R820T

Hi All, When measuring the NF of dongles using this chip with traditional methods one gets very discouraging results. I need to turn up the noise source for NF to 17 dB before I see the desired 3dB increase in received noise power. Fortunately this is an artifact The NF is low, but there is some kind of noise controlled AGC somewhere. If I send one or two carriers into the dongle everything seems normal, but if I send a single carrier plus white noise into it, the intelligent "something" applies automastic gain control to keep the noise floor at a nearly unchanged level. The signal which is at a fixed level seems to decrease while the noise seems unchanged. This is not what we normally want in SDR. When looking at the code I just get confused. It is not obvious to me what gain control is involved. Presumably the RTL2832 chip has some clever algorithm for evaluating the noise floor and the output from that seems to be used for the R820T. I think that for SDR it would be better to disable all those things and allow the user to control gain manually. One of the good usages of RTL-SDR is to monitor the wideband noise floor of microwave systems. Measuring performance by measuring sun noise for example. That is not possible with the current implementation of the R820T. Is there anyone who could give a hint how to disable this feature? Regards Leif / SM5BSZ