11 Aug
2013
11 Aug
'13
10:48 p.m.
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(a)yahoogroups.com [mailto:sdr-radio-com@yahoogroups.com]
On Behalf Of Leif Asbrink
Sent: 10 August 2013 22:30
To: sdr-radio-com(a)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
12 Aug
12 Aug
10:43 a.m.
New subject: About 'internal AGC'
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(a)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(a)yahoogroups.com [mailto:
sdr-radio-com(a)yahoogroups.com]
On Behalf Of Leif Asbrink
Sent: 10 August 2013 22:30
To: sdr-radio-com(a)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
11:48 p.m.
New subject: False alarm about 'internal AGC' in R820T
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
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For more options, visit https://groups.google.com/groups/opt_out. 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(a)yahoogroups.com [mailto:
sdr-radio-com(a)yahoogroups.com]
On Behalf Of Leif Asbrink
Sent: 10 August 2013 22:30
To: sdr-radio-com(a)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
14 Aug
14 Aug
2 p.m.
New subject: Comparing tuners.
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
15 Aug
15 Aug
4:55 a.m.
New subject: About 'internal AGC'
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
5:06 a.m.
New subject: About 'internal AGC'
On Wed, 14 Aug 2013 19:55:03 -0700 (PDT)
m240zz(a)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
2:42 p.m.
New subject: About 'internal AGC'
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
18 Aug
18 Aug
1:10 a.m.
New subject: About 'internal AGC'
On Wed, 14 Aug 2013 19:55:03 -0700 (PDT)
m240zz(a)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
28 Aug
28 Aug
5:50 p.m.
New subject: Obsolete: INCLUDES
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
3916
days inactive
3933
days old
8 comments
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