Lasers are pretty much magic — it’s all done with mirrors. Not every laser, of course, but in the 1980s, the most common lasers in commercial applications were probably the helium-neon laser, which used a couple of mirrors on the end of a chamber filled with gas and a high-voltage discharge to produce a wonderful red-orange beam.

The trouble is, most of the optical power gets left in the tube, with only about 1% breaking free. Luckily, there are ways around this, as [Les Wright] demonstrates with this external passive cavity laser. The guts of the demo below come from [Les]’ earlier teardown of an 80s-era laser particle counter, a well-made instrument powered by a He-Ne laser that was still in fine fettle if a bit anemic in terms of optical power.

[Les] dives into the physics of the problem as well as the original patents from the particle counter manufacturer, which describe a “stabilized external passive cavity.” That’s a pretty fancy name for something remarkably simple: a third mirror mounted to a loudspeaker and placed in the output path of the He-Ne laser. When the speaker is driven by an audio frequency signal, the mirror moves in and out along the axis of the beam, creating a Doppler shift in the beam reflected back into the He-Ne laser and preventing it from interfering with the lasing in the active cavity. This forms a passive cavity that greatly increases the energy density of the beam compared to the bare He-Ne’s output.

The effect of the passive cavity is plain to see in the video. With the oscillator on, the beam in the passive cavity visibly brightens, and can be easily undone with just the slightest change to the optical path. We’d never have guessed something so simple could make such a difference, but there it is.

Implementing PoE is made interesting by the fact that not every Ethernet device wants power; if you start dumping power onto any device that’s connected, you’re going to break things. The IEEE 802.3af standard states that the device which can source power should detect the presence of the device receiving power, before negotiating the power level. Only once this process is complete can the power sourcing device give its full supply. Of course, this requires the burden of smarts, meaning that there are many cheap devices available which simply send power regardless of what’s plugged in (passive PoE).

[Jason Gin] has taken an old, cheap passive PoE splitter and upgraded it to be 802.3af compatible (an active device). The splitter was designed to be paired with a passive injector and therefore did not work with Jason’s active 802.3at infrastructure.

The brain of the upgrade is a TI TPS2378 Powered Device controller, which does the power negotiation. It sits on one of two new boards, with a rudimentary heatsink provided by some solar cell tab wire. The second board comprises the power interface, and consists of dual Schottky bridges as well a 58-volt TVS diode to deal with any voltage spikes due to cable inductance. The Ethernet transformer shown in the diagram above was salvaged from a dead Macbook and, after some enamel scraping and fiddly soldering, it was fit for purpose. For a deeper dive on Ethernet transformers and their hacked capabilities, [Jenny List] wrote a piece specifically focusing on Raspberry Pi hardware.

[Jason]’s modifications were able to fit in the original box, and the device successfully integrated with his 802.3at setup. We love [Jason]’s work and have previously written about his eMMC adventures, repairing windows tablets and explaining the intricacies of SD card interfacing.

People talk about active and passive components like they are two distinct classes of electronic parts. When sourcing components on a BOM, you have the passives, which are the little things that are cheaper than a dime a dozen, and then the rest that make up the bulk of the cost. Diodes and transistors definitely fall into the cheap little things category, but aren’t necessarily passive components, so what IS the difference?

Resistors, Capacitors, Inductors, Transformers, Diodes*, and Memristors

That’s the list. Those are your passive components. Well, it’s not that easy. Also add in a bunch of types of sensors, because they are still passive. A photoresistor is a sensor but it’s still a resistor, even though its resistance changes based on an external influence. Any sensor whose measurement is a change in resistance, capacitance, or inductance still qualifies as a passive device. Also for fun let’s add a piezo buzzer.

The memristor is weird because it has only recently been proven to exist despite being theorized in the 70s, and is still not quite commercially available. There are now theories about meminductors and memcapacitors, which would also be passive devices, but they don’t exist yet.

It Depends on What Your Definition of Active Is

Part of the problem is it seems people have varying definitions of active. Rather than debunk all the wrong ones and spread bad ideas, here’s what’s correct. A device is active if any of these conditions are met:

• It is a source of power
• It amplifies power
• It acts as a switch

Applying this to the obviously active devices, like microcontrollers, it makes sense. It does all of those things on a GPIO pin. A transistor can amplify or act as a switch. A battery is a source of power.

