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

[Chuck Hutton]

Adam:

96 dB would indeed be the limit if there was nothing else in the picture.
However.....

A common technique these days is to oversample and decimate. A 4-times
oversampling with decimation gains you an additional bit of resolution. Do
that several times (to take a 65 Msps down to the width of a ham band or
sub-band or several SW channels or....) and you'll get easily get 18 bits or
more of accuracy.

Here's a fairly simple and clear explanation:
http://www.atmel.com/dyn/resources/prod_documents/doc8003.pdf


Chuck

-----Original Message-----
From: premium-rx-bounces at mailman.qth.net
[mailto:premium-rx-bounces at mailman.qth.net] On Behalf Of Adam Farson
Sent: Thursday, February 15, 2007 10:25 PM
To: sig346 at netscape.net
Cc: Premium-Rx
Subject: RE: [Premium-Rx] The new R&S EM510 Direct-Sampling HF Receiver

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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