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.

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