This article continues on from Riding the RF Gain control – part 2 and explores the operating advice when applied to the next generation of receivers.
Conventional superheterodyne communications receiver with DSP demodulation.
The next generation of receivers was a conventional superheterodyne with a DSP based demodulation stage (initially at quite low Intermediate Frequency to suit the power of the available DSP chips).
Communications receivers were enhanced by replacement of the demodulators with a DSP performing demodulation digitally. The DSP sampled the IF signal and digitised it, and channel filtering and demodulation was performed ‘mathematically’ using the digital data stream as input.
There are two significant differences with this change:
- receiver bandwidth can be determined by digitally synthesised passband filters in the DSP; and
- first step in the DSP process is conversion of the IF signal to a digital stream in an analogue to digital converter (ADC).
Critically, the Analogue to Digital Converter (ADC) had an overflow point, and overflow of the ADC creates serious IMD and major degradation of received signal, overflow has to be prevented at all cost. To limit the power delivered to the ADC, a narrow ‘roofing’ filter usually preceded it, and the channel filter was digitally synthesised. Continue reading Riding the RF Gain control – part 3