Direct Radio Signal Amplification Chip RDA1846
More and more often, when producing budget-class radio communication devices, manufacturers use direct conversion circuitry based on the RDA1846 chip. This solution significantly reduces the cost of the transceiver while maintaining acceptable receiver sensitivity and selectivity.
Main advantages of direct conversion circuitry
- Low price and minimal adjustment complexity of the receiving path; everything is implemented on a single chip
- High-quality FM decoding using the built-in DSP processor (in relation to the RDA1846 chip)
- Compact size
- Higher mechanical reliability of the radio due to the absence of a fragile quartz filter
Main disadvantages of direct conversion circuitry
- Low dynamic range of the receiving path
- Inferior receiver sensitivity and selectivity
- Susceptibility to interference
The main drawback of a direct conversion receiver is low selectivity, meaning insufficient suppression of signals from adjacent stations compared to the tuned station's signal. Therefore, this type of receiver is convenient only for receiving powerful radio stations.
In practice, radios based on the RDA chip, when operating in areas saturated with industrial interference, often experience signal blockage from powerful broadband noise, which the DSP signal processor attempts to convert into a useful signal. In open terrain and in the absence of nearby interference sources, such issues do not occur.
The disadvantages of radios using the RDA1846 stem from simplified circuitry, the absence of tunable input filters, and an overestimated HF amplifier gain. Some negative effects of such circuitry can be partially compensated by additional modules, but this negates the main goal—reducing the cost of the radio. When using a direct conversion receiver on shortwave bands, for example in amateur radio, dual-sideband reception becomes a serious drawback, as narrow amateur bands have many interfering signals from nearby stations. Suppressing the unwanted reception channel can be achieved using a phase compensation method. However, this eliminates the receiver’s key advantage—simplicity of design and tuning.
In terms of reception performance, a classical superheterodyne circuit works noticeably better, providing an acceptable dynamic range for portable radios and high selectivity for spurious reception channels, thanks to properly designed input circuits. A superheterodyne receiver, due to more thorough tuning and design (to prevent the receiver from generating its own interference), also ensures frequency stability.
Brands such as Yaesu, Icom, and some Vector and Adjetrays models use a superheterodyne design, making these radios significantly more expensive than budget ones built on the RDA chip, such as Baofeng, iRadio, Voxtel, and Midland.
The revival of direct conversion receivers began in the 1960s with the introduction of a new component base—operational amplifiers and transistors. This made it possible to use high-Q active filters on operational amplifiers. It turned out that, despite their relative simplicity, direct amplification receivers could show characteristics comparable to superheterodynes.
Related articles: Modulation, DSP signal processor, TDMA, CDMA, dPMR, Heterodyne.