My car, a ’98 VW Golf Mk4, did not have any trouble with a 100W strong RF field emitted from a “magmounted” antenna on top of the roof. However, there were some quite strong RF noise coming from the car electronics. I did not have time to investigate, it could possibly be solved with the usual EMC tactics of ferrite cores on cables and so on, so for working the weaker contacts I simply turned off the ignition.

During the 180km trip, I made 4 check-ins with the reference station, as well as some contacts with other participating mobile stations. The map shows received (R) and sent (S) signal reports and the distance to the reference station for each check-in.

My “shack power supply”, a Palstar SPS8250, is providing me with 13.8V at up to about 25A for various pieces of radio equipment. It is mostly running with a very low load, 1-2A, and don’t really need the active cooling of a fan. Since the fan is a bit noisy, I made a simple thermostat so that the fan could stay mostly off and only go to full speed at high load and temperature. It is really simple, with an AVR and a few bits of C code to read an LM335A temperature sensor through the ADC and then simply switch the fan through a FET. Two potentiometers are also read through the ADCs to supply high and a low thresholds for the hysteresis. In the picture above, you can see the crude veroboard bolted to the fan in the back of the power supply. The two trimmers are at the bottom, just behind three Berg pins; one connected to ground and the other two to the wipers. This way the thresholds can be read simply by a voltmeter and some basic arithmetic (0-5V = 0-100°C). The LM335A can be seen wrapped in heat-shrink tubing and glued to the heat sink. Except for that I run the whole thing off a supplied fan connector on the main PCB, which apparently has some kind of built-in speed limiting for the fan, it works fairly well. I have it set with about 50°C and 35°C as the high and low thresholds. See full source code at github: avr-thermostat and my ugly schematics/working notes below.

Get it as a stand-alone HTML page, in XML (probably the best if you want to add it to your own Google Calendar), iCalendar (for almost all calendar applications) or embed it in your own site.

I recently bought a digital storage oscilloscope (DSO) from DealExtreme for the ridiculously low price of $360 (including shipping!). The Rigol DS1052E is widely known among hobbyists as delivering very good value-for-money – not hard to understand when you compare the price to the basic specifications: 2 channels, 1GSa/s, 50MHz bandwidth. USB for saving screenshots, raw data (CSV), printing and computer control. Nice selection of triggers, filters, zoom and so on. Many time and voltage measurements available. 1Mpts memory depth. FFT and more. Good build quality and feel. And what’s even better: the scope can be modified by software to provide a claimed 100MHz bandwidth. For me, this is a tremendous leap up from my old analogue scope, especially in terms of functionality. But what about the performance? And will the 100MHz modification deliver?

After performing the reasonably easy modification to up the BW to 100MHz, I decided to test one of the most basic properties of an oscilloscope; its bandwidth. Basically how fast a signal you can measure. In practice you need some overhead to be able to see distortions, noise and so on. For digital signals, you need a lot of overhead. A perfect square wave, as in a (theoretically perfect) digital signal, is composed of an infinite number of harmonic sine waves. A suggested rule-of-thumb is to use a BW five times the fastest clock rate, or in other words, enough bandwidth to see the 5th harmonic. A rule-of-thumb for analogue signals is to use a scope with 3 times the bandwidth of the signal. Depending on exactly what you are using the measurements for, and what kind of accuracy you need, the required bandwidth can be both lower or higher than this. There is a really nice paper by Agilent called Evaluating Oscilloscope Bandwidths for Your Application. It explains the rules of thumb mentioned above as well as how you can more scientifically evaluate the necessary bandwidth based on the desired accuracy.

Above is a sweep of an 1GHz BW Agilent scope where a sine-wave signal generator is set to sweep across a large frequency span in a short time. The timebase and triggering on the scope is adjusted so that the complete span is shown in one scope sweep. Pretty neat.

I tried doing the same on my scope, but it looks like the sampling rate at large timebases is slowed down and the measurements didn’t make any sense. Instead I decided to rely on the built-in voltage measurements and manually step the generator through the frequency range until the voltage dropped by 3dB. The measurement functionality on the scope is a little fiddly and changes a bit by varying the timebase. I decided to give the scope the benefit of the doubt and used a timebase displaying more than 10 cycles of the wave as this seemed to produce the most consistent results. (1-5MHz: 500ns, 5-10MHz: 200ns, 10-20MHz: 100ns, 20-50MHz: 50ns, 50-100MHz: 20ns, 100-175MHz: 10ns).

