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

Thursday, August 22, 2013

Spectrum Averaging to Improve Measurement of Signal Strengths

Many grabbers based on Spectrum Lab include a spectrogram along with the waterfall to give some idea of signal strength. I have recently enhanced this feature on my grabber to facilitate accurate measurement of signal strengths. Spectrum Lab makes spectrum determinations at a defined scroll interval which in the case of my 10 minute grabber is 0.7 seconds. The long term average function, selectable in the Spectrum(1) configuration window, averages during the grab interval to produce a dramatic reduction in noise. For the 10 minute grab interval this results in 857 individual spectra being averaged, resulting at a 15 dB improvement in signal-to-noise. Here's what it looks like (yellow is the long term average, red a single spectrum):


There is a problem in reading QRSS signal strengths because of the dual FSK peaks. If the signal was unmodulated, i.e., a straight line, continuous carrier then the displayed peak would be meaningful. However, an FSK signal has two peaks for Mark and Space which under the ideal condition of no QSB would be different due to the different cumulative times of each. To get the true signal level requires a little math to add the two, doing so in linear space rather than dB space. It's necessary to take the antilog then add and take the log of the results to get back to dBs. The following sketch shows how this works:

Figure 2

This is a bit of effort but not too bad once a work flow is established. It's worth it when you bear in mind that we are measuring signal strength over a real dx path with a precision approaching several dBs. Here's a grab I used to compare the signal strength of 4 stations located in England (a,b,c,d) and 2 in the US:

Figure 3

It is not wise to compare signals with large differences in path length because of unequal Doppler broadening as will be discussed below. This table shows the raw data, mark, space and noise, along with the processed results and comparisons:


The noise baseline was subtracted from Ptotal to get the SNR. We could conclude from this data that on the US side, station f was 7 dB stronger than station e while on the European side, how other stations compared to station b which is consistently the strongest EU station seen on my grabber. Repeated observations of the EU stations give about the same results from day to day which would allow antenna experiments to improve one's signal.

This particular grabber screen was designed especially for this kind of measurement with a larger spectrum window for more easily reading the dB level and with a 3 minute grab interval to better follow the QSB.  

There are some problems and limitations to be considered. First, signals must be continuous, either a steady unmodulated carrier or FSK without an off time during the grab period. Second, the signals must have a meaningful, defined peak. Hellschreiber and drifting signals will not work because there is not a defined peak. Third, the width of the signals being compared must be the same otherwise we'd have to consider the area under the curve which is beyond the capability of this technique. Doppler broadening generally increases with path length so comparisons should be made only signals from the same general area. Finally, the signals must be continuous The signal at g is not usable for this reason.

As a bonus, I have adjusted the output of my receiver to make the noise floor always at -110 dB so that day-to-day or perhaps even seasonal comparisons are possible, assuming that the rx noise floor and gain/loss of the antenna/feedline remains constant and I see no reason why they shouldn't. Referring to Figure 2, in the spectrogram the reason there is so much space to the left of the noise baseline is so I can see the -110 dB level to make this adjustment, which I do while substituting a 50 Ohm load for the antenna. As long as I don't fiddle with the gain controls the noise floor is remarkably stable from day to day.

Some stations using the U2 "Ultimate" from QRP labs are transmitting both QRSS and WSPR signals by time sharing where QRSS runs for 8 minutes then shifts to WSPR for 2 minutes. I have made some preliminary comparisons to signal strengths made using the technique described here with the dB levels reported by WSPR and get good agreement. When I have made more comparisons and am confidant in the results I'll present it in another post to this blog.

In case you're wondering, my receiving system is linear because the AGC has been disabled using a technique described in a previous post.

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