Noise determines largely whether you can receive a signal or not. At the HF frequencies where we operate Softrocks it is usually the outside noise pickup by the antenna that limits the weakest signal we can receive. If our radio/soundcard/computer is making more noise than we receive on the antenna then we have a problem.
Noise is also a good indicator of a working radio. As Alan, Bob, and others have noted on this board, the surest and quickest test for receiver sensitivity is to connect an antenna and see if the noise level goes up. If it does then your receiver is as sensitive as it needs to be to use on these bands.
But the ability to accurately measure a noise level is very valuable when you are trying to optimize the performance of a system. So I want to describe a very simple way to accurately measure relative noise to within 1 dB.
For this you need an SDR radio, HDSDR, and a calibrated signal source. I used a homemade S9 calibrator that puts out a -73 dBM (S9) signal and is switchable on all bands.
The steps are:
1) Calibrate the HDSDR S meter so that the -73 dBm signal is reading exactly -73 on HDSDR. Use the spectrum window in the lower right and use the left slider to expand the vertical scale all the way until you see 1 dB increments. Then use the next slider to the right to view the portion of the vertical scale containing -73 dBm. Us the Misc. Options/Calibrate S meter function to adjust so that the peak of you calibrator signal hits exactly -73. It is touchy so you may have to do it a number of times.
2) Now disconnect the calibrator and terminate (put a dummy load on) the antenna connector. Using the second slider from the right, move down the vertical range until you see a screen full of noise. Depending on your audio card/radio this will be anywhere from -145 to -100 dBm. You will see a screen FULL of noise. The center of this mess is the level that represents your noise floor. But you don't have to guess! On the far right of the Spectrum display you will see an "Average" pull down...... do that and select "128". Magically, over the period of a few seconds, that mess of noise will resolve itself into a relatively straight line at a value you can determine and write down. With internal laptop cards mine tend to be in the -140 to -135 dBm range.
3) Now connect your antenna to the radio and you should see this level rise dramatically. Expect to see it go up from 10 to 30 dB depending on the noise levels at your location. Tune to a quiet portion of the band and let the averaging settle and, voila!, you now know the real noise floor of your system, both the noise floor of the card, and the operating noise floor on the particular band. Typically, with some exceptions like WSPR, you will not be able to resolve signals at lower levels than the antenna noise floor.
Important note: you will get different numbers depending upon the mode and bandwidth selections on the HDSDR software. I record all of my numbers with the mode set to USB or LSB and the filter bandwidth set to 2.5 KHz.
You can do all of this without the calibrator and still get noise measurements that are accurate as compared to each other. Calibration at a known level allows you to express the levels as absolute; -145 dBm for example. Uncalibrated, you can still know with confidence your antenna noise floor is x dB above your system noise floor and this is all that is really important.
Warren Allgyer - W8TOD