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Radio Spectrum Watching Expectations

radio spectrum watching expectations

So, are you achieving your radio spectrum watching expectations? Let’s take a look at the details of display optimization.

Most new receivers and transceivers are software defined radios. You will also find that most have spectrum displays, whether they are packaged or PC connected. In October 2009, ARRL Product Reviews added a new metric called Spectral Display Sensitivity. If you read the details, you will notice this is a measurement of display dynamic range. Most packaged gear have spectral sensitivity down to -113 to -126 dBm over a 100 kHz bandwidth. This is more than enough to display all signals above the external noise floor for HF users.

By contrast, most PC SDR software displays down to -150 dBm, which is overkill on HF but useful at higher frequencies where receivers typically have a much lower noise figure. And, depending on your radio spectrum watching expectations, you can make many software adjustments to optimize the display.

Your main FFT adjustments are resolution and display range. Resolution will increase FFT sample size and decrease FFT bin width. Range will enable you to select how much bandwidth you display at one time, as well as the upper and lower amplitude levels. Often, these two adjustments work together.

By increasing FFT size or reducing displayed bandwidth, you can reduce the displayed noise floor and see more signals. Pretty much all software offers these choices, or makes adjustments automatically.

Second, you can select from a variety of FFT windows which reduce spectral leakage and sharpen the display. Some windows are best for measuring amplitude or others frequency resolution. Take some time to read about what the different windows do and experiment.

Lastly, you can reduce instantaneous noise by using FFT averaging and get cleaner signal traces by reducing the display frame update rate.

Radio Spectrum Watching Expectations across different tools

If you are like me, you probably use several SDR applications and several different receivers.

Should your spectrum display be the same across different radios, software applications and antennas? Not likely.

Most radios distribute and manage their gain differently. Using different radios with the same software produces variable results, even with comparable ADC bit depth. On the other hand, using different software with the same radio can come close. But you will find this depends on how you have configured your FFT parameters.

Of course, different antennas can vary in efficiency and effectiveness across different frequencies. The most obvious difference are in patterns and amplitudes. But if you are lucky, you can often achieve similar signal to noise ratios, which is what really counts.


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