Ask Harlan, May 30, 2006

From: Trevor Baker, Coffey

I need a device that can transmit and receive information data. This device would be installed inside a house but needs to be able to transmit multiple miles. Is there such a device? Would a relay point extend the transmittable distance? Can this device be integrated with a computer?

Dear Trevor,
There are quite a few commercial wireless LAN devices that are operated under IEEE Std. 802.11, which will do this. There are also wireless services offered by people like Verizon and Comcast, which may be more appropriate for longer distances. Retailers like Comp USA may be your best source of information.

From: Panagiotis Mourtopallas, Student

I'm trying to find a book pertaining to the theory of injection locked oscillators. Could you provide any guidance? It's for my diploma thesis. Thank you.

Dear Panagiotis, The definitive paper on injection locking is "Injection Locking of Microwave Solid-State Oscillators" by K. Kurokawa, Proc. IEEE, Vol. 61, October 1973, pp. 1386-1410.

From: Jesse Sheinwald, The Aerospace Corp.

Years ago I designed waveguide systems utilizing 'choke' flanges. Today, I am faced with a new waveguide design problem and before I blindly use choke flanges I want a little more theoretical basis on how they work. Unfortunately, I have found little information in the literature. Do you know any good sources on the operation and effectiveness of choke waveguide flanges?

Dear Jesse,
There is a discussion of waveguide choke flanges and other chokes in chapter seven of Microwave Engineering - Passive Circuits, P. Rizzi, Prentice Hall, 1988, ISBN#0-13-586702-9. He also references additional information in Rad. Lab Series Vol. 9, Microwave Transmission Circuits, G.L. Regan. While out of print, the Rad Lab series is still available from Artech House on CD.

From: Paul Calvert, Radiance

Is it possible to encode radar waveforms at X-band in such a way that: At the (bi-static) receiver they can be mixed down to IF, the IF be converted to digital, then match the filter to the data with a 'transformed' code? I don't see how I can match-filter an X-band signal (digitally due to A/D difficulties at 10 GHz), nor do I see how CMDA PN codes can be match-filtered in the analog realm. However, I am thinking codes can be chosen at X-band such that once mixed down to IF, coding will still be present, although perhaps a radically different code than the original. Or am I missing something? I'm totally new to the RF/radar world, so please excuse any 'obvious' questions.

Dear Paul,
Your questions are not obvious. In fact, I'm not sure of the answers myself since this is at the edge of my experience and digital techniques have evolved significantly since I was a working engineer. All that said, I think you can encode at X-band and downconvert to IF and recover the coding in digital form. There are two recent books on the subject that may be helpful to you: Radar Signals, Levanon & Mozeson, Wiley, 2004, ISBN#0-471-47378-2 and Digital Techniques for Wideband Receivers, J. Tsui, Artech House Inc., 2001, ISBN#1-58053-299-3. I'm sorry that I can't be more specific. Perhaps one of our readers can offer an additional response.

From: Tan HB, NTU

I am interested in passive component design for microwave communication systems. I have some rough ideas of designing a Lange coupler. How do I optimize a 3 dB 90 degree Lange coupler to achieve a broadband design?

Dear Tan,
You do not define what you consider "broadband". The basic single section 90 degree coupler (either Lange or some other structure) has a useful bandwidth of one octave. If you design it for exactly 3 dB coupling, it will only have a perfect split at the center frequency. Many people design them for a tighter coupling, for example, 2.75 dB, to improve the performance at the band edges. If your bandwidth is more than an octave, you will need multiple sections.

From: Rajendran N S, United Microwaves P Ltd.

In a 1 GHz monopulse Rx, with a required bandwidth of 10 MHz, I have low noise amplifiers and filters meeting the required noise figure and selectivity. Why do I have to go superheterodyne, when the current log amplifers go well beyond 1 GHz?

Dear N S,
If, as you say, you have the noise figure and selectivity that you require, you do not need to downconvert. You can go directly to the log amp with post detection.

From: Rupreet S., EIL

In your opinion, what are the best book(s) for understanding RF and microwave basics?

Dear Rupreet,
The two books that I usually recommend for basic study include:
High Frequency Techniques, J.F. White, Wiley, 2004, ISBN#0-471-45591-1 and Microwave Engineering, P. Rizzi, Prentice Hall, 1988, ISBN#0-13-586702-9
Both are easy to understand and have extensive additional references.

From: Matthew Hathaway

I am relatively new to the field of microwave technologies. Could you suggest a good list of available references for self-study?

Dear Matthew,
Two of the best texts include:
High Frequency Techniques, J.F. White, Wiley, 2004, ISBN#0-471-45591-1 and Microwave Engineering, P. Rizzi, Prentice Hall, 1988, ISBN#0-13-586702-9. Both books have extensive additional references.