Thursday, 23 May 2013

DATV-Express still alive

For those that have asked the project is still very much alive.
I have been working on the Linux code. At the moment I am
having some difficulties with libusb1.0.x. Under Ubuntu 32 bit
I get the occasional hang at start up. However under Ubuntu 64 bit
I can't get it to work at all. This all worked fine when using Fedora
and an earlier version of libusb. Oh well I will eventually sort it out.

Apart from that I have been working on a GPSDO. I already have
a Z3801A Frequency standard but I wanted something for portable
use that would use less power. I recently installed a proper GPS
active antenna that came from a CSMA base station on the roof
and the GPSDO never goes into holdover now. All my test gear
and most of my radios now have GPS derived timing.

DATV-Express requires a reference frequency of between 20 MHz
and 100 MHz. I have bought a 40 MHz tcvcxo which should do
nicely. I am not going to use the 10 KHz output from an old GPS,
instead I am going to use the 1 PPS signal from an Adafruit
supplied GPS, these units are much lower powered than the old
Jupiter T GPS and have many more receive channels. They also
have a claimed short term jitter of 10nS on the 1PPS signal.
I will post a picture of the completed unit when it is done
(I am waiting on some parts at the moment).

This summer I am thinking of upgrading the antennas the current
favourite is a mesh dish kit from these people RFHAMDESIGN
The biggest issues seem to be; bandwidth of their dish feeds,
whether I should go for a 1.9m or 2.4m dish and do I really want to
add elevation control to the dish for any potential activity on DATV
from the ISS. I am in a coastal location so wind loading and corrosion
will also play a big factor in any choices I make. Although I have space
for the 2.4m dish I will probably end up with the 1.9m one. I have put
up a number of antennas in the past that have been physically too big for
this location.

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