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john john at tonebridge.comUpdate: 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 -------------- next part -------------- A non-text attachment was scrubbed... Name: airband_g28_i2_terminated.jpg Type: image/jpeg Size: 238969 bytes Desc: not available URL: <http://lists.osmocom.org/pipermail/osmocom-sdr/attachments/20150801/f5a5d006/attachment.jpg>