Why exactly 43.9? There’s obviously some significance in that number, and I’m very curious which.
I can only assume that number was suggested as it’s one of the actual gain settings those radios will set that is middle of the road give or take. There is a decent gap between 44.5 and 48.0. Starting at 43.9 gives you some room to wiggle:
0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6
You can of course type in anything you want, but the firmware will set the nearest to the scale above.
And those gain numbers are, well, not all that accurate. There’s a lookup table that maps hardware gain steps to dB values that was calibrated once, on one bit of hardware, at a different frequency, several years ago. Don’t put too much weight in the absolute values.
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I was playing around with some gain settings. The difference of these above 40 is not as much as it looks like. Only if you go below 40 or use “-10” shows a larger difference.
I assume 43.9 is a good starting point to optimize
I think this is what you eluded to measured at 928Mhz, -60dBm input on an r820t (according to the source) - no clue how closely it relates to the t2 even:
Am I looking at the correct thing @obj ?
Yes, that’s what I meant.
I recently did some gain measurements on a (non-rtlsdr) R820T2 at 1090MHz and got somewhat different values (values in dB relative to minimum gain):
.lna_gains = {0.0, 2.7, 6.2, 10.2, 13.4, 15.0, 17.7, 20.2, 22.7, 23.5, 24.0, 24.5, 25.3, 25.3, 26.8, 27.9},
.mixer_gains = {0.0, 1.3, 3.1, 4.8, 6.1, 7.5, 9.0, 10.4, 11.8, 13.2, 14.7, 16.0, 17.3, 17.3, 17.3, 17.3},
.vga_gains = {0.0, 3.4, 6.8, 10.2, 13.7, 17.1, 20.6, 24.0, 27.8, 31.2, 34.6, 38.1, 41.5, 45.2, 48.7, 52.7},
Here’s a comparison of the resulting dB values for each gain step using the gain-setting algorithm that librtlsdr uses. (That is, for the same series of hardware settings, these are estimates of the actual total gain, using the two different measurements of gain-at-given-hardware-setting. There’s no difference in what the hardware does, this is just about what gain value in dB is displayed)
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