Hi This explains why the FM trap filter helps so much. Especially in the VHF band when the FM transmitters are nearby (city).
SG On Sun, Aug 11, 2013 at 11:48 PM, Leif Asbrink <[email protected]> wrote: > 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 > > > > > > > > I use a R820T chipped dongle for receiving aircraft transponder signals > > through a specialised application called RTL1090 from jetvision, the > control > > panel of which allows the tuner and device AGCs to be independently > toggled > > on or off. I have not however used this with SDR-Radio in the past so > cannot > > comment on the optimum settings. > > > > > > > > 73 > > > > Charlie > > > > www.G4EST.me.uk > > > > > > > > From: [email protected] [mailto: > [email protected]] > > On Behalf Of Leif Asbrink > > Sent: 10 August 2013 22:30 > > To: [email protected] > > Subject: [sdr-radio-com] Re: About 'internal AGC' > > > > > > > > > > > > Hello Patrick and All, > > > > The R820T chip has an advanced AGC function that I do not > > think can be disabled. The chip detects the noise floor > > in a wide bandwidth and adjusts the gain to keep the noise floor > > constant. > > > > The wideband AGC has surprising effects. If one tries to measure > > the noise figure with a noise source that is manually switched > > on and off one finds a really bad NF. That result is false however, > > if one measures S/N of a weak signal one finds the true NF which > > is quite good. > > > > To verify the finding one can inject a weak signal together > > with the signal from a noise source. What happens when the > > noise is turned on is that the signal becomes weaker while > > the noise floor does not change. > > > > A 500 kHz wide filter in front of the R820T chip converts the > > noise from the noise source to a narrowband signal which will > > not affect the wideband AGC. > > > > I made some effort to switch this feature off but failed. > > > > The behaviour is probably quite clever for reception of digital TV > > but I find it very disturbing in a general purpose SDR. I did not > > take notes and I did not investigate in detail what types of signals > > will affect the AGC and what types will not. That would be > > a big investigation and I see no reason to do it because there > > are other chips. > > > > The R820T gives good signals many times but I do not like the feeling > > of not knowing what I am doing..... > > > > The "internal AGC" option is another thing as far as I understand. > > The chip has RF AGC as well as IF AGC. > > > > 73 > > > > Leif / SM5BSZ > > > > > Hello Group, > > > > > > When using a DVB-T type dongle (mine is a R820T), do you tick the > > "internal AGC" option or not ? > > > > > > I tried both "internal AGC" desactivated (with more gain) and AGC > > activated (less gain to avoid spurs from my local FM TX) ... I can't tell > > which one is better. Even on weakish sigs it's about the same. > > > > > > What about your experience ? > > > > > > Regards, > > > Patrick > > > > > > > > > > >
