Things I Learned: Winding Toroids; QRP SWR meters; Logarithmic Amplifiers

While my Masters dissertation hasn't been made public domain yet, I found the pages pertaining to the logarithmic amplifier in one of my way back machines—a briefcase I won in science fair that I used to cart things around in graduate school. I also found cassette tapes of talks from the 2000 Java One conference hosted in San Francisco: one on aspect oriented and another on testing object oriented systems . If you ever wondered how people reviewed conference talks before the age of Zoom and YouTube, in this case, it was cassette tapes.

Prepping to construct a standing wave ratio, (SWR), meter for the Rockmite has brought me to the end of years of trepidation I developed reading amateur radio books about winding toroids. After spending a few decades with fears of ‘ruining a toroid’ permeating me like the magnetic field through a FT50-43 ferrite core, I’ve finally come up with a solution: order more magnet wire than I need. Gosh. That was easy.

The instructions I found for toroid winding—I swear I’ve read them before—involve a series of pictures that my fear-addled mind still has a bit of an issue decoding. I’m going to ask the 11, 10, and 7 year-old gang here if I can make a video of them winding the toroids for the SWR meter—once we have more wire. I figure this will solve a number of problems:

They have arguably similar (better?) fine motor control compared to me at this point and smaller fingers. That’s got to help. Right?
I won’t be wrapping the toroid. Yeah, I’m still not up to facing my fear quite that fully.
We’ll wind up with a video of how to wrap the toroid for people who learn better from more dynamic forms of documentation.
The gang will have wrapped a toroid successfully—no matter how many tries that takes—therefore—hopefully—ensuring they don’t think toroid wrapping—let’s face it: an important life skill—is a big deal.

Bwaahahaha! I just used three sets of M-dashes in a single sentence—I answer to no one!

OK, back the logarithmic amplifiers!

Mark Smith KR6ZY mentioned that the issue with QRPp (less than 1 watt) SWR meters is getting a reading at all because in the following circuit taken from his video:

The two diodes have to drop whatever their forward bias voltage is (on the order of 1 volt) before they conduct sending current to the meter. At QRPp levels, the voltage on their anode might never reach a volt. Mark has a solution, logarithmic amplifiers, (also discussed in an episode of the Ham Radio Workbench podcast featuring Mark and this circuit.

Note: I’ve found one source (Lloyd's demos) that indicates the Parts and Kits SWR meter will do just fine at QRPp levels with a little finagling. I’m going to try this version of the project first to see what I see (in large part due to a months old request from Daize, the 11 year old to learn how to use a small LCD screen for graphics).

Pictures from the previously mentioned project on Lloyd's demos.

I used logarithmic amplifiers in a beam positioning monitoring circuit at the Proton Storage Ring in Los Alamos. The purpose of the circuit is to monitor the position of the proton beam, hopefully preventing it from running into the wall of the accelerator chamber scattering radiation all around. My usage involved the Analog Devices 640 amplifier.

AD640 characteristics

The AD 640 has a current output rather than the voltage output of the AD 8307 that Mark discusses. The main reason for using the AD 640—I believe—was because the beam position monitor circuit summed the outputs of the two logarithmic amplifiers attached to two opposing electrodes in the acceleration chamber to determine the position of the beam. Summing currents is kinda easy what with resistors to ground and all.

Anyway:

I came across this great post on a Raspberry Pi web server from the Matt Webb, the person who invented the Anyway section! It’s a great little piece of synchronicity because one of the kids and I are discussing pairing Raspberry Pi Pico web servers with Rockmites to build a networked farm of ham radio transceivers in our nearby forest.

h! Speaking of forests and kids and unschooling: Raptor Day!

Has anyone devised a metric for measuring books by the length of M-dashes—laid end to end of course—they employ rather than word count? Just wondering.

Anyway.

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Copasetic Flow

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