A Spicy Regenerative Reciever

We recently posted a three-part series about using LTSpice to simulate electronic circuits (one, two, three). You might have found yourself wondering: Can you really simulate practical designs with the program? This quick analysis of [QRP Gaijin’s] minimalist regenerative receiver says “yes”.

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Transforming Spice

Spice is a circuit simulator that you should have in your toolbox. While a simulator can’t tell you everything, it will often give you valuable insight into the way your circuit behaves, before you’ve even built it. In the first installment of this three-part series, I looked at LTSpice and did a quick video walkthrough of a DC circuit. In the second, I examined two other parts of Spice: parameter sweeps and AC circuits. In this final installment, I want to talk a bit more about real-world component performance and also look at modeling transformers.

Recap

lowpasssLast time we looked at a low pass filter, but it wasn’t practical because the components were too perfect. Only in simulation do voltage sources and wires have zero resistance. There was no load resistance either, which is unlikely. Even an oscilloscope probe will load the circuit a little.

The resulting AC analysis showed a nice filter response that was flat to about 1 kHz and then started roll off as the frequency increased. Suppose the source had an 8 ohm series resistor. How does that change the circuit response?

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Spice Power

Spice is a circuit simulator that you should have in your toolbox. While a simulator can’t tell you everything, it will often give you valuable insight into the way your circuit behaves, before you’ve even built it. In the first installment of this three-part series, I looked at LTSpice and did a quick video walkthrough of a DC circuit. This time, I want to examine two other parts of Spice: parameter sweeps and AC circuits. So let’s get to it.

schem2In the first installment, I left you with a cliffhanger. Namely the question of maximum power transfer using this simple circuit. If you run the .op simulation you’ll get this result:

--- Operating Point ---
V(n001): 5 voltage
I(R1): 0.1 device_current
I(V1): -0.1 device_current

The power in R1 (voltage times current) is .5 W or 500 mW if you prefer. You probably know that the maximum power in a load occurs when the load resistor is the same as the source resistance. The Rser parameter sets the voltage source’s internal resistance. You could also have created a new resistor in series with V1 and set it explicitly.

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Adding Spice to Your Workbench

Most of us didn’t fight in World War II, drive a race car, or fly the Space Shuttle. But with simulation, you can experience at least some of what it would be like to do those things. Granted, playing Call of Duty isn’t really the same as going to war. No matter what you are simulating, it only goes so far. However, you can get a lot of value from a simulation. I’d bet the average kid who has played Call of Duty knows more about WWII locales and weapons than my high school history teacher.

When it comes to electronics, simulation is an excellent way to get insight into a circuit’s operation. After all, most circuits operate in the abstract–you can’t look at an audio amplifier and see how it works without a tool like a scope. So simulation, when done well, can be very satisfying. You just have to be careful to remember that it isn’t always as good as the real thing.

That’s Spicy

One of the best-known electronics simulators is Spice, which Berkeley created in 1973. In its original form, you had to punch cards that described your circuit and the analysis you wanted to perform. Modern PC versions sometimes replace the deck of cards with a text file. The best modern versions, though, give you a GUI that allows you to draw a schematic and then probe it to see the results.

There are several paid and free versions of Spice (and other simulators) that include a GUI. One of the best for a casual user is the free offering from Linear Technology called LTSpice.

Linear makes LTSpice available and populates it with models for their devices in the hopes you’ll buy components from them. However, the software is entirely usable for anything, and it has a powerful set of features. Linear produces the software for Windows, but I can attest that it runs just fine under Wine on Linux. The Web site will invite you to register, but you don’t have to if you don’t want to.

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Using LTSpice to Measure Total Harmonic Distortion

Audiophiles spend a lot of time and effort worrying about audio specs like Total Harmonic Distortion (THD). Makes sense, because THD affects the quality of audio reproduction. However, THD can also affect interference from radio signals and even losses in power transfer systems. A simplified definition is the THD is the ratio of the sum of the power of all harmonic frequencies to the power of the fundamental frequency.

If a circuit produced a perfect sine wave, there would be no harmonics. There are many ways to measure THD in practice, but [Michael Jackson] has an interesting video showing how he easily visualizes THD using LTSpice. Assuming you already have the system in question in LTSpice (or you could use another simulation tool, if you prefer) it is fairly straightforward.

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A Breadboard In A Browser

[Flownez] sent in a tip that a port of the venerable Falstad circuit simulator is now available that doesn’t require Java (it uses HTML 5). This is a welcome port since some modern browsers (particularly Chrome) make it difficult to run Java applets and prevented the Falstad simulator’s execution.

spice2Like the original simulator, this one is great to show a classroom circuits and encourage building or studying circuits in the browser. There’s no extra software to install, which is handy for an impromptu demo. Another cool feature is the visualization of current flow as animated dots. The dots move in the direction of the current flow and the speed of motion is proportional to the amount of current. Watching a capacitor charge with the moving dots is very illustrative. You can also view data in a scope format or hover the mouse over things to read their values.

You can open a blank circuit and add quite a few components (use the right click button on your mouse or the menu to add components and wires). However, you can also pick from a number of predefined circuits ranging from the simple (a voltage divider, for example) to the illustrative (a PLL frequency doubler comes to mind). There’s even an AM radio (see below) that you can tune to find several “stations” by varying the tuning capacitor’s value. Circuit elements include many types of analog and digital components.

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Sine Waves, Squares Waves, and the Occasional FFT

I became aware of harmonics and the sound of different shaped waveforms early in my electronics career (mid 1970’s) as I was an avid fan of [Emerson Lake and Palmer], [Pink Floyd], [Yes], and the list goes on. I knew every note of [Karn Evil 9] and could hear the sweeping filters and the fundamental wave shapes underneath it.

bil1

I remember coming to the understanding that a square wave, which is a collection of fundamental and (odd) harmonics frequencies, could then be used to give an indication of frequency response. If the high frequencies were missing the sharp edges of the square wave would round off. The opposite was then true, if the low frequencies were missing the square wave couldn’t “hold” its value and the top plateau would start to sag.

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