Your Amplitude May Vary

A signal generator is a very useful thing to have in an electronics lab. These devices can generate sine, square, and often many other waveforms, at a given amplitude and frequency. Just the thing for measuring frequency response in filter circuits — or generating a clock signal to drive a digital circuit.

An Agilent 33220A signal generator. (Click for larger.)

However, there’s a hidden “gotcha” on many of these devices. They don’t actually measure the amplitude of the signal they put out. They rely on an internal voltage divider and calibrated amplifier stage, and assume a given load impedance — typically either 50 ohms or high-Z (effectively, an open circuit). If the load impedance matches what the signal generator has been set for, all is well, and the amplitude will be more or less correct.

If the load impedance is significantly different from the menu setting on the signal generator, though, the amplitude can be off by as much as 100%. For example, if a signal generator is set for 1.0Vpp at 50 ohm impedance, but is actually connected directly to an oscilloscope with 1Mohm input impedance, the actual output amplitude will be right about 2.0Vpp.

The moral of the story? Never trust a smiling signal generator — at least as far as impedance is concerned. Whenever you change the load impedance (which can happen by simply changing frequency, if you have a reactive component to the impedance), it’s important to actually measure the real output amplitude — for example, with a ‘scope.

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Granddad’s Altimeter

One of the most interesting mementos that I have from my grandfather is a surplus mechanical altimeter from the early days of United Airlines. It was an unusual object for a kid to see on someone’s bookshelf — even given that Granddad was a maintenance supervisor for United. I would look at it and wonder what it was and why it was there — until one day I asked him about it.

Granddad's altimeter / barometer. (Click for larger.)

Granddad’s altimeter / barometer. (Click for larger.)

He told me that it was an aircraft altimeter — and that it still worked well, although it hadn’t been calibrated in a long time. Pressure altimeters, such as this one, work by comparing the ambient air pressure to the current theoretical sea level air pressure for a given location (measured by weather stations at airports.) Since air pressure decreases with altitude (at sea level, by about one inch of mercury for every 1000 feet), the difference in air pressure can be used to determine an aircraft’s current pressure altitude. When corrected for local weather-related changes in air pressure, a reasonably good estimate of the aircraft’s actual MSL altitudeĀ can be obtained.

Granddad, as it turned out, was using the altimeter somewhat backwards — as a barometer. Since the altitude where he lived was nearly at sea level, he could set the altimeter’s barometric setting to the standard value of 29.92, and then use it to monitor changes in the local air pressure. If it showed zero altitude — sea level — the local air pressure must be 29.92 inches of mercury. If it read lower, that meant that the air pressure was higher than average, and fair weather could be expected. When it started reading higher than sea level, the air pressure was lower than average, possibly indicating stormy weather.

I found that to be one of many valuable lessons that Granddad taught me over the years I knew him: Just because something wasn’t intended to do something, doesn’t mean it can’t do a perfectly good job.

Besides, it looks a lot cooler than a typical barometer — and makes such a cool, practical,
conversation piece.

Posted in Aviation, Nostalgia, Science | Leave a comment

Ladyada interviews Paul Horowitz

Now here are two celebrities actually worth watching. Limor “Ladyada” Fried, electronics geek extraordinaire and founder of Adafruit Industries, interviews Paul Horowitz (of Horowitz and Hill fame)! (Limor’s apparently interviewing Hill on the next one.)

Included at 3:15 is a shocking revelation:
Paul Horowitz never took an electronics course!!

Now — invite Hill, Jeri Ellsworth, and Steve Wozniak, and let’s really get this party going!

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Third Edition

The Book Is Retired. Long Live The Book!

Earlier this month, Electronics geeks around the world felt a disturbance in the Force. A presence they had not felt since 1989. This disturbance, however, was most welcome. The long-rumored Third Edition of The Book was actually here.

“The Book,” of course refers to H&H. The Bible. TAoE. The Good Book.
The Art of Electronics, by Horowitz and Hill.

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TAoE is technically a textbook, but finds its best use as a reference.


It’s a testament to how well-written and comprehensive TAoE is, that a quarter century after the second edition was published, it was, in many geeks’ minds, stillĀ the book on electronic circuits. Now, finally, H&H have updated the book, incorporating many new modern topics. The fundamentals haven’t changed since 1989, but digital electronics certainly has.

If you’re into electronics and don’t have one, go get one. It’s worth it.

Posted in Analog, Current Events, Design, Digital, Electronics, Fundamentals, HOW-TO, Nostalgia, Resources, Reviews, Troubleshooting | Leave a comment