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@yahoogroups.com [mailto:sdr-radio-com@yahoogroups.com] On Behalf Of Leif Asbrink Sent: 10 August 2013 22:30 To: sdr-radio-com@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
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 leif@sm5bsz.com 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: sdr-radio-com@yahoogroups.com [mailto:
sdr-radio-com@yahoogroups.com]
On Behalf Of Leif Asbrink Sent: 10 August 2013 22:30 To: sdr-radio-com@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
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@yahoogroups.com [mailto:
sdr-radio-com@yahoogroups.com]
On Behalf Of Leif Asbrink Sent: 10 August 2013 22:30 To: sdr-radio-com@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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Hi All,
An attempt to install rtlsdr under Mageia 3 (32 bit) failed with a message saying INCLUDES is obsolete and should be replaced by AM_CPPFLAGS
That means that line 4 in rtl-sdr/Makefile.am and rtl-sdr/src/Makefile.am should be changed.
I have verified that this is ok for Ubuntu 9.04 which is the oldest version on my multiboot system.
Regards
Leif
Hi All,
The rtlsdr dongles come with different tuner chips. Have a look at this video: http://www.youtube.com/watch?v=cVU5X1d2XYU
Four dongles are run simultaneously with the same input signal. A seven port resistive combiner is used to combine a noise source, a signal generator and a sweep generator and to distribute the combined signal to the four dongles.
The screen shows three instances of SDRsharp running the three tuners plur one instance of Linrad with the E4000 tuner for which Linrad allows a different gain distribution that provides a much better suppression of false signals.
The Linrad screen is pixel-oriented so the video should be looked at with high quality.
Regards
Leif / SM5BSZ
yes, I am afraid you are assuming all of us are as smart as you on the topic so if you have time please have another go at it. You are in a unique position to do this since you have all the equpiment at hand.
The thing that confuses me is why is it during the sweeps due the fft look so different in each of the windows and it seems like the timing of the sweeps are different. The sweeps happen at the same time for each dongle, right? The dongles are all listening in parallel at the same time? If so, seems like they would look more alike at the same time. My apologies for the density on this end.
73 wa5ngp
On Wed, 14 Aug 2013 19:55:03 -0700 (PDT) m240zz@gmail.com wrote:
yes, I am afraid you are assuming all of us are as smart as you on the topic so if you have time please have another go at it. You are in a unique position to do this since you have all the equpiment at hand.
OK. I will try to make things more clear.
The thing that confuses me is why is it during the sweeps due the fft look so different in each of the windows and it seems like the timing of the sweeps are different. The sweeps happen at the same time for each dongle, right? The dongles are all listening in parallel at the same time? If so, seems like they would look more alike at the same time. My apologies for the density on this end.
The dongles suffer from various problems. Overload and inadequate anti-alias filtering. A lot of false signals are generated but that is different between the different dongles.
The input is identical to all the dongles and you can see that in the beginning of the video where signal levels are low. At high signal levels they behave quite differently because false signals dominate (But not in Linrad.)
73
Leif / SM5BSZ
On Wed, 14 Aug 2013 19:55:03 -0700 (PDT) m240zz@gmail.com wrote:
The thing that confuses me is why is it during the sweeps due the fft look so different in each of the windows and it seems like the timing of the sweeps are different. The sweeps happen at the same time for each dongle, right? The dongles are all listening in parallel at the same time? If so, seems like they would look more alike at the same time. My apologies for the density on this end.
The purpose of the vido was to demonstrate that dongles are different. The sweeps indeed happen at the same time for all the dongles. The very point is that they do not look very much alike. It is non-trivial to know in advance which one will perform best in a real life situation.
It was the first video. It just shows that dongles are (very) different. I am uploading some more videos as a response to the feedback I received so far. (Upload is extremely slow from here...)
It would be trivial, but it would take a lot of time, to use a dongle to provide a wideband map from 50 to 500 MHz or whatever on a particular antenna in a particular location. With that knowledge it would be easier to decide what dongle to choose for a particular situation. It is trivial to add a selective filter and that can make a big difference for the dongles.
73
Leif
thanks for all the explanations. Since I'm new to this I was thinking I was missing something bigtime. I guess not. I was expecting to see a single spike sweeping thru there. When I saw all that stuff it kinda reminded me of the intermod that you see on vhf stuff where several frequency signals beat together to make an image back on the channel that you are listening to. Based on those results I am surprised that the whole thing is usable as a receiver but I suppose real life listening there aren't that many strong signals that create those artifacts.
Somehow on my screen the linrad thing was not very visible due to contrast or something so I couldn't quite tell what I was looking at.
If the bandwidth &/or gain is turned way down, say 200 khz window, do those false signals due to the sweep and subsequent aliasing get minimized?
73 don
-
Leif Asbrink -
m240zz＠gmail.com -
Sdr Guru