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Leif Asbrink leif at sm5bsz.comHi All, I falsely interpreted the phenomenon I described as an action of the LNA AGC. It is a striking effect experimentally, but tha cause is not the noise generated by the temperature limited vacuum diode. As it turns out, the phenomenon is caused by the DC, about -1.3 V, that is present at the output of my Magnetic type 123 noise source. Obviously the designers of the ezcap USB 2.0 DVB-T/DAB/FM dongle decided they could save a cent or so by not adding a DC blocking capacitor on the input.... The data sheet I have indeed tells that there is a wideband LNA AGC but is does not behave as I was misled to believe on the basis of experiments with the 123 noise source. 73 Leif / SM5BSZ > > 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: sdr-radio-com at yahoogroups.com [mailto: > > sdr-radio-com at yahoogroups.com] > > > On Behalf Of Leif Asbrink > > > Sent: 10 August 2013 22:30 > > > To: sdr-radio-com at yahoogroups.com > > > 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 > > > > > > > > > > > > > > > > > > > -- > You received this message because you are subscribed to the Google Groups "Ultra Cheap SDR" group. > To unsubscribe from this group and stop receiving emails from it, send an email to ultra-cheap-sdr+unsubscribe at googlegroups.com. > To post to this group, send email to ultra-cheap-sdr at googlegroups.com. > Visit this group at http://groups.google.com/group/ultra-cheap-sdr. > For more options, visit https://groups.google.com/groups/opt_out.