Bandpass Filters From The CNC Mill

July 14, 2018 by 30 Comments

A bandpass allows a certain electrical signal to pass while filtering out undesirable frequencies. In a speaker bandpass, the mid-range speaker doesn’t receive tones meant for the tweeter or woofer. Most of the time, this filtering is done with capacitors to remove low frequencies and inductors to remove high frequencies. In radio, the same concept applies except the frequencies are usually much higher. [The Thought Emporium] is concerned with signals above 300MHz and in this range, a unique type of filter becomes an option. The microstrip filter ignores the typical installation of passive components and uses the copper planes of an unetched circuit board as the elements.

A nice analogy is drawn in the video, which can also be seen after the break, where the copper shapes are compared to the music tuning forks they resemble. The elegance of these filters is their simplicity, repeatability, and reproducability. In the video, they are formed on a CNC mill but any reliable PCB manufacturing process should yield beautiful results. At the size these are made, it would be possible to fit these filters on a business card or a conference badge.

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Milling A Flow Sensor To Safeguard A Laser Tube

September 29, 2017 by 16 Comments

Powerful lasers get powerfully hot and if you don’t keep them cool you’ll pay the price. After two such experiences [NixieGuy] got smart and milled this flow sensor as a failsafe.

Laser cutters are awesome. But acquiring one can be expensive, and keeping them in working order is no small feat. From the gunk that builds up as a byproduct of vaporizing the cutting stock, to keeping the optics focused correctly, it’s a game that forces you to become a laser cutter operator and not merely a user. One of the worst things to deal with is having to replace a burnt out laser tube. They do have a life to them but in this case the filter on the water cooling system clogged and the tube cooked itself. Twice.

Flow sensor shown in the upper right.

This flow sensor now acts as an interconnect with the laser enable line. Starting with an acrylic rod, [NixieGuy] machined out a center hole for a magnetic stopper, then milled three channels for water to pass around it. Each end of the rod was turned on a lathe to interface with plastic tubing of the water cooling system, and a slot was milled on the outside for a reed switch.

The demo video is below. You can see that when water flows it pushes the magnetic stop up (against gravity) where it engages the reed switch, allowing the laser to operate. If something impedes the flow of water (even if the pump still runs) the laser will be disabled and (hopefully) prevent future tube loss.

Want to see some of the oops moments faced by many a laser cutter operator? Check our guide on how to fail at laser cutting.

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Bessel Filter Design

July 29, 2017 by 28 Comments

Once you fall deep enough into the rabbit hole of any project, specific information starts getting harder and harder to find. At some point, trusting experts becomes necessary, even if that information is hard to find, obtuse, or incomplete. [turingbirds] was having this problem with Bessel filters, namely that all of the information about them was scattered around the web and in textbooks. For anyone else who is having trouble with these particular filters, or simply wants to learn more about them, [turingbirds] has put together a guide with all of the information he has about them.

For those who don’t design audio circuits full-time, a Bessel filter is a linear, passive bandpass filter that preserves waveshapes of signals that are within the range of the filter’s pass bands, rather than distorting them in some way. [turingbirds]’s guide goes into the foundations of where the filter coefficients come from, instead of blindly using lookup tables like he had been doing.

For anyone else who uses these filters often, this design guide looks to be a helpful tool. Of course, if you’re new to the world of electronic filters there’s no reason to be afraid of them. You can even get started with everyone’s favorite: an Arduino.

Real World RF Filter Design And Construction

March 30, 2017 by 13 Comments

We bet when [devttyS0] made his latest video about RF filter design (YouTube, embedded below), he had the old saying in mind: in theory, there’s no difference between theory and practice, but in practice, there is. He starts out pointing how now modern tools will make designing and simulating any kind of filter easy, but the trick is to actually build it in real life and get the same performance. You can see the video below.

One of the culprits, of course, is we tend to design and simulate with perfect components. Wires have zero resistance, capacitance, and inductance. Inductors and capacitance have no parasitic elements in our rosy design world. Even the values of components will vary from their ideal values and may change over time.

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Don’t Fear The Filter: Cascading Sallen-Keys

March 29, 2017 by 22 Comments

In the last edition of Don’t Fear the Filter, we built up two examples of the simplest and most-used active filter of all time: the two-pole Sallen-Key lowpass. This time, we’re going to put two of these basic filter blocks in a row, and end up with a much sharper lowpass filter as well as a bandpass filter. For the bandpass, we’ll need to build up a quick highpass filter as well. Bonus!

I claimed last time that the Sallen-Key lowpass would cover something like 80% of your filtering needs. (And 72.4% of all statistics are totally made up!) These two will probably get you through another 10% or so. Honestly, I’ve never built a standalone active highpass, for reasons we’ll see below, but the active bandpass filter that we’re building it for is a great tool to have in your belt, especially for anything audio.

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Low Parts Count ARM SDR

November 14, 2015 by 4 Comments

[Alberto di Bene] wanted to build an SDR for relatively low frequencies. Usually, you’d start with some front end to get the radio frequency signal down where you can work with it. But [Alberto] practically just fed an antenna into an STM32F429 Discovery board and did all the radio processing in the onboard ARM chip.

There is a little more to it than that, but only a little. If you open the PDF file on [Alberto’s] site, you’ll see there is a simple front end filter (a transformer, along with a few capacitors and inductors). This low pass filter prevents high frequencies from reaching the ARM processor’s analog to digital converter. In addition, a capacitor and a couple of resistors ensure the converter only sees positive voltages.

The CPU digitizes the incoming signal and processes it, demodulating several different types of radio transmission. The recovered audio is sent through the onboard digital to analog converter.

In addition to an input filter, the output also needs a filter to prevent high frequencies from reaching the speaker. Unlike the input filter, this one is a bit more complicated. The inductors needed for a passive filter were too large to be practical, so the output filter is an active one with a few transistors. The only other external circuitry is the power supply for the Discovery board.

The document does a great job of explaining the rationale behind the design choices and how the whole system works. It also includes simulations of both analog and digital filters used in the design.

This is really bare metal SDR and reading the code is educational. However, if you want to start with something simpler, consider GNU Radio and either an SDRPlay or a cheap RTL-SDR dongle.

 

Building Your Own SDR-based Passive Radar On A Shoestring

June 5, 2015 by 80 Comments

Let’s start off with proof. Below is an animation of a measurement of airplanes and meteors I made using a radar system that I built with a few simple easily available pieces of hardware: two $8 RTL software defined radio dongles that I bought on eBay, and two log-periodic antennas. And get this, the radar system you’re going to build works by listening for existing transmissions that bounce off the targets being measured!

I wrote about this in a very brief blog posting a few years ago. It was mainly intended as a zany little side story for our radio telescope blog, but it ended up raising a lot of interest. Because this has been a topic that keeps attracting inquiries, I’m going to explain how I did the experiment in more detail.

It will take a few posts to show how to build a radar capable of performing these types of measurements. This first part is the overview. In later postings I will go through more detailed block diagrams of the different parts of a passive radar system, provide example data, and give some Python scripts that can be used to perform passive radar signal processing. I’ll also go through strategies to determine that everything is working as expected. All of this may sound like a lot of effort, but don’t worry, making a passive radar isn’t too complicated.

Let’s get started!

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