[Skimmertalk] SNR Measurements

[Jim Baremore]

My good friend Stan, K5GO, raised the question about the SNR readings from
CW skimmer at a remote location and asked if they were correlated to the
power of the transmitting station.  Some readings offered by others did not
seem to correlate well to each other.

 

Understanding the CW Skimmer is offering a Signal to Noise ratio and not a
signal strength as such, led me to the following quick experiment:  I used
my old Cushcraft CE-6 service monitor as a signal source and varied the
input to the SDR-IQ while observing the SNR ratio on the CW Skimmer display
connected to my SDR-IQ. I used the readings from the 'calibrated attenuator'
to record the level being inputted to the SDR-IQ.  After I was done, I
connected the Cushman to my Anritsu spectrum analyzer and measured the
levels at each of the points I had previously recorded from the Cushman
dial.

 

I entered the results in an Excel spread sheet and did a quick plot.  What I
found was that for a SNR from 2 to about 7, there was a distinct slope of
the SNR vs. dBm of received signal that was different from SNR readings of 8
to 35.  I also had a slight bias in one of my two instruments as the signal
levels were both straight lines but were not the same slope. One of the two
is calibrated slightly differently than the other.  I think I believe the
Anritsu more than the Cushman, but who knows.

 

Using the Cushcraft data for the low SNR ratios, I found the slope of the
line was about .6 SNR per dBm.

 

>From SNR = 8 to 30, the Cushcraft showed about 1.4 SNR per dBm.  The Anritsu
showed about 1.77 SNR per dBm.

 

In other words, I found a distinct knee for the low SNR ratios as it took
about 1/.6 or about 1.7 dBm to increase the SNR by one unit.  

 

Above a SNR or 7 to 8, it changed and it looks like the average of the two
instruments would be about 1/1.55 or only about .65 dBm to increase the SNR
by one unit.

 

A final note is my old Cushcraft is pretty old and may not be the purest
signal possible.  It could have some flutter which could affect the SNR
ratios as detected by CW Skimmer. 

 

Finally, remember this was a single signal into the SDR-IQ and is in
contrast to an environment where it is connected to antenna listening to a
lot of signals as well as atmospheric noise. I also noticed the absolute
values of the SNR, at a given signal input level, varied considerably if I
changed the bandwidth of the CW filter.  This says if you want to compare
your SNR at two different remote locations, you should be sure they are both
using the same CW bandwidth.

 

 

OK,  nuff for now

 

73

 

Jim K5QQ