Ask Harlan, June 7, 2006

From: Devendra Chitguppa, BEL Bangalore

What is the relation between CW (unmodulated carrier power) and modulated carrier power in the case of BPSK, QPSK, 16QAM, 32QAM, 64QAM, 128QAM and 256QAM?

Dear Devendra,
The theoretical modulation losses for various schemes are a function not only of the modulation type but also bandwidth and allowable BER. They vary from 10 to 30 dB plus an implementation loss of about 2 to 5 dB. There is a good discussion of this along with a table of theoretical losses for 22 different modulation types in chapter three of Radio System Design for Telecommunications, R. Freeman, Wiley, 1997, ISBN#0-471-16260-4.

From: Jean Almira, Eudyna Devices Asia

My inquiry is regarding balun design. In the papers I've read, they often mention power dividers/combiners at the same time. However, I'm not sure what the relation is between a balun and power divider. How does one relate to both issues? What are the similarities or differences between the two?

Dear Jean,
A power divider/combiner provides that function in a uniform line. A balun is a transformer from a balanced to an unbalanced transmission line and frequently divides or combines power in the process. There is a good discussion of baluns in Chapter 3 of Classic Works in RF Engineering, Walker, et al., Artech House, 2006, ISBN#1-58053-056-7.

From: Maria Siddiqua

I am desiging a passive crossover using ADS software. I did it by cascading two 90 degree hybrids. According to theory, the signal applied at port 1 should appear diagonally at the output port, but I am not sure about the phase. Should there be any phase difference between i/p and o/p? What should the phase difference be between the o/p ports?

Dear Maria,
The output arms of a 90 degree hybrid have a relative phase relationship of 90 degrees. However, there is an absolute phase shift in your crossover, which is the sum of the hybrids plus any interconnecting line lengths. For your crossover configuration, the output phase relationship of the two outputs will be the same as the relationship of the two inputs, since the phase shift for each signal through the crossover is the same, assuming they are at the same frequency.

From: Ramaraju Biruduraju, HELA

We have been trying to realize an SP6T switch using one SPDT and two SP3T MMIC switches in the frequency range from 6 to 18 GHz. The combined insertion loss is coming out to be about 6 dB, which is very expensive for us. Could you suggest a direct SP6T switch in a MMIC version, if available?

Dear Ramaraju,
The best approach is to use a single pole, six throw, series junction with follow-up diodes in each arm to provide the needed isolation. Such switches are commercially available as MIC structures with insertion losses typically around 2 dB. I don't know of any commercial MMIC product. If you must use MMIC, you may have to have a custom design made for you. We don't recommend specific manufacturers; however, you can check our buyer's guide under switches and MMICs.

From: Gao Wei, the University of Electronics and Technology of China

What kind of antenna should I utilize for a unit of array operating at better than 35 GHz? Could you direct me to any references regarding this topic?

Dear Geo,
Since you haven't specified the purpose or characteristics of your array, I can't give you a specific answer. I suspect that it is a commercial application that will require a low cost array such as microstrip. There are a number of appropriate microstrip elements. A good reference is Microstrip Antenna Design Handbook, Garg, et al., Artech House Inc., 2001, ISBN#0-89006-513-6. A good general text is Modern Antenna Design, Milligan, Wiley, 2005, ISBN#10-471-45776-0.

From: Manoj Das, Student

How does the driver for operating the PIN diode switch work?

Dear Manoj,
A PIN diode switch works by having either a forward DC current bias or a reverse DC voltage applied to the PIN. A driver works by supplying the appropriate bias given a command input (frequently TTL). Whether the forward bias causes low loss or isolation depends upon whether the diode is in series or shunt. In some cases both configurations are used and the driver must be able to supply both at the same time.

From: Joseph Moreno, University of Glamorgan - School of Electronics

I am an MSc student in mobile communications and currently researching some information regarding beam steering systems for mobile phones (3G and 4G) as well as wireless LAN applications. Could you assist me with some references?

Dear Joseph,
There are a number of good antenna design references. Three that may be helpful to you include:
Microwave Antenna Theory and Design, S. Siver, Perigrinus, 1984, ISBN#0-86341-017-0; Modern Antenna Design, Milligan, Wiley, 2005, ISBN#10-471-45776-0; and Antenna Theory Analysis and Design, Balanis, Wiley, 1997, ISBN#0-471-59268-4.