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For the results of the tests presented: The tests and data have nothing to do with input filtering or very little with input match. Everything within each data set is within the same band. Of course, any input filtering will pass all signals close to the target signal.
Another point seen especially in data sets Airspy7_0dB.jpg and to a lesser extent in data set Airspy 9_0dB.jpg: At issue may be digital saturation. In an SDR when all available bits are utilized as with possible reception of a strong signal, the output becomes garbage and unpredictable. This is highly evident in the Airspy7_0dB.jpg. Attenuation between the antenna feedline and the input to the receiver, in this case, is mandatory. I wrote an article some 4 or five years ago in the Journal of the Society of Amateur Radio Astronomers (SARA) addressing this issue for reception around 20 MHz (when solar conditions were pretty much the best of Cycle 24).
The issue of 'bogus' signals in the FM broadcast band: This is common in nearly all receivers be they the older technology or the newer digital receivers like the Airspy and others. The problem is that within the 88 to 108 MHz FM broadcast band, there are many truly strong signals, especially true in or near large cities. This is required to minimize the noise and maximize the level of quieting in detection of a wideband signal. The input stage(s) (front end) acts as a mixer as its driven into unintended non-linearities by all the strong stations within that 20 MHz of spectrum. The strong stations mix with each other to produce bogus or false signals. Again, too much signal requires either a stopband filter (if you're not interested in receiving FM), or insertion of an appropriate amount of attenuation between the antenna feedline and the input of the receiver. Any input passband filter present for the FM band, specifically, will aim at passing the whole band on to the mixer or ADC. Again, an attenuator is required due to too much in-band strong signal levels.
The suggestion has been made to understand how receivers are tested and the basics of their internal design. The information is on the Internet, so you don't have to visit a library. For proper evaluation of the newer SDR technology, to properly design tests and evaluations of the item, a knowledge far deeper than just the basics of the design will be required.
In conclusion, your measurements and data do not address input filtering and/or input match. To properly conduct the tests you desire, you must first understand what you are evaluating.
Dave - WØLEV
On Fri, Oct 18, 2019 at 5:07 AM jdow <jdow@...> wrote:
Please look at "https://en.wikipedia.org/wiki/Radio_noise". It is a simple
Dave - WØLEVJust Let Darwin Work