Regular reports of my grabber activity and that of others, plus information on QRSS software, hardware and technique that comes my way

Friday, April 15, 2011

WA5DJJ's new mept

Dave, WA5DJJ, has brought together some ideas to produce the ultimate QRP Labs mept modification that is stable in both time and frequency to make it adequate for image stacking.  To jog ur memories, it is possible to stack successive grabs using an astrophotography program called Rot'N'Stack to pull really weak signals out of the noise. To do so the message must appear at exactly the same place on the grab from one to the next, i.e., must be stable in both time and frequency. The SNR improves as the square root of the number of images stacked.  I'll leave it to Dave to describe his mods but basically it involves using the 4-component temperature controller I described in Part 2 of my series here on this blog and the change to a PIC keyer designed by Mike, N0QBH.  This latter mod allows a 32 kHz clock xtal to stabilize the timebase.

Last night I recorded the signal from Dave' new rig and the results were nothing short of phenomenal.  Over a period of 4 hours he drifted only 1 Hz, which as I told him could easily be my rx...the wx is wonderful now and we're leaving the windows open in the shack resulting large temperature changes.  The times base was virtually jitter free and started exactly on time every 10 minutes.  Figure 1 shows his signal as recorded on my 5 hr grabber. 

Figure 1.  5 hour grabber showing Dave's signal

Each of the short pulses represents a 10 minute grab as normally seen on my regular grabber.  I used 25 of the 10 minute grabs, going from 0717z to 1114z, in the Rot'n'Stack stacking program to produce the averaged image shown in Figure 2.  Even though Dave's signal was strong and didn't need stacking to bring it out of the noise, this provided an acid test of his rig's stability in both time and frequency.  The degree to which it is off will appear as blurring or jitter.  The timing is virtually perfect and what little frequency blur there is is caused by the 1 Hz drift over the recording time.

Figure 2.  Stacked average of 25 grabs

In addition to Dave's signal, several surprises popped out of the noise.  VK6JY is using a stabilized Hewlett-Packard signal generator which responded quite well to the stacking process.  There was no hint of N4FRE or ZL1EE even on the 5 hr grab which has a much better SNR than the 10 minute grabbs.  Both have highly stabilized signals via atomic clocks and/or GPS.  FRE usually drops completely out this time of night and if any of his signal was left it was completely obliterated by P29ZL.  It's ironic to be using sophisticated software and equipment to extract a 'local' station out from under one so far away (with an antenna  up in a banana tree no less).  However ZL's mept is not stabilized in time and thus averages at the same time FRE's builds up.  QRSS does have it's moments.

Congratulations to Dave on his new rig.  It looks like all the mods can be added directly to the rig so it will still fit in an Altoids tin.  Now if everybody adopts those mods then they too can pop out of the noise when stacking is applied.

de w4hbk

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