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Ask Harlan: October 7, 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.
FROM: Wayne Conley, Alameda County Communications
Hi, I have a need to provide field techs with test equipment to easily test the gain performance of wide band RF amps in the 1 GHz band. Do you know of procedure and test equipment? Thank you, Wayne
Virtually all of the major test equipment manufacturers make field test sets. I suggest that you check out the manufacturer's directory on our web-site. There will also be an updated print directory in the supplement that will accompany our upcoming November issue of MWJ.
FROM: Greg Wampler, AFBM
After 20+ years as a microwave engineer I have had a 5 year hiatus. I'm trying to get back in the swing of things and am stumped in getting a good explanation of trellis code modulation (TCM). I understand that it is a predictive or statistical type of coding but I'm interested in the details. Can you help me out?
This is not one of my areas of strength. However, according to the Communications Standard Dictionary by Martin H. Weik, Chapman and Hall, 1996: trellis coded modulation (TCM) is,"Modulation that (a) is a modification of continuous phase modulation (CPM), (b)improves performance without changing bandwidth, (c) when filtered, does not have the constant envelope of CPM, and (d) uses expanded signal sets that make it useful in the design of spectrally efficient communications systems." There is a discussion of TCM on PP183-184 of Digital Communications, A. Bateman, Addison-Wesley, 1999, ISBN 0-201-34301-0
It is also related to Viterbi decoding, which is discussed in chapter 8 of Error-Control Coding and Applications, Djimitri Wiggert, Artech House, 1978, ISBN 0-89006-066-5.
FROM: Paul Holes, Anritsu Ltd
Hi Harlan, I have a requirement to measure AM to PM conversion of mixers.
What kind of test equipment do you think I would need, and is there any literature around that may assist me. Many thanks,
The basic instrument is a vector network analyzer that will display phase. AM/PM conversion is a function of power level and needs to be specified at a given power level. I searched for some literature but could not find anything, however, a friend said that the operating manual for the other "brand A" VNA has a procedure.
FROM: Adeel Malik, Computel
Good day, Our company has developed an OFDM modem for point to point microwave links supporting data rates in access of 34Mb/s. We want to develop a point to point microwave link having following charactersitics. 1. Frequency Around 7 GHz 2. Range Around 60km max We want to build the link but do not have much RF expertise. Can you tell us the following please 1. Any companies which provide a transceiver for our job? 2. How to get the link designed in a cost effective way? 3. Where to look for assemblies with which we can attain our goal? Best Regards, Imran Raouf AERO
We do not recommend specific suppliers as part of this service. I suggest that you consult the manufacturer's directory on our website. With regard to the design of the link, if you do not have some technical expertise within your organization, I think you will be better off purchasing a turnkey system.
FROM: In Kim
Do you know anything about GMSK modulation? If you do, then do you know how much different it is from the FSK modulation? Thanks,
GMSK is Gaussian Minimum Shift Keying, which is a continuous phase modulation and is unrelated to FSK or Frequency Shift Keying. An extensive explanation of GMSK is given in Chapter 4 of the book, Robust Modulation Methods and Smart Antennas in Wireless Communications, Bruno Patten, Prentice Hall, 2000 ISBN# 0-13-022029-9
FROM: Randy Van Steen, Raytheon
What VSWR should we use as a pass/fail criteria for an RF transmission line between an active GPS (1575.42 and 1227.60 MHz) antenna and its receiver? I would have said 1.5:1 (14 dB return loss), since that's what the spec'd values are for the receiver and antenna (and quarter-wave stub transient suppressors between them). But my colleague believes we should look for more like 1.22:1 (20 dB return loss). The manufacturer training for the test equipment points to 30 dB return loss across the cable sections, and 20 dB across the connectors.
Also, is there a technology to translate an RF signal (say, 1.2 to 1.6 GHz) to lightwave, so that an system could use optical fiber, as opposed to coaxial cable, to transfer signals from a receiver antenna to a receiver several hundreds of feet away? What say you, Harlan?
The problem with multiple mismatches is that they will multiply and create VSWR peaks. There is no reason to accept a 1.5 for a transmission line. At your frequencies a transmission line should be readily available with a VSWR well under 1.1.
You can modulate a laser diode at the microwave frequency and then demodulate it at the other end of the fiber. I'm sure there are a number of manufacturers that can supply a commercial product that will do the job.
FROM: Kah Boon Loo, Agilent Technologies
Hi Harlan, I am interested in designing the cavity resonator using the "Sperry Gyroscope" data curves. I tried to search for this data curve which printed long ago but in vain. Would appreciated if you could help to get the data curve. Paper that I am studying now: Title: Design of Cylindrical Re-entrant Cavity Resonator Author: M B Kelly and A J Sangster Resource: from IEEE web
The Sperry Gyroscope curves for a re-entrant cavity are on pages 188-192 of The Microwave Engineers' Handbook Volume #1 by T. Saad, 1971. The book is still available from Artech House and may also be in your company library, since it is a classic.
FROM: Chimp 007, Micro-Effort
How many microwaves were sold in the United States in 2002?
By "microwaves", I assume you mean "microwave ovens". Our industry is microwave electronics and has no connection with the oven business, so I can't answer your question. Sorry.