[Premium-Rx] The new R&S EM510 Direct-Sampling HF Receiver

[Adam Farson]

Hi AW,
 
You made an excellent point. The basic problem with the direct-sampling
approach is the ADC headroom. A 16-bit ADC, no matter how fast, has 16*6 =
96 dB theoretical dynamic range. (I understand that some device
manufacturers are claiming SFDR > 96 dB, but am not sure how that is
accomplished.) There is a practical lower limit to the RF bandwidth which
the RF filters present to the ADC. It will not take too many strong signals
falling within this bandwidth to over-range the ADC; hence the
"low-distortion" mode, which bypasses the RF preamp and/or inserts
attenuation ahead of the ADC input - obviously degrading the noise figure.
 
The other side of the coin is that direct sampling eliminates the mixer, LO,
roofing filter etc. The ADC clock is the main determinant of the receiver's
"phase noise" performance; R&S gives a figure of -140 dBc/Hz (typical) at 1
kHz offset - pretty impressive. So it would seem that direct sampling is a
technology worth pursuing further. Its performance is bound to improve as
better ADC's are developed.

Cheers for now, 73,
Adam VA7OJ/AB4OJ



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