False alarm about 'internal AGC' in R820T

Hi Charlie, The controls for AGC do not affect the peculiar wideband LNA AGC in the R820T chip. The RTL1090 uses the same rtlsdr.dll as is used by SDR#. I do not know how to evaluate noise levels and signal levels in RTL1090, but with SDR# one can see funny things directly on the waterfall. The LNA is followed by a VERY sensitive power detector that is somehow followed by filters and amplifiers. It is arranged in a way to not react on narrowband signals, but already a 3 dB increase of the noise floor causes a loss of gain through AGC action. Most striking is this experiment: Connect a combiner to the input of the dongle and use it to combine a signal generator and a noise source. The noise source needs a filter that assures that it does not add any noise on the frequency of the desired signal. It is OK to use a T-connector if you do not have a wideband combiner. I used a 100 MHz low pass filter connected to a vacuum diode noise source cabable of delivering 17 dB excess noise combined with a signal generator on 144 MHz by use of a T connector. I used SDR# to look at the spectrum around 144 MHz. Without "RTL AGC" and without "Tuner AGC" the noise floor does not change when the noise source is switched on or off. That is expected because the noise source can not send any noise through the 100 MHz low pass filter. That is true at modest gain settings, but if the gain is set at maximum (49.6 dB) the noise floor increases by 3 dB when the noise source is turned on. A small but unexpected effect. The signal however is attenuated by 23 dB for a total loss at max gain of S/N of 26 dB!!!!! Please note that the true S/N is not affected at all. There is no noise added at 144 MHz. If I switch on "RTL AGC" or "Tuner AGC" or both, S/N still changes the same way. Depending on the signal level of the 144 MHz signal one can see the signal go down or the noise go up. Or both. The way sensitivity is lost due to out-of-band noise is invisible to the user. There is no warning about overload. The noise power from 0 to 100 MHz is -174+17+80 = -77 dBm ( -174 dBm/Hz = room temp) ( +17 dB is excess noise) ( +80dB is 100/MHz/1Hz) There is some filter loss and the dongle presumably has a high pass filter so one can assume that the noise power is -80 dBm RMS. I have tried to activate the LNA AGC by use of narrowband signals in the 50 to 100 MHz range. Even two signals at -30 dBm each do not have any effect regardless of the frequency spacing. It seems the "intelligent" power detector of the LNA AGC can reject narrowband signals even if they are much stronger than the noise floor. The wideband LNA AGC in the R820T may cause problems when an up-converter is used in front of the dongle. The noise floor of the up-converted HF spectrum may cause unexpected loss of sensitivity in the upper part of the HF spectrum where the noise floor is low. Adding a filter for the desired HF band. With some gain to ensure that the noise floor is higher in the desired frequency range than elsewhere could perhaps make the R820T dongles behave much better. 73 Leif / SM5BSZ control toggled cannot sdr-radio-com(a)yahoogroups.com]