SDR# - noise floor jumping about

[Feccy]

Slight newbie question - when scanning with the frequency scanner plugin, the noise floor seems to jump about a fair bit - the noise floor does seem to increase as the frequency climbs (assume that's normal) but it jumps either up or down (so far down it disappears off the screen for a bit) erratically. I'm using SDR# v1.0.0.1784. 'RTL AGC' and 'Tuner AGC' are unchecked and it's running on Windows 7

I would appreciate any help here

 

[morfis]

I don't see much change in the noise floor as frequency increases.



If you mean the main spectrum display, where the red arrow is, then check the 'Range' slider setting in sdrsharp. Using the Range and Offset you can alter the spectrum display to suit your use. It's possible you have it set with a range that is too small for the actual received signal.
Actual noise figure minima and maxima would be useful info - a disadvantage of having a display you can zoom in on is that it can make an insignificant difference look huge.

If you mean where the scanner visualisation is, blue arrow, then you are currently limited by the way the plugin was originalluy written by Vasili

For sdrsharp there is a web/mailing list at groups.io with much useful information.

 

[Feccy]

I don't see much change in the noise floor as frequency increases.

View attachment 95140

If you mean the main spectrum display, where the red arrow is, then check the 'Range' slider setting in sdrsharp. Using the Range and Offset you can alter the spectrum display to suit your use. It's possible you have it set with a range that is too small for the actual received signal.
Actual noise figure minima and maxima would be useful info - a disadvantage of having a display you can zoom in on is that it can make an insignificant difference look huge.

If you mean where the scanner visualisation is, blue arrow, then you are currently limited by the way the plugin was originalluy written by Vasili

For sdrsharp there is a web/mailing list at groups.io with much useful information.

It's the red arrow area. For example, when I switch between some stored frequencies the noise floor drops below the bottom of the window for a brief second then bounces back up again. This happens whilst scanning also - I'm assuming its because I'm running on a not-so-new laptop?

 

[morfis]

It doesn't sound like the noise floor is changing and may be right that your laptop may be sruggling to run the program.
What is the processor and how much RAM is avaialble? What elese is running?
What else is installed to your sdrsharp software (eg accessibility plugin, zoom plugins)?

A pre-Telerik version of sdrsharp might run better on your laptop - there is a link to some older versions in the scanner plugin thread you posted to and the sdrsharp download page might contain a link to the last pre-telerik version

Depending on the requirements at the time turning off display elements might help free up a bit of performance - eg. turn off the waterfall display if you aren't really using it (eg. if scanning airband it's not going to be of much use) or at the very least reduce it's refresh rate, turn off the spectrum display as well - these two items hog a lot of juice.

 

[WB9YBM]

Slight newbie question - when scanning with the frequency scanner plugin, the noise floor seems to jump about a fair bit - the noise floor does seem to increase as the frequency climbs (assume that's normal) but it jumps either up or down (so far down it disappears off the screen for a bit) erratically.

The "jumping" noise floor can be caused by any number of things: computer controls (used on everything from toasters to washing machines) turning off/on (some aren't as clean as others) to ignition noise from someone's souped-up hot rod rocketing past your house. I've even heard electric motors (especially those with worn brushes) from washing machines, to air conditioner compressors throw out electrical hash.

As for noise increasing as frequency goes up, yeah, to a point. Here's why: every time you have two or more transmitters in a given area, even if they run a theoretically perfectly clean signal, you're going to have heterodynes--that's where frequencies on the same frequency beat against each other. And just like in a radio receiver where the signal coming down the antenna beats against the first oscillator's frequency (in the process of converting the RF energy coming through the airwaves to frequencies capable of being heard by the human ear), you're going to have a total where frequency 1 + frequency 2 = frequency 3. (there's also frequency 1 - frequency 2 = frequency 4, but we don't need to worry about that right now).

So the more "frequency #3" you get (like in a big city with lots of people using radios) there can be plenty of noise on higher frequencies. And if there happens to be a licensed, legitimate user on frequency #3, you might be out of luck (depending on the strength of each signal).

A good example of this happened to me when I worked a Chicago repeater from my mobile: from the 'burbs I was able to work the repeater with no problem, but the closer I got to the repeater the less intelligible it became.

The quality of the receiver has something to do with it (or a cheap receiver with filtering), although even the best receiver cannot suspend the laws of physics.

 

[prcguy]

A lot of SDRs overload or their front ends go into compression if the gain is up too high or you have a lot of antenna in the air. As the front end preamp overloads much of the spectrum will bounce up and down as the strong signals causing the overload go on and off.

 

[merlin]

I might suggest it is a cheaper dongle with no bandpass filtering. You get a 2 Mhz spectrum with SDR# but the front end still picks up 50 to 1300 Mhz and signals out of band wreak havoc with the AGC.
A flaw I have seen with SDR# also is the spectrum noise floor changes sliding over 2 Mhz prompting adjustment of the floor.
Sadly, I am 300 yards from a major cell and repeater site so noise is strong above VHF like S9 or above.
You may be getting the same effect even though you are scanning lower frequencies.
Prcguy is on track here.
My solution is working up combline banpass filters to get rid of this out of band junk I have here. Been robbing filters from some old 800 Mhz Ericsson radios and re-tuning them for 900Mhz, it does help, but looking into a far better SDR radio.

 

[Ubbe]

but it jumps either up or down (so far down it disappears off the screen for a bit)

First check without antenna connected where the noise floor are and that it doesn't jump around in signal level.

The noise floor might go up 5dB or so when you connect the antenna, or you have too much gain set or you are too close to the transmitters. SDR dongles are not the best receivers for sensitivity or strong signal handling when they cost $10. The gain setting (without any AGC enabled) are probably dependent of the antennas frequency range and the frequency band you are monitoring. As mentioned the more transmitters you have in a frequency band the more will the noise level rise and you should probably reduce gain to keep that noise floor down. The important factor are the relation between signal and noise, the bigger difference the better, and not the signal strenght itself.

/Ubbe

 

[Feccy]

Well...I installed SDR# in a different laptop and no issues - so yes, it was hardware that was causing the issue. New laptop on its way so that'll sort it. Thanks for all the info

 

[Ubbe]

Well...I installed SDR# in a different laptop and no issues

If the PC isn't powerful enough you can reduce the load, and lower the frequency range that the spectrum displays, by clicking on the cogwheel and set the sample rate to 1.024MSPS which is more than enough for single frequency monitoring, that you do while in search mode.

/Ubbe