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...

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. :-(

Raspberry PI from Farnell

Yesterday I received an email from Farnell stating that my RPi had been shipped (order #ORP051xxx). Since I ordered it the same day I submitted my order to RS, I was expecting a similar 11 weeks delivery time. But it got on my way about 50 days early. Could the recent increase in production capacity have something to do with it? Or most people ordered their copy from RS, since the communication was a tad better?

There is apparently no way to know which express courier Farnell used, nor to get a tracking number. On the other hand, RPi @ RS was paid right away, while Farnell charged my card on the shipping day.

Still no shipping news from RS, though.

DX's EzTV668 lacks ESD protection diode too

It is amazingly easy to open the EzTV668 RTLSDR USB dongle bought at DealExtreme: make a little pressure with a small flat screwdriver on both sides of the USB connector and the cover will pop up.

I wanted to check whether my dongle has the ESD protection diodes installed...

I was not surprised to find an empty U7 location next to the antenna connector. BAV99 should be the part missing, which costs next to nothing. Plus shipping, of course.

I.F.R. current

After adding the tuning knob and on request of a fellow builder, I checked the IFR current. The DVM read 16mA with a LCD without backlight. If you need a backlight then add the LED current, 10 to 20mA.

In cases where the power budget is strict, select an LCD that can be used in both cases (like the classic black/green) and add a switch for controlling backlight (or a trimpot).

Raspberry PI: ordered

The "invitation to order" email came one day late and I have just paid for my RasPI. Order #127xxx, despatch expected within 11 weeks (or 77 days, or mid September).

COUNTDOWN TIMER WIDGET

(up here is a countdown widget in Flash, in case you don't see it)

avreal32 and lock bits

I feel dumb. I was expecting to see AVR's lock bits LB1 and LB2 amongst FUSES in avreal32 programmer, so I never saw that there is a dedicated option for preventing the readback of the chip content.

The command for locking an ATmega168 is (add your usual programmer definition and options):

avreal32 +mega168 -l2 (programmer parameters)

Neat.

SDR# on a Centrino CPU

Not a big achievement, but I want to report that I could use the RTLSDR dongle with SDR# on a laptop running WinXP with an Intel Centrino 1.6 GHz CPU. I have reduced the sampling to 1 Msps and the machine was running at 50% CPU: not bad for a 2004 computer! I listened to both WFM and SSB signals. There is probably room for a higher sampling rate, but keep in mind I have been listening to decoded output and not fed it over to another software (which in turn uses its share of CPU cycles).