John's Geekblog

Monday, September 29th

Loran results with LPF installed


Over the weekend, I built a low pass filter for the Loran receiver. Details are at http://www.febo.com/geekworks/antennas/amrad-lpf.html.

It seems to have solved the dirurnal phase jump problem once and for all, and although I only have about 12 hours of data so far to go on, it looks like the trace is generally cleaner now -- which makes sense since I've now removed the 30dB attenuator that I had in place before. The net result is that the interfering signal is much reduced, but the desired signal is 30dB stronger than it was.
jra on 09.29.03 @ 01:35 PM EDT [link]


Sunday, September 14th

Temperature plotting


By the way -- I now have my indoor/outdoor temperature plotting up and running. The graphs are updated every 15 minutes and uploaded to http://www.febo.com/geekworks/therm.html.
jra on 09.14.03 @ 03:38 PM EDT [link]


HP5065A Offset


Here's a plot of the HP5065A with the current C-field setting. The offset is 1.27x10-12. I believe the near-sine-wave phase variation in this plot is related to outside temperature variations, possibly due to phase shift in the active antenna. It doesn't correlate to the temperature in the basement where the equipment is.
jra on 09.14.03 @ 03:17 PM EDT [link]


Wideband power measurement


Yet more on the Loran phaseshift issue. Yesterday I fired up my HP3586C selective voltmeter and wrote a GPIB program to log data from it. The 3586C can measure wideband power between 20Hz and 32MHz with a resolution of 0.01dB. I logged data for 24 hours at 1 minute intervals, and here is the result.

It shows that the daytime power ievel is about -6dBm, which is tough enough for any front-end to deal with; when the local AM stations shift to night-time operation, the power level increases abruptly to about -3dBm. Then at daylight, the power level drops back down again. This means that the Austron receiver front-end has to deal with suddenly seeing twice as much power, and this is almost certainly the cause of the phase shift. (And actually, the situation is worse than this because I'm measuring power off a simple transformer splitter; when the meter is disconnected, the power level at the receiver will go up by 3dB.)

I have the parts for a 500kHz low pass filter ordered, and will get that built soon. It should solve the problem nicely.

I've continued to run the Austron receiver with a 30dB attenuator in front of it, and although this solves the sudden phase jump problem, I still have a diurnal phase change of about +/-50 nanoseconds. I suspect this is temperature related due to the sine-wave shape of the plot, which looks a lot like the outside temperature plot. More on this later.
jra on 09.14.03 @ 12:31 PM EDT [link]


Saturday, September 6th

HP5065 Offset update


I just did a check of the offset on a fairly linear piece of data at the end of the run, and came up with an offset of -4.5x10-12. That's substantially more, and in the opposite direction, than the measurements I made last week. This sample covered only about 12 hours. Because I'm still seeing the diurnal effect, I'm not sure if these measurements are what they should be. I really need to get a comparison against GPS to validate.

In any event, I'm adjusting the frequency up one minor division, to 1.84, and will start a new run with that setting.
jra on 09.06.03 @ 11:22 AM EDT [link]