The generator I borrowed for this experiment is specified with an accuracy of +/-1dB and a flatness across its frequency range of 0.3dB at 0dBm output. I connected it to the scope through a short, good quality coax cable, a T-adapter and a 50ohm terminator (as the scope only has a 1Mohm input). I set the output level to 0dBm.

$$0dBm = 224mV_\mathrm{RMS}$$

At 1MHz, it gave a result of 218mV on the scope which I suppose was a combination of generator inaccuracy, cable/connector loss and scope measurement inaccuracy. The cable loss should however be pretty flat across the bandwidth (estimated <0.1dB at 150MHz).

$$218mV_\mathrm{RMS} – 3dB = 154mV_\mathrm{RMS}$$

As you can see from the screenshots above, there is a bit of distortion to the sine wave at 120MHz (the frequency counter shown is a little bit off). It is almost like the signal is a bit amplitude modulated.

I went on to manually sweep the signal generator from 1MHz to 175MHz and record the readings off the scope. All this hard work resulted in the graph below. -3dB at 127MHz and -2dB attenuation at 100MHz. Not too bad! Although the flatness could have been much better over the lower frequencies, I guess the DS1052E does deliver what it promises – and even more with the software modification.

The complete set of measurements and code (Octave/MATLAB and gnuplot) for generating the graph are available on github: ds1052e-measurements.

On the subject of typography, I’ve also added a plugin for producing stunningly beautiful mathematical expressions. MathJax is a really impressive JavaScript library that interprets TeX and LaTeX (or MathML) syntax for mathematical formulas and outputs code that successfully degrades depending on your browser’s capabilities. MatJax comes in a nice plugin called MatJax-LaTeX.

$$\frac{4}{\pi}\left (\sin(\pi ft)+{1\over3}\sin(3\pi ft)+{1\over5}\sin(5\pi ft) + \cdots\right )$$

Lastly, I’m using the SyntaxHighlighter Evolved plugin which employs Alex Gorbatchev’s SyntaxHighlighter to produce nice looking syntax highlighted code snippets. Behold the beauty of Duff’s device:

 register n = (count + 7) / 8;      /* count > 0 assumed */

 switch (count % 8)
 {
 case 0:        do {  *to = *from++;
 case 7:              *to = *from++;
 case 6:              *to = *from++;
 case 5:              *to = *from++;
 case 4:              *to = *from++;
 case 3:              *to = *from++;
 case 2:              *to = *from++;
 case 1:              *to = *from++;
                    } while (--n > 0);
 }

Cold draft from a pair of doors.

This weekend I got the chance to borrow the thermographic camera from work. The idea was to do an inspection of the house insulation, but it’s also a fun tool to just play around with. Below, my cat. The coldest spot? The moist nose.

Hedy (the cat)

Finally! The Galileoscope arrived today! I’ve never used a telescope before and have been looking forward to trying out this very affordable ($15) product. I’ve uploaded a few photos from the unboxing to flickr.

Assembly was very easy and I had no problem locating Jupiter and spotting four of its moons with the 25x magnification. Using the 50x magnification was a lot harder and probably needs some more practice. Below: getting a shot of Jupiter with a compact camera through the telescope turned out to be next to impossible, so I used Stellarium to create a simulated image of approximately how Jupiter looked through the Galileoscope (the second object from the right is no moon).

1. Take one nice and wide elastic band and pull the ends through the venting holes on a Peltor G22 hard hat.
2. From the inside, adjust the size of the elastic loop to fit the player and tie the ends together with a wire tie.
3. If you want, add some friction between the player and hat surface by attaching a big furniture feet pad. I used a felt pad, rubber would be better.

32 bottles of Coopers Real Ale ready for capping and bottle fermentation.

While waiting for the ferry in Hirtshals last summer, I shot a few photos. Below is a panorama stitched together with Hugin from 29 images. After cropping, the pixel count is ~46M. I think Hugin does a really good job, it is hard to notice any “stitch lines” or artifacts.

Click the image above to get the full resolution.

Apache VirtualHost configuration

<Proxy>
    Order deny,allow
    Allow from all
</Proxy>
ProxyRequests off
ProxyPass /cc/ http://localhost:3333/
ProxyPassReverse /cc/ http://localhost:3333/

CruiseControl.rb: config/environments/production.rb

ActionController::AbstractRequest.relative_url_root = "/cc"