Designing a High Performance Parallel Personal Cluster

Kristina Kapanova is a PhD student at the Bulgarian Academy of Sciences. Her research is taking her to simulations of quantum effects in semiconductor devices, but this field of study requires a supercomputer for billions of calculations. The college had a proper supercomputer, and was getting a new one, but for a while, Kristina and her fellow ramen-eating colleagues were without a big box of computing. To solve this problem, Kristina built her own supercomputer from off-the-shelf ARM boards.

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Unlock the Phase Locked Loop

If you want a stable oscillator, you usually think of using a crystal. The piezoelectric qualities of quartz means that it can be cut in a particular way that it will oscillate at a very precise frequency. If you present a constant load and keep the temperature stable, a crystal oscillator will maintain its frequency better than most other options.

There are downsides to crystals, though. As you might expect, because crystals are so stable it’s hard to change the frequency much when you want a different one. You can use a trimming capacitor to pull the frequency a little, but to really change frequency, you have to change crystals.

There are other kinds of oscillators that are more frequency agile. However, they aren’t usually as stable. To combine flexibility with crystal-like stability, you can use a Phase Locked Loop (PLL). Many modern systems use direct digital synthesis, but the PLL is a venerable and time-tested technique.

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Oscillator Design by Simulation

[Craig] wanted to build a 19.2 MHz crystal oscillator. He knew he wanted a Pierce oscillator, but he also knew that getting a good design is often a matter of trial and error. He used a 30-day trial of a professional simulation package, Genesys from Keysight, to look at the oscillator’s performance without having to build anything. He not only did a nice write up about his experience, but he also did a great video walkthrough (see below).

The tool generates a sample schematic, although [Craig] deleted it and put his own design into the simulator. By running simulations, he was able to look at the oscillator’s performance. His first cut showed that the circuit didn’t meet the Barkhausen criteria and shouldn’t oscillate. Unfortunately, his prototype did, in fact, oscillate.

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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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