False alarm about 'internal AGC' in R820T

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Leif Asbrink leif at sm5bsz.com
Mon Aug 12 21:48:49 UTC 2013


Hi 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
> > > >
> > >
> > >
> > >
> >
> >
> 
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