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Industry News

Ask Harlan, January 30, 2007

Harlan Howe has 34 years experience as a microwave design engineer and fifteen as publisher and editor of Microwave Journal ® , and is an IEEE

January 30, 2007
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Published January 30, 2007

From: Brian Fernandes, Avago Technologies

Dear Harlan,
Why is there a frequency limitation on the GTEM for measurement of radiated emissions? And how can this be physically measured?

Dear Brian,

A GTEM cell is a closed cell measuring device that creates a TEM mode field for EMI testing. I do not believe there is a physical frquency limitation; rather, the limitation is set by international regulations. Measurement techniques can be accessed at www.tdkrfsolutions.com/WebDataSheets/EMI-GTEM-LABWEBSheet.htm.

From: Amir Mohammad Khezri, IUST

Dear Harlan,
YIG filters have many variations due to both temperature variations and the hysterisis effect. How can I overcome this problem? Setting a frequency by moving step by step on a hysterisis graph takes a lot of time. Is there another way to avoid the hysterisis error?

Dear Amir,

Both temperature and hysterisis errors can be compensated using digital drivers with "look-up" tables incorporated in them. Since the YIG tuning speed is slower than the driver speeds, there is no reduction in tuning speed.

From: Rouzbeh Fereidouni, Student

Dear Harlan,
I want to build a low phase noise source. My reference is 1 GHz and I want to build a low noise 8 GHz using a MP7100M sampling phase detector from mpulse. I do not know how much the additive noise is of SPD for my application. I want to know how much a sampling phase detector would degrade the phase noise? What is the phase noise of SPD at different offsets from a 1 GHz carrier?

Dear Rouzbeh,

Since you are using a specific commercial SPD, you need to contact the manufacturer for the specific data related to their product. Two general texts that may be helpful to you include:

Design of RF and Microwave Amplifiers and Oscillators, P, Abrie, Artech House Inc., 1999, ISBN# 0-89006-797-x, and RF and Microwave Oscillator Design, M. Odyniec, Artech House Inc., 2002, ISBN# 1-58053-320-5.

From: Gholamreza Moradi, Amirkabir University of Technology

Dear Harlan,
Would you be able to provide me with a reference that shows the procedures for measuring the permeability of materials?

Dear Gholamreza,

I am not aware of any reference on the subject. Measurements of this sort are dependent on test fixtures specifically designed for the purpose. While I do not normally recommend specific companies, there is one that specializes in such fixtures and procedures. They may be able to help you. Try contacting Nick Damaskos at www.damaskosinc.com.

From: Thomas N.C. Wang

Dear Harlan,
For a "MIMO antenna" to be used for a sectorial BTS application, will it be workable for a TDD system (i.e., the downlink and uplink have the same carrier frequencies)?

Dear Thomas,

If the Multiple Input Multiple Output (MIMO) antenna has sufficient bandwidth to support the BTS application, it should be OK if both signals are at the same carrier frequency. There is a good discussion of MIMO techniques in Wireless Communications over MIMO Channels, V. Kuhn, Wiley, 2006, ISBN#0-470-02716-9.

From: Boon Wei Kang, Agilent Technologies

Dear Harlan,
I am designing a microwave circuit from DC to 6 GHz. How can I determine the values of the DC block/ bypass caps and RF choke that I should use?

Dear Boon Wei,

Since I do not know the type of circuit you are designing, I cannot give you actual values. They will depend on the reactance that you need for each function. The equations for reactance (in ohms) are:

XL = 2*pi*F*L for inductive reactance

XC = 1000/(2*pi*F*C) for capacitive reactance

where L is in nH, C is in pF and F is in GHz

From: Alok Joshi, Texas Instruments

Dear Harlan,
How do I calculate optimum load impedance for a nonlinear power amplifier such as a class-F amplifier?

Dear Alok,
The optimum load impedance will be dependent on the S-parameters of the active output device under the operation conditions of the design. There is a good discussion of load impedances in Design of RF and Microwave Amplifiers and Oscillators, P. Abrie, Artech House Inc., 1999, ISBN#0-89006-797-X.

From: Tim Ressel, Veris Industries

Dear Harlan,
I saw a schematic for a 0-1G dual conversion receiver with a 1221.4M first IF. A diplexer on the IF side of the first mixer used what looked to be four-port hybrid couplers in a race-track configuration. Can you explain how that works and what kind of couplers those were?

Dear Tim,
What you describe is probably what we call a rat-race hybrid. It consists of a ring that is 1 1/2 wavelengths long with four arms spaced 90, 90, 90 and 270 degrees. A signal entering the middle 90 arm splits evenly to the adjacent arms in equal phase, while a signal entering the fourth arm splits to the same two outputs with a phase difference of 180 degrees. There is a good description of this circuit with design information and performance curves in Chapter 3 of Stripline Circuit Design, H. Howe, Artech House, 1974, ISBN#0-89006-020-7. This book is now out of regular print, but is still available POD.

Harlan Howe, Jr. received his BS degree in optics from the University of Rochester in 1957. He has been actively engaged in the microwave industry for 48 years, first as a design engineer and then as an engineering manager. In 1990 he became the publisher/editor of Microwave Journal. He retired as publisher in 2001, but remains the editor. He is a Life Fellow of IEEE, past president of MTT-S and the recipient of an IEEE Third Millennium Medal in 2000 and the MTT-S Distinguished Service Award in 2005.

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