Mobile field-day 2011

Yesterday I participated in what has become an annual event, a mobile field-day where radio amateurs in the district drive around in the countryside and make radio contacts on HF, mainly 80m (3.5MHz), as well as on the higher bands. Some of the participants stop on the side of the road and put up temporary antennas, usually low-hanging dipoles for NVIS, others have antennas mounted on the car. This was the third time I participated, and the first time I had an HF-antenna mounted to my car. The organizing team put up a reference station, with the call sign LA1G/P, for comparing antenna systems and radio coverage, as well as serving as a final destination for the trip to meet up and have a chat with other people. This year, like previously, the reference station was set up in the parking lot at Vindfjelltunet.

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.

Fan thermostat with an AVR

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.

NAC calendar

I’ve created a public calendar for the amateur radio Nordic Activity Contests on VHF/UHF/SHF. The regular Tuesdays and Thursdays are set to auto-repeat, but the fifth Tuesdays are updated manually (for now, about a year into the future).

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.

Assessing the Rigol DS1052E digital oscilloscope

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.

Frequency sweep over the full BW of an Agilent scope

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.