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Author Topic: A *few* questions  (Read 1322 times)

Offline trenchcoat

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A *few* questions
« on: January 03, 2019, 0355 UTC »
So I am new to SDR. I got mine recently and have been playing around with it, even put up a random wire room antenna (Just goes around the perimeter of the top of my room. Fancy, right?). Onto the questions:
  • I get these odd tones that scroll in the opposite direction of where I am scrolling when I am scrolling. What is this and/or what is the cause?
  • Sometimes I lose the ability to receive anything and I just get static. Why does this do that?
  • Any activity I do see is just random noises when I tune into them. Like a digital signal. Is this just because no one is doing anything on the frequencies I can receive?
  • I often see stations that are in the FM band repeated multiple times on different frequencies. Like a radio stations at 89.7 MHz is being received at 200 MHz. What on is the reason for that?
Like I said I am new to SDR, so anything is appreciated.

P.S. If you have any tips, tricks, or useful info for someone who is receiving in the Omaha NE area, feel free to message me or reply here.
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Offline Josh

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Re: A *few* questions
« Reply #1 on: January 03, 2019, 2157 UTC »
1. When you have the lines scrolling in opposition to your tuning, this is a pretty good indication that you are receiving aliases, reduce the gain on the front end and see if they go away or reduce. See here for sciencey stuff;

2. This sounds to me like the front end amp has gone into compression because the gain is set too high or a very strong signal is in the passband and causing the agc to make the receiver go deaf. In this case reduce the gain on the front end. Another cause could be you've tuned outside the tuner's ability to tune.
The low cost sdrs could use a fm band notch filter, as the fm band has literal megawatts of rf lighting up everything. Even a cheap radio shack fm notch filter will help.

3. This might be because you've tuned to a digital system, any more, v and uhf is populated with digital signals, especially near densely populated areas. Most ems/fire/police have gone to apco25 trunked systems so you might look into getting an apco35 decoder for the sdr, that way you can listen in on whatever's out there not actually encrypted. You might also be tuning into spurs created because the sdr is overloading.

4. All of these issues are common to low cost sdr rigs, they can be reduced by making sure the antenna isn't feeding to strong a sig into the sdr, reduce the gain of the antenna or sdr or both to get the most out of what you have.

Here's my review of the RTL V3;
It has some tips on using the V3 on hf, didn't cover vhf and above. One thing available on v/uhf in the rtl sdrs that isn't available on hf is the tuner has a built in preselector of sorts, using an sdr app that enables this preselector/bandpass filter should make the sdr happier.
« Last Edit: January 03, 2019, 2200 UTC by Josh »
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Offline Josh

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Re: A *few* questions
« Reply #2 on: January 10, 2019, 2007 UTC »
Also, in some cases the sdr noise blanker may be on by default, and/or set to too high a value, thus causing distortion to appear.

In such case simply reduce the nb level or disable it, if temporarily. I usually set nb level to cut off the very high level random spikes like a car going by with a bad plug wire and leave it on all the time. In a sdr rig, you can normally increase the nb value till all noise melts away, and it takes the band with it. If you had only very weak signals to listen to this would be a good use for all that noise blanking, but in the real world, too high a level will make the sdr inoperative.

I suspect this nb level setting is one of the number one causes of new sdr user complaints; they try the sdr, it pukes and they can't find a sig worth listening to due to all the spurs because the nb level is too high, and they hate sdr from then on.

One way to "set and forget" adjust it is to find a strong ambc or hfbc station, tune a few kc off so you get splatter, and adjust the nb to the point of distortion, then back it off. That should put the nb level optimally.

Additionally, make sure you're not running the sdr wide open as far as RF and IF gain. In the RTL SDR V3 and RSP2 sdrs, I always disable IF gain and agc, and set the RF gain accordingly.

Another thing to consider is Decimation. If the sdr application offers Decimation, crank in as much as you can, this is a good thing and too much is just right. With Decimation, you trade bandwidth for dynamic range and sensitivity. If you want to view 10MHz of spectrum you're not going to be able to use much if any Decimation, if all you want to do is listen to 6KHz or less of spectrum, crank in the Decimation.

I don't use sdr# so don't know if it offers Decimation, but HDSDR and SDRuno do, oddly HDSDR offers more Decimation levels in Zero IF mode than High IF mode. In SDRuno with a Decimation at 32, the screen has about 200KHz of spectrum, the Decimation increases the dynamic range markedly over the hard wired 12bit adc dynamic range.
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