Tuesday, September 28, 2010

Free Beer Tomorrow!

My Dad taught me early that nothing in life is free, but over the years I have found quite a few free EE design tools that have helped me out a lot. These tools are offered free, but are providing marketing value to the companies distributing them, so my dad is still right!

Free Tool #1:
While designing and supporting embedded systems I am always in need of quickly finding component pricing and availability. I came across www.oemstrade.com website while searching for End-of-Life component crosses.

This website allows you to type in a manufacturers part number and it automatically searches through 34 different electronic components distributor's websites. Digikey, Newark, Mouser, Avnet, Arrow and Future are all included in the search (NuHorizons needs to get on board as well). This simplifies searching for "in-stock" components. I also just use this website as a part number checker to make sure I've entered the P/N correctly in my BOMs; if the part is found at some distributor chances are it is a valid part number.

Free Tool #2:
You don't have to be an RF Engineer to need PCB trace impedance calculation tools; with DDR & DDR2 memory traces requiring 50 Ohm characteristic impedances, Ethernet requiring 100 Ohm differential traces, and USB requiring 90 Ohm differential traces even the Embedded Designer is in need of calculating PCB Trace Impedances. I came across the Saturn PCB toolkit (http://saturnpcb.com/pcb_toolkit.htm) software package several months ago and I've been putting it to some good use. This tool uses IPC-2221, IPC-2251, IPC-2152, and IPC-2141 for calculation of: trace impedance, differential traces, via impedance, trace & via current capacity and much more. This tool has its limitations, the IPC standards don't take into account all the different field effects especially if you are exceeding the Trace height and width ratios, but this tool is a lot less expensive  than a $15,000 Field Solver. All trace impedances should still be checked by your PCB supplier for accuracy.

Free Tool #3

I like to do a lot of hands on testing, but every once in a while a quick simulation will answer my questions about a circuit and I can save the trip to the lab for the next problem. SPICE simulation tools have come a long way over the years and as long as you have a basic understanding of what the outputs should be (so you know if the simulation is inaccurate) these tools are very helpful. I have been using LTSPICE from Linear Tech (http://www.linear.com/designtools/software/) for years now and I really like it. It is not as pretty or easy to use as ORCAD, but I have gotten used to the quirks.

Linear Tech provides LTSPICE with a library full of Linear Tech parts and some various passives. They obviously want you using their parts, but I've simulated Analog Devices opamps and Texas Instruments comparators by creating new components that reference the other manufactures SPICE file. LTSPICE is definatly worth downloading and giving a try. There is also a Yahoo News Group that specifically discusses LTSPICE activities and issues.

If you have any other Free tools that have been helpfully please add the links as a comment to this post.

Saturday, September 25, 2010

Unwanted Positive Feedback

A few years ago I was hired as a contract engineer to help a local medical company redesign and update their 100Watt, 900MHz, Microwave Generator that was used for RF Thermo therapy. The Microwave Generator consisted of a frequency synthesizer, RF Power Amplifier, Dual Directional Coupler, and a microcontroller based feedback/control board. The microcontroller controlled the frequency and output power of the synthesizer and measured the forward and reflected output power of the Power Amplifier via the Dual Directional coupler.   

The Power Amplifier consisted of three RF amplification stages and was in need of a complete redesign as it was based around several now obsolete parts. I redesigned the Power Amplifier based around one of Freescale’s newer LDMOS Power Transistors. It was a similar three stage RF amplifier that had roughly 50dB of gain. 

My problems started in the lab after my first Power Amplifier prototypes showed up. The output stage of my amplifier was continuing to “Burst into Flames”. These made exploding Tantalum Capacitors look like child’s play in comparison. Every time the output stage blew up it would take a chunk of the Arlon PCB with it…. Embarrassingly this happened often enough with this project that I got quite good at repairing the burnt traces with copper tape, some solder and some patience.  Needless to say I was stressed out, this was my first High Power RF design and I needed a “Grey Beard” to help me out. I got a hold of an older RF Engineer named John, who like most good RF engineers was a HAM Radio nut and had more electronic equipment in his basement than the Pentagon. 

John and I hooked up my RF power Amplifier to a sig gen and slowly cranked up the output power until we noticed the frequency spectrum of the output started going unstable and multiple frequencies popped up. We quickly backed the power off. This was my first clue as to what I was doing wrong…. Before I was never monitoring the output of the RF Amplifier with a spectrum analyzer, I was just using a RF Power meter…. it is equivalent to using a multimeter to measure a voltage when you really should use a scope, so you can see more than just the “average” information. My amplifier was turning into an oscillator due to some unwanted positive feedback.  
 “50dB of gain is a lot of gain to have in one house under one roof” John told me; we were obviously getting some feedback between our amplification stages. A few cuts to our GND plane, isolating each stage a bit better and retuning each stages gain down a little and our Amplifier stayed an Amplifier.  I am over simplifying the process we went through, but a few weeks later we had a non-oscillating RF Amplifier. John taught me a few other RF tips & tricks which I‘ll share in later Blogs.

This project was probably one of the more challenging projects I’ve ever worked on, but it was probably the most rewarding.