Ask Harlan: November 18, 2003
YOUR RF & MICROWAVE TECH Q&A RESOURCE
You may know Harlan Howe from his twelve years as publisher and editor of Microwave Journal ®, or from his 34 years as a Microwave design engineer and engineering manager, or from his service as an IEEE fellow and past president of MTT-S.
Now, although semi-retired, Harlan is available to answer your questions about RF and Microwave engineering. If he doesn't have the answer, he will find an industry expert who does.
[Editor's Note, 18 November, 2003: If you submitted a question to Harlan in the last several days, or plan to submit one this week, please be patient as we enter the Holiday season. The next Microwave Flash is scheduled for the week beginning 8 December and the next installment of Ask Harlan will be published at that time. Thank you.]
FROM: Dave van Alstine, Electro-Radiaion
I see that many RF receivers bring the signal down to a low IF. My question is what are the negatives on mixing all the way down to baseband and using the IQ mixer output to maintain the full bandwidth information. Is there a fundamental reason that signals can't be mixed down to zero (in effect a zero IF) as they do in the digital world?
The signal can be mixed to Zero and there are a number of Zero IF systems. The principal drawback is 1/f noise, which can seriously degrade the performance.
FROM: Matteo Biggi, Officine Pasquali
Dear Mr Harlan,
I would like to know if there are some issues of "Microwave Journal" that concern the directional coupler design theory.
Maybe there are some old issues and so I don't know how could I get them.
Can You help me?
Thank You Mr Harlan
The subject of directional couplers has been covered frequently in Microwave Journal since its inception. I can't give you all the references, since we are talking about 46 years of monthly publication.
If you know of a specific paper and can provide the date and author to use, we can send you a copy.
I would suggest, however, that you look at two books: Microwave Transmission Line Couplers , Malherbe, Artech House, 1988, ISBN# 0-89006-300-1 and Microwave Filters, Impedance Matching Networks and Coupling Structures , Matthaei, Young and Jones, Artech House, 1980, ISBN# 0-89006-099-1.
To see a sampling of online articles matching "directional coupler" since 1995, click here.
FROM: Foo Dominic, AVX / Kyocera (S'pore) Pte Ltd
What is the significance of VSWR (Voltage Standing Withstanding Ratio)?
VSWR (Voltage Standing Wave Ratio) is a measure of the reflections on a transmission line due to discontinuities and impedance mismatches. A high VSWR can cause distortion, poor power transfer, high power breakdown and damage to some circuits.
FROM: Francisco Sierra, Broad Telecom, S.A. (BTESA)
Dear Mr. Harlan,
Along the last years we have used some off-the-shelf class-AB UHF amplifiers employing LDMOS technology, and we have also designed our own amplifiers. Inherent to class-AB, these amplifiers suffer a crossover distortion in the region close to small-signal. This is a problem for signals covering a wide dynamic range. How can this effect be corrected or overcome? I am talking about something inside the amplifiers, not a pre-distortion mechanism in the system.
Cross-over distortion in Class-AB amplifiers is largely related to the bias levels and techniques used. An excellent discussion of AB amplifiers can be found in: RF Power Amplifiers for Wireless Communications , Cripps, Artech House, 1999, ISBN# 0-89006-989-1.
FROM: Chengheok Kang Madam, DSO National Laboratories
I'm testing components, namely power dividers/combiners, and would like to know what are the specifications for acceptance and types of failures which usually occur to passive dividers. What is the carrier level to be tested? Thanking you for your feedback.
The acceptance or rejection levels are a function of the system requirements needed. There are no universal rules. The most common failure of these devices is burnout of the termination resistors. Thus, while the basic performance can be done at low power levels, some testing should be done at the operating levels of the system.
FROM: John Cowles, Analog Devices
I would like to understand how CCK modulation used in 802.11b WLAN is a spread-spectrum technique. The low rate 1 & 2 Mbps 802.11b standards use 11-bit codes to spread the spectrum by 11 in the way that CDMA does it.
The CCK modulation scheme, to the best of my understanding, has a processing gain of 2x by mapping 8 data bits onto 8 complex bits. The advantage of CCK seems to be in its resistance to fading, etc.
However, is this processing gain?
I believe that CCK (Complimentary Code Keying) is a processing gain scheme. While I was not able to find a specific reference in our library, the book: Digital Modulation Techniques , Xiong, Artech House, 2000, ISBN#0-89006-970-0 may be helpful to you.
FROM: Nikolay Tchamov, Tampere University of Technology
I am trying to use 802.11g for two-ways Video communications using video camera and the things do not quite work as they were expected. Maybe we are too far away, or moving by walking too fast.
What would be the best way and Radio Tx/Rx you would suggest to be used, and how to proceed buying it; the price is not the most important argument; the performance is, and that is mostly:
- Very Portable (min Size-Weight-Battery) for example anything below 1kg would be good to consider.
We will appreciate your advice very much.
I'm not sure what type of modulation you are employing or the type of range and antenna coverage you need, so it's hard to give you a specific answer. There are a number of manufacturers of 802.11 hardware available. I suggest that you discuss the specific requirements of your system with them. 802.11g is relatively new and approaches to it are changing rapidly.