Heatmap with unwanted vertical lines

Nikolay Dimitrov picmaster at mail.bg
Wed Jul 15 13:03:43 UTC 2015


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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