Deep down, your SDR receiver creates numbers. Lots of them. Where do these numbers come from? What do they mean and how are they used?

When you listen to your SDR, you are listening to numbers. When you watch radio signals and noise, you are watching numbers. Your SDR receiver creates numbers by transforming voltages into digits. These digits represent signals and noise. This magic is accomplished by the analog to digital converter, ADC.

ADC compares an input voltage, from your antenna, against a reference voltage and measures the difference. Typical ADC operate at an input range from 1.5 to 2.5 volts peak-to-peak. Input voltages are continuous; output digits are discrete, a series of steps. You will find that the size of each step depends on the number of bits. If your ADC has 14 bits, each step will be the maximum voltage divided by 2^{14} or 16384. This means it can measure a voltage change of 153 μV. Adding a preamplifier of 20 dB means that your ADC can measure voltages down to 15 μV. Smaller signals than this will just get lost in the (quantization) noise without further processing by the FFT.

The range of frequencies you can convert into numbers depends on ADC sampling rate. Most other receiver characteristics – sensitivity and dynamic range depend on the number of usable bits in your ADC. More bits means better dynamic range and ability to distinguish signals from noise.

You want your SDR receiver to use as many of its bits as possible. This is one reason for a preamp or attenuation at the start of the signal chain. Boost the signal above the ADC quantization noise floor, or decrease your signal below its maximum (clipping) level.

After digitization, the FFT can provide spectrum visualization, and convert the data into the frequency domain for demodulation. More on FFT performance later.

## SDR Receiver Creates Numbers – Understanding Noise Figure

Ideally, as your SDR receiver creates numbers, it should not make your signal-to-noise ratio worse. This means that neither your LNA nor ADC should add noise as they process input voltages. Engineers define the Noise Figure (NF) as the amount by which the received SNR deteriorates. Ideally, NF should be below the level of atmospheric, galactic and man made noise at your antenna.

However, most ADC have a poor Noise Floor, typically 30 dB, which might be fine on lower HF but is too high for upper HF and beyond. Placing a low noise amplifier ahead of the ADC not only amplifies, but more important, reduces the NF of your entire (cascaded) system. In effect, the LNA determines your radio’s overall Noise Figure, not the ADC.

So, the ADC in my Flex 6300 has a NF=28 dB, which is fine for a system Noise Figure below 10 MHz. Above that, activating the preamp reduces the Flex 6300 Noise Figure to 19 dB while adding 10 dB of gain. (You might want to play around with this Cascaded Noise Figure calculator.)

Because of the way your SDR receiver creates numbers, you can watch and use radio signals with a high degree of measurement accuracy and resolution.