5. Digital Radio and Receivers on The HamStudy Amateur Extra Class HamBook (2024 - 2028)

5.1 Converting Signals to Data

Mon, 01 Jan 0001 00:00:00 +0000

Section 5.1: Converting Signals to Data #

In a software-defined radio (SDR), software does the filtering and detection. First, though, an analog-to-digital converter, or ADC, has to turn the signal into numbers. Some SDRs mix the signal down to a lower frequency before converting it, much as a superheterodyne receiver does. Others leave even that frequency conversion to software.

Key Information: Direct sampling in software defined radios means incoming RF is digitized by an ADC without being mixed with a local oscillator signal.

5.2 Digital Signal Processing

Mon, 01 Jan 0001 00:00:00 +0000

Section 5.2: Digital Signal Processing #

A receiver that samples everything from near DC to 30 MHz captures the whole HF spectrum at once, as tens of millions of numbers every second. To hear one SSB voice, though, you need only about 3 kHz of it. Digital processing has to pick out that channel and drop the data it doesn’t need. Its tools are simple: delaying samples, scaling them, and adding them. The same tools can also generate a signal for transmitting.

5.3 Noise and Sensitivity

Mon, 01 Jan 0001 00:00:00 +0000

Section 5.3: Noise and Sensitivity #

Tune to an empty spot on the band, and you still hear a soft hiss. Part of that noise comes in through the antenna, and the receiver itself generates the rest. A weak station is readable only if it stands out above that background. More gain lifts the hiss along with the signal, so judging a receiver means measuring its noise.

5.4 Receiver Limits and Distortion

Mon, 01 Jan 0001 00:00:00 +0000

Section 5.4: Receiver Limits and Distortion #

A receiver can hear an extremely weak test signal and still struggle on a crowded band. Nearby strong signals place different demands on its filters, amplifiers, mixers, and converter. Some interference enters through the antenna; some is created inside the receiver itself.

The Other Frequency That Fits #

A superheterodyne receiver’s mixer converts two input frequencies to the same IF: the wanted signal and an image twice the IF away. Once the image reaches the IF, even a sharp IF filter can’t separate it, so the input circuits must reject it before the mixer. With a 455 kHz IF, the image of a 14.000 MHz signal is only 0.91 MHz away, too close for a broad input filter to remove. With a 9 MHz IF, it is 18 MHz away.