Senstivity comparison to Airspy HF+, trying to understand the data
Apologies in advance since this is my first KiwiSDR, and I am NOT an EE. I am trying to understand why it seems significantly less sensitive than the Airspy HF+ SDR I have. Aside from what my ears were telling me, I decided to make some objective measurements to share, on the chance someone can help me understand what I am doing wrong. Or guide me on some better experiments to verify that my KiwiSDR does not have a hardware issue.
With no antenna connected, AM mode (at 10 kHz bandwidth) the 0-30 MHz spectrum is reasonably clean, with a typical reading of -116 dBm. If I zoom in to a narrow slice, say 30 kHz wide, and enable spectrum display, the visual noise floor level is about -153 dBm. This makes sense to me since the power bandwidth for the spectrum display will be much smaller than for the 10 kHz detector bandwidth. Also, since I am using a switching wall wart power supply (shame on me, but it's all I have available for now), I also powered the Kiwi via an external USB battery....with no discernible impact on the noise floor. Question #1 - are these observed values typical? They seem OK to me, so far so good. Or is it?
Next, I attached a -126 dBm signal source (TinySA, low output, attenuated from -76 dBm to -126 dBm, unmodulated carrier at 11.500 MHz). While the dBm the Kiwi reports doesn't budge from -116 dBm, the zoomed spectrum display does show a peak about 6 dB above the noise floor. It's noisy, but enough signal to hear in CW mode, or USB mode if I lower the tuned frequency to 11.4995 MHz (500 Hz offset). in sharp contrast, with the Airspy HF at this level the signal sounds much less noisy, and the S/N ratio (displayed in SDR# is anywhere from 16-34 dB, depending on the sampling rate I set, i.e. at the highest 912 kbps rate the S/N is 16, increasing to about 34 dB at a 24 kbps sampling rate. Why the difference versus sampling rate is a separate mystery, but regardless of that, even at the lowest S/N I can hear a large difference versus the KiwiSDR. I have other data collected for higher signal levels (up to -76 dBm), but the -126 dBm level illustrates the pattern I am seeing. Question #2 - is the KiwiSDR response to a -126 dBm input signal typical?
My next experiment will be attach the outdoor antenna to both SDRs and pipe the audio (virtual audio cables) into two separate instances of WSJT-x running on a Windows PC, and see how the WSPR decode counts compare. That seems like another way to compare the weak signal performance.
Open to any and all suggestions and/or editorial comments!