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Ask Harlan, August 25, 2006

Harlan Howe has 34 years experience as a microwave design engineer and fifteen as publisher and editor of Microwave Journal ® , and is an IEEE Fellow and past president of MTT-S. He's here to answer your questions on RF and Microwave engineering.

August 25, 2006
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Published August 25, 2006

From: Melanie Barclay, Scientific Research Corp.

Dear Harlan,
I am familiar with four-port couplers, but I am having trouble understanding the operation of a four-port, 20 dB coupler with two inputs. The port connections are as follows: J1=main signal; J2=output from coupler; J3=terminated (50 ohms); J4=secondary input from a harmonic generator circuit. My problem: A co-worker says that only the secondary signal is attenuated 20 dB, and not the main signal. I do not understand. Could you explain this?

Dear Melanie,
If I understand your port designations, J1 and J2 are the mainline (thru arm) ports. Thus, there will be no attenuation, other than insertion loss, from J1 to J2. J3 is terminated on the coupled arm and J4 couples to J2. If a secondary input is injected at J4 it will be terminated by J3 and will be 20 dB down at J2.

From: Alexandru Muntoiu, Internet Link

Dear Harlan,
My understanding of OFDM means multicarrier modulation, special tranceivers and multipath NLOS propagation, but a lot of 802.11a and g specifications say they make OFDM. Is this true?

Dear Alexandru,
Yes, OFDM is the preferred modulation for multicarrier applications. However, it also has specifications for 802.11a and g. A good discussion of OFDM can be found in OFDM for Wireless Multimedia Communcations, Van Nee & Prasad, Artech House Inc., 2000, ISBN#0-89006-530-6.

From: Shruti P., Vedangradio Pvt. Ltd.

Dear Harlan,
We are well into the RF research that we are conducting. Could you suggest accessories that would be required to measure RF PCB cards for all types of components (for example, 0603 capacitor (pf) and 0603 inductor (nh), COG and resistors)?

Dear Shruti,
The type of test fixtures are determined by the mechanical features of the components to be measured. Most component manufacturers will provide either information or fixtures for their products. I suggest that you contact them.

From: Dave Howie, SAAB Avitronics

Dear Harlan,
I am very familiar with the manufacturer range of pin diode limiters that can cover 0.1 to 18 GHz. These often incorporate a shunt inductor. I am unable to locate a manufacturer that covers the frequency range from 30 MHz to 3 GHz. Is this a limitation of the shunt inductor or of PIN diodes?

Dear Dave,
It is certainly not a limitation of the PIN diodes and there are plenty of inductors that are appropriate for that band. I suspect it may be more of a market limitation. I am sure any of the limiter manufacturers can make one for you.

From: Mario Leib

Dear Harlan,
Why is a beam-led diode called beam-lead?

Dear Mario,
Unlike a conventional diode, which is typically mounted on a stud with a wire bond to the active side, a beam-lead has both terminals on the same side. They are made by plating and etching and provide two "beams" in the same plane.

From: Mohammed Rahman, RFS

Dear Harlan,
I am involved in a product that will have Antenna Interface Standard Group (AISG) functionality. One of the AISG requirements is the inrush current and the following is the spec from AISG. I cannot interpret this spec and was wondering if you could provide any insight. 6.4.4. TMA inrush current A TMA (including its optional ALD modem) shall exhibit the circuit equivalent of a DC power consumer with a current consumption of maximum 1 A in parallel with a capacitor of maximum 0.5 ìF.

Dear Mohammed,
I have no idea. This is outside my area of knowledge. I would go back to the people who wrote the spec or perhaps one of our readers can help.

From: Ling Wang, University of Electronic Science and Technology of China

Dear Harlan,
I am interested in learning more about a fast-speed pulse modulator. Which type of diodes can realize a fast-speed pulse carrier signal that allows a rise/fall time of 100~200 ps? How much time is the intrinsic response time of Schottky diodes?

Dear Ling,
The intrinsic response time of a Schottky diode is related to its capacitance. There are Schottky diodes available that should be able to handle your desired rise/fall time.

From: Jidesh Shankar, ISRO

Dear Harlan,
Is there a substantial change in design for phase shifters, attenuators, etc., in the microwave band for space use when they are hermetically sealed or not exposed to "space" per se?

Dear Jidesh,
There is no change in the design despite the fact that space is a vacuum. The hermetic sealing is needed to protect the devices before and during the process of getting them into space.

From: Akhilesh Jain, RRCAT

Dear Harlan,
For a solid-state RF amplifier, would you recommend using a conductive heat compound, just below golden color LDMOS heat sink, or any dielectric-based heat sink compound? Our logic seems to indicate that we should improve the current path between the source of LDMOS to solid metal enclosure.

Dear Akhilesh,
I am not sure what your "golden colored" heat sink is. If it is conductive to ground, then a conductive heat compound may be helpful. If it is an insulated heat sink, then a conductive compound will not do any good.

From: Jan Browns, K&L Systems

Dear Harlan,
I am making a cable assembly and I have the calculated phase length available. Can you tell me how to figure out actual phase length on a cable during assembly? I know that VOP keeps varying from cable feet by feet. How do I figure out practical actual phase length of cable assembly and how do I test the phase length and what test setup is required?

Dear Jan,
Phase matched and phase stable cables are difficult to calculate without measured data on the cable. You can do a rough calculation based on the manufacturer's specs, but that will only get you close. It is also important that you temperature cycle the cable assembly to help stabilize it. Measurements require the use of a VNA. Quite a bit of experience is needed. My suggestion is to buy the cable assembly from one of the companies that specializes in phase stable cable assemblies.

From: Ahmet Isdan, KL Ltd./Eng.

Dear Harlan,
We are planning to make a solid-state power amplifier for C-band. There are amplifiers implemented with discrete GaAs FETs or PHEMTs. Also, there are MMICs that perform amplification. Which one do you recommend? Is it better if we create the amplifier utilizing discrete elements or an MMIC?

Dear Ahmet,
It depends on a number of factors such as power, bandwidth, cost and available size. Generally MMICs will be smaller and if commercial chips will do the job, less expensive. Discrete devices tend to be easier to heat sink if you are dealing with very high power but require more external circuitry, which also requires more up front engineering and assembly time. If the production quantities are high, MMICs will be more cost effective, whether you use commercial chips or have to design and produce your own.

From: John Pritchard, Qinetiq

Dear Harlan,
How do you relate SNR to pre-detection and post-detection bandwidths for linear detectors?

Dear John,
If the signal and noise bandwidths are the same, there will be no change. If either changes the effect will be linear with the bandwidth change. The best discussion of noise is Noise in Linear and Non-linear Circuits, S. Maas, Artech House, 2005, ISBN#1-58053-849-5.

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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