15 Jul
2015
15 Jul
'15
1:52 p.m.
Hello,
When I use heatmap.py with output from rtl_power I get regularly spaced
vertical lines that do not appear to be related to any signal. They
look they like repeat at the dongle bandwidth (2048000Hz in this case).
The crop option for rtl_power reduces the presence but I am not sure f
that is intended by that option. Even at -c of 70% they are still there
(see attachment).
Is this because of small bin width? If I use a larger bin (32k) they
are still there. In this case there is no frequency legend along top so
can't compare if they happen more often.
Are these lines expected? Can they be removed?
John
15 Jul
15 Jul
3:03 p.m.
Hi John,
Put a 50- or 75-ohm termination on the rtl-sdr antenna connector and
redo the plot, to see whether the beat-frequencies are generated inside
or outside your dongle.
Next you can put a bandpass filter in front of your rtl-sdr dongle, in
order to reduce the out-of-band signals that probably overload your
front-end. In practice, we shouldn't be using any RF device without
input and output bandpass filters.
Next, you can also try putting an rf choke/ferrite (a common-mode
transformer) on the USB cable, in order to reduce the noise coming from
the USB-host and through the cable.
Next, putting the dongle inside a metallic enclosure will help
screening the RF circuits, and will allow it to receive signals only
through the input connector (and preferably through an input bandpass
filter). You can create an effective "poor man's enclosure" by cutting
and soldering pieces of double-sided PCB.
Finally, you can test your dongle with Linrad with its patched version
of librtlsdr. Linrad uses a different gain distribution and there's a
big chance that it can satisfy your needs. You can do similar
experiments by reducing the RF gain and AGC on rtl_power and see
whether it influences positively your measurements. Please try these
and share your experience.
Regards,
Nikolay
On 07/15/2015 02:52 PM, John wrote:
Hello,
When I use heatmap.py with output from rtl_power I get regularly spaced
vertical lines that do not appear to be related to any signal. They
look they like repeat at the dongle bandwidth (2048000Hz in this case).
The crop option for rtl_power reduces the presence but I am not sure f
that is intended by that option. Even at -c of 70% they are still there
(see attachment).
Is this because of small bin width? If I use a larger bin (32k) they
are still there. In this case there is no frequency legend along top so
can't compare if they happen more often.
Are these lines expected? Can they be removed?
John
2 Aug
2 Aug
1:03 a.m.
Update:
Termination: My dongle has 75 ohm antenna connection so I used 75 ohm
terminator. The lines remained. The lines are much diminished or it
could be due to fact "background" signal is very prevalent. I have
attached this one image for reference. I was expecting to see no signal
at all (or no single where the lines are not).
Bandpass filter: ave not done this. I have a broadcast FM notch filter
I could try.
RF Chokes: Tried these on 8" usb cables feeding this RTL dongle,
another RTL dongle and an 802.11 dongle. No apparent change in lines.
Enclosure: The RTL dongles were already in aluminum enclosures. They
are right next to an Odroid single board commuter that has no case. All
of this is in a tin box.
Linrad: Have not tried this.
All my debugging was air band. When I switched to 450-470Mhz the
problem goes away. Since this is my range of interest I will not be
working on debugging this anymore.
I can provide other images if someone is interested themselves.
Thanks again,
John
On 07/15/2015 08:03 AM, Nikolay Dimitrov wrote:
Hi John,
Put a 50- or 75-ohm termination on the rtl-sdr antenna connector and
redo the plot, to see whether the beat-frequencies are generated inside
or outside your dongle.
Next you can put a bandpass filter in front of your rtl-sdr dongle, in
order to reduce the out-of-band signals that probably overload your
front-end. In practice, we shouldn't be using any RF device without
input and output bandpass filters.
Next, you can also try putting an rf choke/ferrite (a common-mode
transformer) on the USB cable, in order to reduce the noise coming from
the USB-host and through the cable.
Next, putting the dongle inside a metallic enclosure will help
screening the RF circuits, and will allow it to receive signals only
through the input connector (and preferably through an input bandpass
filter). You can create an effective "poor man's enclosure" by cutting
and soldering pieces of double-sided PCB.
Finally, you can test your dongle with Linrad with its patched version
of librtlsdr. Linrad uses a different gain distribution and there's a
big chance that it can satisfy your needs. You can do similar
experiments by reducing the RF gain and AGC on rtl_power and see
whether it influences positively your measurements. Please try these
and share your experience.
