23 August 2012

How to measure coax cable velocity factor

A recent (2010 AD!) discussion on an Italian forum tried to answer the question: "how to measure coax cable velocity factor?" Depending on the cable dielectric and size, vf is between 0.6 and 0.9.

If you have an antenna analyzer, such as the MFJ-259, it's easy, since the device will do it for you. Just follow the instructions on the user's manual.

If your instrument doesn't allow that, then you will have to proceed with trial and error. For example connect a known non-inductive impedance (resistance) on one end and look for the frequency where the same impedance is shown on the other side of the unknown cable. This condition occurs at multiples of lambda/2, so choose your frequency accordingly. The resistance must be different from the line impedance.
Edit 2013-10-22. OM Don adds that due to the intrinsic feed line attenuation, the resistance seen at the generator's side will be less than the face value. Obviously "how much?" depends on line length and test frequency, but could be in the order of ohms or fractions.

With a two-channel oscilloscope another comparison can be done, on two sections of the same cable, same length. Feed cables with a power splitter and observe the signal at both far ends: if waveforms are out of phase, then your two cable sections have different vf.. From this point you can use your imagination to work out other measures the o'scope can provide...

09 August 2012

Raspberry Pi arrived and tested

My RasPi from Farnell arrived 3 days after the shipping notificaton. It came with regular mail from UK.

Setting it up was not so straightforward:
  • pushing the operating system ISO to the SD card was not so easy with MS Windows
  • I had no USB keyboard at home
  • I have no HDMI-capable screen, at home
  • I had no DHCP-enabled network to connect it to
With the help of a colleague I got a working SD card and put my RPi on the net. I ran few apt-get's to update it and install gcompris and childsplay packets. Unfortunately there isn't much choice of software ported to the ARM processor. No problems to get a vncserver running at a decent resolution or 1280x900 over the 100 Mbps network link.

At home I used the composite video out to test how functional it can be, but the result is not encouraging. Both childsplay and gcompris require a minimum screen resolution that is larger than what is available on TVout, while they run OK on the existing home computer + LCD screen.

On the other hand it may work as a digital media center with OpenELEC, but I haven't had the time to test it in the real world. But I can imagine the burden of wires around/behind the TV if an external hard-disk is added, then a USB-to-WiFi adapter, a powered USB hub ... not charming at all!

My RasPi test drive impressions are that it has enough computing power to do interesting things, with an excellent performance/price ratio. I am a bit skeptical about its usefulness in bringing youngsters closer to in-depth computer science and programming. And I doubt I will have much use for it. :-(