databits into different constellation points from different constellation diagrams. The function associates two code data elements and transforms those elements into a format for transmission on two separate signal tones.

From: Richard Mayo, ATRAD P/L

Dear Harlan,
ATRAD operates in the rather "boutique" RF domain of high power pulse radars (10 to 100 kW at up to 10 percent duty) in the VHF (30 to 50 MHz) and MF (2 to 3 MHz) bands. These radars are used for atmospheric research in the Mesosphere-Stratosphere-Troposphere (MST) regions. We often use phased yagi arrays for our antenna systems and hence meet the problem of inter-antenna coupling, which means our power amplifiers having to deal with the resultant poor VSWR. An isolator would be of great benefit to us but I have always assumed that ferrite-based isolators were only practical at higher frequencies than the ones we use. I have recently heard suggestions that it is possible to build low band VHF isolators but have not been able to find anything on the web. Can you suggest anything?

Dear Richard,

VHF circulator/isolators have been around for quite a while. They are generally called "Gyrators" and require lumped element tuning structures at each port for the given frequency band. Try contacting one or more of the major ferrite isolator companies listed in our buyer's guide with your specific requirements. It may have to be a custom design.

From: Henderson Neves de Lima, TDCC

Dear Harlan,
There is a Petrochemical Process that does not allow heating fluids with steam, due to incompatibility with water, despite the appropriate heater exchanger equipment. I am looking for a microwave system to heat process fluid without contact with the fluid. Is there some development in this area? If the answer is no, I believe that it is an area of opportunity. How can I move forward to develop a model for this system? I work within the Petrochemical industry.

Dear Henderson,

It should be possible to heat some other liquid besides water in a heat exchanger. However, I am not aware of any system that does so. The organization that specializes in microwave heating and industrial processing is the International Microwave Power Institute (IMPI). They can be contacted at www.impi.org and may be able to help you.

From: Maurizio Brignoli, Telewire Italy

Dear Harlan,
Why is there a difference when I design a filter (HPF or LPF) with EDA (MWO, free software, etc.) comparing sample prototype PCB using a lumped component (R,L,C)? Typically, the filter prototype is cut before simulation. Why is there a difference when comparing prototype and simulation?

Dear Maurizio,

Although simulation software is better than ever and keeps improving, the structures or elements that are actually used in the prototype construction are not perfect, particularly for high Q circuits where small differences in available Q will have a significant effect. This is one of the reasons that Microwave Journal requires measured data in the papers that we publish.

From: Johana Yan

Dear Harlan,
Why 50 ohms?

Dear Johana,

The use of 50 ohms as the industry standard for characteristic impedance is a compromise between 44 ohms, which is the Zo for the highest power capability in coaxial air lines, and 93 ohms, which provides the lowest attenuation in coaxial air lines. For a teflon-filled line the lowest attenuation is at 64 ohms. There are a number of variations to these figures due to other factors; however, 50 ohms was established many years ago and it has remained the standard.

From: Mustafa Karakoc, Kocaeli University

Dear Harlan,
I am PhD student at the University of Kocaeli. I have some questions regarding multi-user downlink strategies. Could you please help me? My questions are as follows: 1.) What is the numerical value of downlink power of users that have a different downlink data bit rate? For example, I have a user with a data rate of 9.6 kb/s (basic rate or R), and another user with a data rate of 38.4 kb/s (for times more than basic rate or 4R). How do I choose the numerical downlink power values? I do not think it is proportional. Do you know if there are any sources that give numerical values for users with different data bit rate?

2.) In downlink at the mobile user, what is the Eb/NoIo threshold value of data for the user with different data bit rates? For example, there is a user with a data bit rate request of 9.6 kb/s (basic rate or R). It is clear that the user is not only receiving its signal, but multipath signals related to the desired signal are also getting another user's signals. After downlink analysis, the signal's BER level has to compare with reference predefined threshold level. For example, S=Eb / (No+Io) level is 5.5 dB for 9.6 kb/s 3.6 dB for 38.4 kb/s. Is there more predefined threshold levels for different data bit rates? For example, what are the data bit rates for 76.8 kb/s or 28.8 kb/s?

Dear Mustafa,

Your question is a communications systems question that is outside my area of expertise, so I cannot answer you directly. However, I can recommend several texts, which may be helpful:

Digital Techniques for Wideband Receivers, J. Tsui, Artech House Inc., 2001, ISBN#1-58053-299-3;

Advanced Techniques for Digital Receivers, P. Page, Artech House Inc., 2000, ISBN#1-58053-053-2;