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Jochen Arndt j.arndt at sis-germany.comDuring implementation of signal strength calculations I came across some problems with E4000 tuners. The RTLSRD library uses these E4000 gains: const int e4k_gains[] = { -10, 15, 40, 65, 90, 115, 140, 165, 190, 215, 240, 290, 340, 420 }; These are the gains from Table 6 of the data sheet plus the mixer gain of 12 db for the last value and 4 db for all others. But the last gain of 30 db from Table 6 is only valid when LNA gain enhancement is enabled. Cite from datasheet section 1.16 LNA Gain enhancement: "The LNA gain numbers quoted throughout this document assume that this register is programmed to the recommended value." The RTLSDR libary actually does not change the LNA gain enhancement register AGC11 with manual gain mode and clears it with automatic gain mode. So the last two values of the e4k_gains array must be: 290 = 250 + 40 (max. LNA gain + low mixer gain) 370 = 250 + 120 (max. LNA gain + high mixer gain) With enabled LNA enhancement, the last value would be 420 = 250 + 120 + 50 (max. LNA gain + high mixer gain + enhancement) However, enabling LNA gain enhancement did not work (like the larger LNA gain step size of 5 db). I have verified the above by measuring pure carrier signals of different strength using FFT and subtracting the total gain. Using the corrected values, the changings in the measured signals correspond to the changings of the transmitted signal. With automatic control, the values has been also compared with the input power calculated from the RSSI indicator and LNA gain. The second to last value of Table 6 is also 25 db. I assume that this is a misprinting and the correct value is 22.5 db. Then the corresponding e4k_gains array entry must be changed from 290 to 265. Mixer Gain (automatic mode) In automatic mode, the mixer control register AGC7 is set to 0x01. This will enable autonomous control with a threshold value of zero resulting in always using the high mixer gain of 12 db. When the received signal is inside the automatic gain range (> -50 dbm), the output voltage of the LNA is so high that applying any amplifying afterwards usually results in overdrive samples. Therefore, the automatic mixer gain should be disabled or set to switch to high mixer gain at max. LNA gain by setting control register AGC7 to 0x1D. For the same reason, the total IF gain should be set to less than 10 db. Applications using automatic gain control can then increase the IF gain when the input signal is below -50 dbm to extend the sensitivity range. NOTE: Section 1.15.2 of the datasheet states that writing 0x15 to AGC7 corresponds to a LNA gain threshold of 7.5 db. This is obviously wrong (verified by reading the mixer Gain2 register with different signals). I hope this is helpful Jochen Arndt