A circuit remains passive until a single active component is added, so an RC or LC network is still passive. A piezo buzzer has an equivalent circuit of entirely passive elements, so it is also a passive device.

As a side note, every circuit has at least one active device (a source of power). Also, an electromechanical device like a physical switch is considered passive.

The Diode

There is an exception with the diode. The vast majority of the time, it is a passive device, so it’s handy to just add it to the list of passive devices and mostly forget about it. It wouldn’t be interesting, though, unless we delve into what makes it sometimes active for that single, and rarely used exception, and to do that we have to get into quantum tunneling.

The tunnel diode is very fast (microwave frequencies), and is used in frequency converters and detectors, especially in space where its resistance to ionizing radiation, low voltage, high frequency, and longevity are desirable qualities. There is a specific condition of the tunnel diode in which it has negative resistance so that increasing voltage results in decreased current. Even the tunnel diode acts like a normal passive diode everywhere except this special region.

A charged particle moving across a barrier needs enough energy to get over the barrier or else it can’t cross. With a normal diode there is a PN junction that acts as the barrier. A power supply gives enough energy (called the forward voltage) for the electrons to get over that barrier, and the current flows through it. According to quantum mechanics, though, there is a non-zero probability that the electron will just jump to the other side of the barrier without going over it. This is quantum tunneling. In most diodes the barrier is high enough (controlled by the doping of the PN junction), that the tunneling is unlikely, so no current will flow until there is enough forward voltage to get the electrons over the barrier. In a tunnel diode, the PN junction has a lot more doping, increasing the likelihood of tunneling. These diodes work at much lower voltages than normal diodes because of the high doping.

At really low voltages, the electrons tunnel frequently and there is some current. As the voltage increases, tunneling increases to a peak and starts going down. It goes down because the electrons on one side of the barrier have more and more energy, but there are not the same holes on the other side of the barrier to accept them from tunneling. Once the forward voltage is high enough, the electrons have enough energy to get over the barrier without tunneling, and the tunnel diode acts like a normal diode again. This behavior allows the tunnel diode to act as an amplifier or as an oscillator, which puts it into the active category. We covered negative resistance in the tunnel diode a few months ago, and a post on diodes kicked off the active/passive debate in the comments.

Does it Matter?

Nah, not really. This is well into the realm of the esoteric, and has no practical use other than to annoy people at parties and probably below in the comments. Active and passive are generic terms for components and whether a particular component is classified as one or another doesn’t change how it is used. Quantum tunneling is neat, though, and the fact that we have harnessed it makes me wonder how close we are to warp speed and teleporters.

An easy way to conceptualize active filters is thinking about audio speakers. A speaker crossover has a low-pass, high-pass and band-pass effect breaking a signal into three components based upon frequency. In the previous part of this series I took that idea and applied it to a Universal Active Filter built with a single chip opamp based chip known as the UAF-42. By the way, it’s pretty much an older expensive chip, just one I picked out for demonstration.

Using a dual-ganged potentiometer, I was able to adjust the point at which frequencies are allowed to pass or be rejected. We could display this behavior by sweeping the circuit with my sweep frequency function generator which rapidly changes the frequency from low to high while we watch what can get through the filter.

In this installment I’ll test the theory that filtering out the harmonics which make up a square wave results in a predictable degradation of the waveform until at last it is a sine wave. This sine wave occurs at the fundamental frequency of the original square wave. Here’s the video but stick with me after the break to walk through each concept covered.

It’s all about that edge

When looking in the opposite direction using the high pass filter (and bandpass) we see that there is energy right at the rising edge of the square wave. This is the infamous “edge” or what I have referred to as “the bite” meaning that the energy is this rising or falling edge is what leaks out and gets into FCC emissions or picked up by analog circuitry.

This rising edge can be described as representing part of an angular frequency, or Δv/Δt (pronounced “Dee Vee Dee Tee”) which is the change in voltage vs. the change in time. The faster the voltage change in a set period of time, the higher the equivalent frequency and energy.

There is a whole slew of semiconductor devices that freak out a bit when faced with very rapid transitions: SCR’s, TRIACs, transistors, MOSFETs and even diodes have been known to turn on with high Δv/Δt. The good news is you can find this susceptibility in the appropriate datasheets.

