Many of you may be familiar with the HF+ Discovery, Airspy's excellent, highly sensitive, high dynamic range, software-defined radio. I acquired one about three years ago to compare with my SDRPlay RSP1a. There is no competition there, the Discovery wins hands down in virtually all categories.
SDR Sharp, known as SDR#, is Airspy's SDR software made for their product line of receivers, including the HF+ Discovery. It is an outstanding piece of tech and includes many frills like the innovative Co-Channel Canceller which can effectively cancel or null one signal amongst several others on the same channel. I use SDR# exclusively here for reception of mediumwave and shortwave.
Herein we will describe a tip on how to accurately check signal-to-noise ratio using Airspy's SDR# receiving program. More Airspy SDR# receiving tips may appear in this blog, so check back often. I have several of them in mind.
In all the talk about signal strength and signal-to-noise ratio (SNR) lately, the community is finally coming around to realizing that SNR is what makes the difference between copying a station or not. Surprisingly signal strength, usually measured as the strength of the received carrier, may have little to do with it.
Noise floors on today's receivers are high, though there are ways to mitigate this using low-noise antennas and other sundry items in our toolkit. The goal is to set up an antenna and feed system environment with a low noise floor and capture enough signal to rise above that. Don't focus on S-meter readings and brute signal strength. The oft-held directive usually heard is, "Allow the antenna to set the noise floor". Translation: We want enough antenna to capture the signal, but not too much antenna to raise the background noise floor to unacceptable levels.
Signal-to-Noise Measurements
So first, how do we measure SNR? And second, how much signal-to-noise difference do we need? We are not talking about tedious lab measurements here, but simply, "How far above the noise is this signal I'm receiving?". Casual measuring of signal-to-noise ratio while receiving on an SDR is often not done correctly, and sometimes SDR software itself does not report it correctly. It's a value that's interesting to know, and tells us a lot about the signal, our background noise level, and the recoverability of the audio we might be hearing.
To get a sensible reading we must set our bandwidth properly and position it correctly in relation to the signal's carrier. The SNR reported will be the signal power contained within the currently set bandpass compared to the average noise floor also within that bandpass. The measurement problem arises when the station's carrier is contained within the filter bandpass. As we will soon see, the carrier does not convey intelligible information, but its presence may add as much as 20 dB or more to our SNR reading!
So therein lies the mistake: Don't center the bandpass on the carrier! In fact, keep the carrier outside of the bandpass altogether.
dBm Talk, And How We Measure Signal Strength
dBm is the common scale used to quantify received signal power. The dB part, or the decibel, is a logarithmic ratio of one value to another. The 'm' part means relative to a milliwatt, or decibels below (can also be above) one milliwatt expended power into 50 ohms. Let's be clear here about dBm and signal measuring. Most SDR receiving software, and SDR# is one of them, does not have calibrated spectrum scales in dBm. The scale shown is in dBFS, or dB relative to Full Scale. Full Scale is where the SDR's analog-to-digital converter overloads. The meat of our testing here will refer to relative values in dB when comparing loops to each other. That is the important thing to note in the discussion which follows.
Let's talk about the makeup of an AM modulated signal, that which we usually receive on our SDR.
The AM Modulated Carrier
The total AM modulated signal consists of the lower sideband (LSB), the carrier, and the upper sideband (USB). They occupy bandwidth on the RF spectrum. Any time information is impressed upon a carrier it occupies bandwidth. The carrier itself is very narrow, and may only occupy 100 cycles (Hz), maybe even less, depending on how pure it is. All three are easily seen on an SDR spectrum display if we zoom in on the signal a little. Find a signal on your SDR and expand your bandwidth out to about +/-5 KHz either side of the carrier peak and you will see the sidebands fluttering up and down. The audio intelligibility is contained in the sidebands, not the carrier, and can be extracted in total from either sideband. Also note the lower and upper sidebands are mirror images of each other.
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| AM modulation showing sidebands |
If the carrier is fully modulated, called "100% modulation", the carrier (the spike in the middle) will contain 50% of the station's power, and each sideband will contain 25% of the power. If there is no audio signal modulating the carrier, then there will be no power in the sidebands.
To properly measure the signal-to-noise ratio of the signal we're trying to hear, we need to measure how far above the noise the entirety of one of the sidebands is. Thus, we must isolate one of the sidebands in the bandpass.
Here's how, using SDR#:
1. Be sure to use "snap to center" tuning, that is, when you click on the spectrum, the tuning point is always centered in the spectrum window.
2. It's always good to be in DSB (double sideband) tuning mode.
3. Take a look at the zoomed in signal and note how wide the modulation is from the carrier peak to the general edge of the audio variations on either side.
4. With your mouse, grab one of the edges of the bandpass and drag it to reduce the total bandwidth to this edge. It may approach 10 KHz, or +/-5 KHz.
5. Note the bandpass width and roughly divide it in half. Example: If 10 KHz, use the mouse to narrow it further to 5 KHz. We want the width of one of the sidebands. Important! Stay away from encroaching splatter from adjacent channels.
6. Retune the receiver, offsetting the main tuning either positively (for an upper sideband check), or negatively (for a lower sideband check), so that the bandpass contains only the sideband. Important! Make sure the carrier peak is just outside the bandwidth stripe. Try to keep a margin of at least 100 cycles (Hz). It's imperative to not include the carrier in the SNR test. It conveys no intelligible information and greatly inflates the SNR reading to a meaningless value for our purposes.
7. Last, hover your mouse within the bandwidth stripe on the spectrum display. Let the SNR reading settle in for a few seconds. You will see several values. The SNR value will be your signal-to-noise value for the set bandpass width. The Floor value will be the noise floor.
Let's try some examples.
In the example, we see WPIE-1160 in Trumansburg, NY, a 5 KW station at 94 km distance. Our real signal-to-noise ratio is 17.1 dB with a noise floor of -116.6 dBFS. We have 17.1 dB of signal poking its head above the noise. That's plenty, but not overly strong. Our original screenshot of us hovering over WPIE's carrier with the bandpass centered would have us believing our SNR was 44.1 dB. That would be considered an exceptionally strong SNR. But it includes the power in the carrier. Remember, the carrier itself carries no audio information.
Click image for larger version.
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| WPIE-1160 |
One more example, WSKO-1260 in Syracuse, NY a 5 KW station at 121 km distance. WSKO is extremely weak, and marginally copyable just out of the noise. Our real signal-to-noise ratio is only 8.2 dB with an extremely low noise floor of -126.4 dBFS. That is minimal and about the weakest signal which we are able to extract copyable audio. Our original screenshot of us hovering over WSKO's carrier with the bandpass centered would have us believing our SNR was 29.6 dB, under normal circumstances a very comfortable listening level.
Click image for larger version.
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| WSKO-1260 |
You will find that signal-to-noise ratios of 9-10 dB to be about minimum for extracting audio intelligibility from a signal. 15 dB becomes comfortable, 25 dB armchair copy.
Remember, centering our carrier in the bandpass is our mistake in measuring SNR. A carrier with no audio modulation or one which is weakly modulated may show an SNR of 40 dB. It's a meaningless value unless you wish to know the SNR of the carrier itself to the noise.
Try this method of checking signal-to-noise ratio. It will give a much more accurate representation of the receiving condition.
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