This article continues on from Riding the RF Gain control – part 3 and explores the operating advice when applied to the next generation of receivers.
Direct sampling SDR
Lets jump a generation to the direct sampling SDR configuration.
In this category, I am covering receivers that do not convert the receive signal to an intermediate frequency, the ADC samples the signal at its off-air frequency.
The receivers may or may not have the following elements ahead of the ADC:
- bandpass filters;
Because there may be no amplification prior to the ADC in some operating configurations, a voltage controlled attenuator may be used to prevent overflow of the ADC, this is the ‘analogue’ part of the AGC system.
Like the DSP demodulated receiver, the demodulator channel bandwidth may be less than the bandwidth presented to the ADC, much much less in this case and again, to be effective the AGC system must have two control actions:
- prevent overflow of the ADC; and
- level the demodulated channel output.
The optimum configuration for operation on the lower HF bands where external noise is much greater than receiver internal noise is to operate the receiver with no preamplification having the benefit of eliminating IMD from within the preamplifier with negligible reduction in realisable S/N ratio, indeed an improvement is likely when IMD noise is factored in.
The receiver should try to automatically protect the ADC from overflow using an AGC controlled attenuator, and in extreme cases additional fixed attenuation (probably using a voltage controlled attenuator) may need to be selected.
Again, good implementations work fairly well and it is doubtful that setting max AF gain and riding the RF gain control improves S/N on most signals.
The next part is a summary of the four parts so far.