Regards,
Nikolay
On 07/15/2015 02:52 PM, John wrote:
> Hello,
>
> When I use heatmap.py with output from rtl_power I get regularly spaced
> vertical lines that do not appear to be related to any signal. They
> look they like repeat at the dongle bandwidth (2048000Hz in this case).
> The crop option for rtl_power reduces the presence but I am not sure f
> that is intended by that option. Even at -c of 70% they are still there
> (see attachment).
>
> Is this because of small bin width? If I use a larger bin (32k) they
> are still there. In this case there is no frequency legend along top so
> can't compare if they happen more often.
>
> Are these lines expected? Can they be removed?
>
> John
5:25 a.m.
Hi John,
Heatmap steps the frequency of the dongle and places spectra with
a bandwidth of about 2 MHz side by side. Depending on hardware and how
you configure it you might have a center spur that repeats.
You cover 118 to 137 MHz which is nearly a factor of 10 above the
bandwidth of the dongle so you loose 90% of the data on each frequency
with an associated higer noise floor (5 dB perhaps) that makes weak signals
more difficult to find.
It seems you have 10 sub-bands (based on the regions of low loise level.)
I have not followed this thread so I do not know what tuner you are using.
The image does not give any hint.
All the dongles have many things that can be controlled by software.
As a first step I suggest you use a "normal" SDR software that uses a single
center frequency. SDRsharp, Linrad, HDSDR or whatever. Look at the spectrum
and play with available settings.
Regards
Leif
Update:
Termination: My dongle has 75 ohm antenna connection so I used 75 ohm
terminator. The lines remained. The lines are much diminished or it
could be due to fact "background" signal is very prevalent. I have
attached this one image for reference. I was expecting to see no signal
at all (or no single where the lines are not).
Bandpass filter: ave not done this. I have a broadcast FM notch filter
I could try.
RF Chokes: Tried these on 8" usb cables feeding this RTL dongle,
another RTL dongle and an 802.11 dongle. No apparent change in lines.
Enclosure: The RTL dongles were already in aluminum enclosures. They
are right next to an Odroid single board commuter that has no case. All
of this is in a tin box.
Linrad: Have not tried this.
All my debugging was air band. When I switched to 450-470Mhz the
problem goes away. Since this is my range of interest I will not be
working on debugging this anymore.
I can provide other images if someone is interested themselves.
Thanks again,
John
On 07/15/2015 08:03 AM, Nikolay Dimitrov wrote:
Hi John,
Put a 50- or 75-ohm termination on the rtl-sdr antenna connector and
redo the plot, to see whether the beat-frequencies are generated inside
or outside your dongle.
Next you can put a bandpass filter in front of your rtl-sdr dongle, in
order to reduce the out-of-band signals that probably overload your
front-end. In practice, we shouldn't be using any RF device without
input and output bandpass filters.
Next, you can also try putting an rf choke/ferrite (a common-mode
transformer) on the USB cable, in order to reduce the noise coming from
the USB-host and through the cable.
Next, putting the dongle inside a metallic enclosure will help
screening the RF circuits, and will allow it to receive signals only
through the input connector (and preferably through an input bandpass
filter). You can create an effective "poor man's enclosure" by cutting
and soldering pieces of double-sided PCB.
Finally, you can test your dongle with Linrad with its patched version
of librtlsdr. Linrad uses a different gain distribution and there's a
big chance that it can satisfy your needs. You can do similar
experiments by reducing the RF gain and AGC on rtl_power and see
whether it influences positively your measurements. Please try these
and share your experience.
Regards,
Nikolay
On 07/15/2015 02:52 PM, John wrote:
> Hello,
>
> When I use heatmap.py with output from rtl_power I get regularly spaced
> vertical lines that do not appear to be related to any signal. They
> look they like repeat at the dongle bandwidth (2048000Hz in this case).
> The crop option for rtl_power reduces the presence but I am not sure f
> that is intended by that option. Even at -c of 70% they are still there
> (see attachment).
>
> Is this because of small bin width? If I use a larger bin (32k) they
> are still there. In this case there is no frequency legend along top so
> can't compare if they happen more often.
>
> Are these lines expected? Can they be removed?
>
> John
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4 comments
4 participants
participants (4)
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john -
John
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Leif Asbrink -
Nikolay Dimitrov