The rise time of a signal also has something to do with how much of the higher frequencies are present. Simply put the faster the rise time the higher the amount of energy present in the RF spectrum. If you’re interested in the math, I have seen this represented as F_KNEE = .5/T_Rise. F_KNEE is the point in the frequency domain where the spectra of energy rolls off (6.8db in this case) and T_RISE is the standard time it takes to get from 10% to 90% of full signal.

You might find surprising the number and accessibility of good filter design programs available directly on the web. Texas Instruments’ Webench starts out by asking what kind of filter; low, band, or high pass and then allows you to pick the attributes you are designing for in general. The next step allows you to observe firsthand the effects of different types of filters and the results are instantly available. In the old days, (before VisiCalc) we might have to crank through the equations repeatedly searching for the best compromise. Finally the program shows a schematic and a Bill Of Materials of standard parts. Again this eliminates the need to try and keep solving for the right set of parts where real resistor values could be used (typically 1% resistors).

Analog Devices Filter Wizard does essentially the same thing allowing the hobbyist or small lab the ability to define an accurate, effective filter without doing a single math equation using imaginary variables (such as the square root of negative 1). A good text back in the day was [Don Lancaster’s] Active Filter Cookbook, though my first exposure was from flipping every single page of the National Semiconductor Analog Databooks.

Finally, if you want to keep exploring this concepts here’s a breadcrumb to guide you: Did you know you can use the phase shift of a filter to create a sine waveform generator or that you can reverse bias the emitter-Base junction of many common transistors to create a white noise source useful for testing filter (or sound system) responses?

Today I am experimenting with a single chip Universal Active Filter, in this case I made a small PCB for the UAF-42 from Texas Instruments. I chose this part in particular as it facilitates setting the filter frequency by changing just a pair of resistors and the somewhat critical values that are contained on the chip have been laser trimmed for accuracy. This type of active filter includes Operational Amplifiers to supply gain and it supports various configurations including simultaneous operating modes such as Band Pass, Low Pass and High Pass make it “Universal”.

Filter Basics

Looking at the block diagram you can see where I have inserted a dual-ganged potentiometer to change both resistors simultaneously which should allow a straight forward adjustment for our purposes here.

Looking into the components of a simple RC filter which can easily implement a simple Low Pass or High Pass filter, we see that the math is fairly straight forward and swapping the components with each other is all that is needed to change the type of filter.

To tell the difference between the high and low pass examine the circuit shown where the capacitor is in series with the signal. Simply put, no Direct Current (DC) can make its way through a capacitor in series, so DC (0Hz) and other low frequencies are rejected or attenuated. Likewise when the capacitor sits across the signal (from the signal to ground) it can act like a battery; it stores energy and resists a change in voltage allowing slower, lower changes or frequencies to get through while resisting faster changes in voltage.

Whats Your Frequency Response

To demonstrate the frequency response of a system a Sweep Frequency Generator can be used. Here my old and venerable HP 3314A Function Generator is set up to sweep from 80Hz up to 5kz and pass it through the filter circuit. Looking at the input you can see the sweep start at a visibly low frequency and then sweep to a high frequency. Looking at the output one can see the amplitude of the different outputs and due to the way the oscilloscope is set up the output is synchronized with the start of the frequency sweep. This means that we can estimate the signal frequency based on its location on the scope.

When a Picture is Worth a 1,000 Hertz

Looking at the image we can see that the blue trace of the High Pass has an initial “blip” (a nonlinearity, possibly of the “ripple” variety) and that the amplitude of the signal on the right shows that it’s passing the higher frequencies. Likewise the green trace is passing the lower frequencies on the left and the pink trace a pass region in the middle.

Due to the adjustability provided by the dual resistors, I can adjust the filter frequency easily from low to high using my potentiometer. And if you watch the video you can see the effects of these adjustments, I tend to think of an analog synthesizer when I think of interacting filters sweeping the audio band, perhaps I can automate the adjustment of these filters in a following video.

Check back soon to catch Part 2 where I test some of what we talked about in earlier episodes about Square waves and harmonics. I also talk a little bit about the real-life math involved.