Heatmap with unwanted vertical lines

john john at tonebridge.com
Sat Aug 1 23:03:58 UTC 2015


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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