The 2020-2024 Extra License question pool asks for the definition of direct digital conversion in a software defined radio:
E7F01: What is meant by direct digital conversion as applied to software defined radios?
A. Software is converted from source code to object code during operation of the receiver B. Incoming RF is converted to a control voltage for a voltage controlled oscillator C. Incoming RF is digitized by an analog-to-digital converter without being mixed with a local oscillator signal D. A switching mixer is used to generate I and Q signals directly from the RF input
This direct digital conversion question gets at the essential difference between a classic heterodyne receiver and a modern software defined receiver (SDR) that uses direct digital conversion. Let’s start with a brief description of simply “digital conversion,” and then we will contrast the “direct digital conversion” of an SDR with classic heterodyning.
In the process of digital conversion an analog voltage signal is sampled to measure instantaneous voltage values over time. Each sampled voltage value is translated into a digital value that may be stored in a digital array and subsequently manipulated or analyzed by digital processing algorithms. An electronic module that performs digital conversion is called an analog-to-digital converter (ADC).
Usually, the sampling rate for an analog AC signal is at least double its frequency, or greater, in order to obtain a digitized set of data that accurately represents the analog waveform. A sampled AC waveform may be reproduced from the set of stored sampling values and the application of filtering or smoothing techniques to “fill in the gaps” between sampled values. You can read more about digital signal processing and the digital conversion process in our DSP article.
Direct digital conversion refers to the position within the receiver’s RF signal processing stream where such digital conversions take place in a software defined radio. First let’s review from the General License Course the processing sequence associated with classic heterodyne receivers, and then we will highlight the difference with direct digital conversion.
Recall that in a heterodyne receiver (aka superheterodyne), the received RF signals are mixed with a local oscillator AC signal to produce sum and difference product signals – that is, the sum of the received frequency and oscillator frequency, and also the difference between the received frequency and the oscillator frequency. The difference frequency is lower than the received frequency, and this product signal is passed along as the intermediate frequency (IF) in the processing stream. The IF is amplified and further mixed down to extract the audio frequencies that provided the RF modulation for the received signal when it was originally transmitted, thereby reproducing the audio of the received station’s signals.
In early digital receivers the intermediate frequency was subjected to digital conversion in order to ensure that the achievable sampling rate was greater than the signal frequency. Thus, the ADC was positioned after the IF stage in receiver processing. However, as ADCs were engineered to have much higher sampling rates, it was feasible to make sufficiently high samples of the received RF signals themselves. With such high sampling rates there is no need to mix the RF signal down to a lower IF to affect digital conversion. As such, an RF ADC component of a software defined radio may completely replace the mixer/local oscillator and subsequent processing components of the classic heterodyne.
In a software defined radio using this technique, the RF signals are directly digitized at their received frequency and never mixed with a local oscillator signal to generate a lower intermediate frequency. This is direct digital conversion.
The answer to 2016-2020 Extra Class question E7F01, “What is meant by direct digital conversion as applied to software defined radios?” is “C. Incoming RF is digitized by an analog-to-digital converter without being mixed with a local oscillator signal.”
-- Stu WØSTU