Modern SDR offer panadapter performance that measures signals accurately. See what’s going on at a glance.
Since the 1990’s, we have gradually embraced digital over analog radios. By converting radio waves to numbers, software defined radios provide unparalleled measurement and display of signals. Once you get hooked on SDR, it’s hard to go back.
Above, you can see the entire spectrum of signals in the 22 meter shortwave broadcast band in SDR-Console. The top half of the panadapter display shows them in real time, while the lower half shows the past 30 seconds of signal history. Since the radio is sampling one million times per second, it’s easy to display a wide range of frequencies.
The magic involved here is the Fourier transform, a math trick that converts time-based signals into frequencies. In the current display, the FFT contains 65,536 little filters called “bins”. Each bin is 15.3 Hz wide. Each bin contains two numbers – the magnitude and phase of a radio signal on that exact frequency. You can easily convert the magnitude number to a measure of signal strength in voltage or power. Then, you can display that measurement as an S Meter scale or referenced to one microvolt (dBμV) or milliwatt (dBm). It’s just math and something computers do well.
I know that I am making this sound easy, but it’s not. Precision depends on many factors, including the number of bits in each sample. Signals smear across “bins”. The various transforms cause distortion and provide gain. Fortunately, engineers have figured out the corrections that need to be made. As long as the software writer understands the maths involved, the result can be very accurate. You will find that panadapter performance on cheap SDR on the market today is similar to expensive lab-grade spectrum analyzers from the past.
Panadapter Performance – Something Old is New Again
And, speaking of the past, panadapters emerged during World War II. Known by different names such as radio spectrum scope and panoramic receivers, they were used for signals intelligence. Perhaps the most famous of these was the RBV used by the US Navy and the BC-1031 used by the Army Signal Corps. On the consumer front, Hallicrafters introduced the Skyrider Panoramic in 1946. And, most hams my age will remember seeing the Heathkit SB-620 in action.
These early panadapters were completely analog. You just connected an oscilloscope across the IF stage of your receiver, and typically displayed ±100 Hz bandwidth around your signal of interest. In most cases, though, calibration was very rough, and the early spectrum scopes were more about spotting signals than measuring them.