Ask Harlan, July 26, 2006

From: SM Hassan, Virginia Tech

Dear Harlan,
How and why does the selection of an RF choke affect the proper operation of the amplifier in RF circuits?

Dear SM,
Chokes are used to separate RF signals from other circuit functions like bias networks. The degree of isolation that they provide affects the operation of the primary circuit by undesired loading the circuit or the introduction parasitic responses. There are also considerations such as harmonic responses of chokes that can affect the circuit. A good rule of thumb is to design a choke to provide at least 20 dB of isolation over the operating band of the amplifier.

From: Akhilesh Jain, RRCAT

Dear Harlan,
I am working on the design and testing of a solid-state RF amplifier at 350 and 700 MHz. Presently, we are testing 350 W CW power modules. From an RF radiation point of view, how safe is our body while testing and tuning a few capacitors like Jhonson 8054 cap. Should I wear some conductive cloth gloves while handling it? My amplifier circuit uses coaxial line components and the size of the amplifier unit is around 250 by 300 mm with 40 mm in height.

Dear Akhilesh,
I am hesitant to make recommendations where safety is involved. My guess is that the stray fields are probably very small or you will have oscillations. My best advice is to buy or rent a field strength meter and make measurements to be sure that the radiation is within the safety limits of the ANSI-C-95 standard. This standard has been recently updated and is available from IEEE at www.ieee.org.

From: Souren Shamsinejad, IUST

Dear Harlan,
I am trying to locate a transistor for use in low noise amplifier (LNA) design. This LNA is used in a 3G/UMTS receiver at a 2 GHz frequency. Could you recommend a manufacturer?

Dear Souren,
There are many good devices available; however, we do not recommend specific manufacturers, I suggest that you look at our buyer's guide or scan the ads in recent issues of Microwave Journal.

From: Muhammad Ismail, Digital Net Services

Dear Harlan,
I am trying to find additional information regarding xMax Technology. I found some information at http://www.xgtechnology.com and it sounds interesting. I suspect it for the R&D and other technical factors due to skepticism in the technology. Moreover, MRT Magazine wrote a few articles about xMax Technology. The links are to follow:
1. http://mrtmag.com/mag/radio_good_true/
2. http://xgtechnology.s2webtrix.com/Vision%20-%20xMax.pdf
3. http://www.techworld.com/mobility/news/index.cfm?NewsID=4722&inkc=0
Could you provide any additional detail regarding this technology?

Dear Muhammad,
xMax technology is just emerging publicly. Since it was originally developed for stealth use, I have not seen any technical papers on the subject. I suspect that there are classified reports, which are not available without a security clearance. A "Google" search for "xMax" brought up hundreds of hits. Some of them may be helpful to you.

From: Mohammed Alwadi

Dear Harlan,
How can I design a single rectangular microstrip patch antenna fed by a microstrip line utilizing microstrip simulation?

Dear Mohammed,
The basic procedure for designing a patch antenna is described in Microstrip Antenna Design Handbook, Garg, et al., Artech House Inc., 2001, ISBN#0-89006-513-6. Once you have a basic design, you can simulate it with microstriplines using the procedures found in Microwave Circuit Modeling Using Electromagnetic Field Simulation, Swanson & Hoefer, Artech House Inc., 2003, ISBN#1-58053-308-6.

From: Karin

Dear Harlan,
Can you manufacture a sample from a negative or positive picture of a circuit board/breadboard? If so, I will need the cost for making the samples and for manufacturing. If not, can you refer me to a manufacturer you might know of someone who can?

Dear Karin,
We do not recommend specific manufacturers. However, it is unlikely that you can fabricate a microwave circuit board to the required accuracy from a photograph of a breadboard. Normal tolerances for stripline or microstrip circuits are ± 0.001 to 0.002 inches. Actual dimensions will vary as a function of substrate thickness and dielectric constant of the board material.

From: Amir Mohammad Kherzri, IUST

Dear Harlan,
I want to design a broadband low noise (noise figure <2 dB) amplifier for a receiver front-end with a high power output (about 30 dBm). Due to arbitrary and unknown signal levels, the existing transistors I do know do not have both of these simultaneously. Some of them are high power and some of the others are low noise. Could you please guide me as to how I can combine both of these characteristics in a single circuit?

Dear Amir,
You do not need both low noise and high power in the same device. Low noise transistors at the front end will establish the noise figure of the amplifier. The higher noise figure of the output transistors will be masked by the gain of the early stages. There is a good discussion of noise sources and the equations for cascaded noise figure in Noise in Linear and Non-linear Circuits, S. Maas, Artech House, 2005, ISBN#1-58053-849-5.

From: Adhikarla Jogeswara Rao, Kavveri Telecom Products Ltd.

Dear Harlan,
I would like to ask your advice regarding filter power handling: 1. What is the maximum power handling capacity for microstrip interdigital filters? 2. What is the maximum power handling capacity for the lumped bandpass filter if all shunt capacitors realized in the form of pads of PCB and the series capacitors are realized with the PCB (parallel plate capacitors) inductors utilizing Cu wire? 3. Could you recommend any reference books relating to this subject?

Dear Adhikarla,
There is no simple answer to your questions. Peak power handling is related to voltage breakdown and is dependent on the dimensions, surface finishes and dielectric properties of the circuit elements. Average power handling is largely a function of the allowable temperature rise due to losses in the circuit. There is a discussion of voltage breakdown in Mircrowave Engineering - Passive Circuits, P. Rizzi, Prentice Hall, 1988, ISBN#0-13-586702-9. I have included calculations for thermal rise in striplines, which can be modified for microstrip in my book Stripline Circuit Design, Artech House Inc., 1974, ISBN#0-89006-020-7.

From: Maria Siddiqua

Dear Harlan,
I am trying to work on a BPSK direct modulator. I have found only one topology that is a reflection topology using PIN diodes. I cannot fully understand the basic functioning of the PIN diodes and how they work in the circuit. Could you please explain this? Is there any other topology regarding direct modulation?

Dear Maria,
Binary phase shift keying (BPSK) is a two state phase modulation which lends itself to PIN diode usage. PIN diodes have two conduction or reflection states depending on their bias. They have been used for many years in different circuit configurations to build microwave phase shifters. The definitive text on PIN diode phase shifters is Microwave Semiconductor Engineering, J.F. White, Noble Publishing, 1995. There are other modulation circuits based on mixers, which can be used as well. There is a good discussion of BPSK in Digital Modulation Techniques, F. Xiong, Artech House Inc., 2000, ISBN#0-89006-970-0.