When working for a firm that made thick film amplifiers, I was given a task of designing a low-noise amplifier with a noise figure of 2.5dB, a gain of 15dB, and an input and output VSWRs of two to one or 9.54dB return loss. The frequency range was from 10 MHz to 300 MHz. That meant I could not use a double resistor series-shunt feedback resistor since that circuit configuration would raise the noise figure. The amplifier was to operate on a 15V supply drawing about 20 milliamps.
I decided to design the amplifier using a non-dissipated feedback consisting of a directional coupler incorporating a dual aperture core with number 32 bifilar wire color coded red and green. I built the directional coupler on a PC board and adjusted the turn's ratio to give the required tap for the gain of 15dB. I calculated the thick-film resistors to bias the chip transistor for the correct Class A DC operating voltage and current. I then had someone install the coupler on the thick-film ceramic carrier that was to be mounted on a 3-pin TO-8 header using silver epoxy.
After the unit was assembled and prior to installing the cover that was to be hermetically sealed, I tested the unit using an HP network analyzer. The gain and VSWRs all met specifications, so I approved the design for production. Three days later, 50 units were ready to be tested by the technicians in the quality department prior to sealing the caps.
Early in the testing, the technician came into my office and said that the gain was 13dB and the VSWRs were four to one and out of specification. I asked the tech to bring the units to my network analyzer where I tested them and they all met specs. I went to his network analyzer and sure enough, they exhibited high VSWRs and 13dB gain.
Looking over the analyzer setup, I discovered the output level of the network analyzer was set to its maximum of +20dBM. This meant the input signal was so high as to saturate the amplifier, and cause the gain to be low and the VSWRs to be high. I showed the tech how to lower the output level to -20 dBM wherein he tested the units and found them to be in spec.
I guess we all assume test equipment setup is automatic, but one has to be careful concerning the levels in and out of test equipment.
This entry was submitted by William J. Garner and edited by Rob Spiegel.
William J. Garner is an RF microwave engineering consultant with 47 years design experience. He has published papers in trade journals and holds seven patents.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Since I had 6 years in the Navy as an ET (primarily vacuum tube schooling) I had fun in the labs while getting my engineering degree with the other students. I would be in and out in 10 minutes while they spent the three hours trying to figure out how to turn the test equipment on. I helped where I could, but they needed to find out for themselves how things worked.
I have lost many a meter due to idiots (bless their hearts) doing idiot things. But I know it is really my fault for not taking the time to check them out and train them properly. Still...
I smoked one of those HP rotary step attenuators with about 100 watts of VHF. I was lab testing a radio for spurs and had a coupler, a notch filter and this external attenuator hooked up. The radio was supposed to transmit through the coupler to the load, the coupler feed a notch filter, the attenuator protect the spectrum analyser input. I mixed my cables up. I noticed as I adjusted the attenuator to increase signal, it instead was going down. I lifted PTT just in time for "Ode of Ohmite". A $500 attenuator was smoked but the analyser was saved. I haven't blown up too much, save the expensive rare switching power supply I once was trying to fix went "Chernobyl" on me.
The entry on the corner horn speaker reminds me of a trip to a stereo store about 40 years ago. I set up an amp to drive a 1 watt signal into a (made-in-USA) famous folded horn speaker, and proved to my satisfaction that 98 or so dB/w figure was likely under-rated. Nearly put the front windows of the store into the parking lot. I'm not sure I've ever seen a stereo store owner move so fast in my life: likely approached Mach 1. I think he could tell who caused the problem, as I was the guy with the eyes like pie plates. Probably not going to let that happen again. The speaker survived, so far as I can tell. Lesson learned.
He didn't say the test gear's default setting was maximum output... just that it was set to +20 dBm [100 mW] (Its maximum output? Perhaps the value required the last time it was used?) whereas what he needed was -20 dBm [0.01 mW].
His bad for not specifying test input power (not that I've *EVER* ommitted a key spec or dimension <grin>)! Four orders of magnitude generally will make a differance!
The one thing that I have found to be worse than those who don't have "a grasp on the obvious" is those who don't understrand about how much they don't have a grasp on. The fact is that most folks do not learn as much in six months as a good engineer learns in five years of school plus ten years of experience.
Of course it would have made a lot of sense to specify an input signal level for the testing. I did have some techs who seemed to delight in asking about t test conditions whenever I forgot to specify something. I always made a big deal of thanking them for catching my error, since that usually prevented disasters, they wound up making all of us look good.
I was testing a newly purchased speaker for the low cutoff. Speaker was for a folded corner horn I designed. I was not getting any signal out so I kept increasing the input. Then I thought to check for loose connections. Well the cone did not fly across the room as I have heard but but one thump and that was it for that speaker. It hurt in the wallet. I did successfully test its mate and also obtained a new mate. The folded corner horn which I found documented a magazine and referenced in my report got a B.
Have to agree with you. Too many engineers really do not know how to operate instruments. Ever watch a guy using Auto Trigger trying to capture an event or leaving the trigger set to down slope while looking for an event transitioning to high? Mant (most) have no clue about using A delayed B triggering, or even how to look at differential signals. Some days I think the technology is doomed!
Your experience is exactly why my policy is to involve me in testing all power amps on the kw output range. I had a brilliant (and I do mean very talented) design engineer plug our brand new HP RF Network Analyzer directly into the output of a 800w rf amp. The analyzer input was 27dBm (1/2w) max! A careless moment can cost thousands of dollars and weeks of lost time.
My rules are: "Don't just do something. Just stand there! (And think.)", and "Double check your math before hitting power on, not after the smoke comes out".
Another, luckily somewhat uncommon, lapse is the use of an underwattage attenuators. Think! If you need to pad down a 1000w amp output by 3dB, why would you use a 1watt, 3dB attenuator? The answer is always, "I didn't think about that!" By they way, an exploding attenuator teaches a great lesson by itself.
I have long claimed that if I could get a nickle a pound for all the stuff my guys have destroyed I could be a retired millionaire today.
So very true, I often see people hit the "auto" button on a scope rather than configuring for what they need. I had another Engineer that would use single sweep over and over rather than using normal triggering. It was so frustrating to watch I wanted to push him aside and just reselect the trigger.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 5
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
A quick look into the merger of two powerhouse 3D printing OEMs and the new leader in rapid prototyping solutions, Stratasys. The industrial revolution is now led by 3D printing and engineers are given the opportunity to fully maximize their design capabilities, reduce their time-to-market and functionally test prototypes cheaper, faster and easier. Bruce Bradshaw, Director of Marketing in North America, will explore the large product offering and variety of materials that will help CAD designers articulate their product design with actual, physical prototypes. This broadcast will dive deep into technical information including application specific stories from real world customers and their experiences with 3D printing. 3D Printing is
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
5 
