[hpsdr] An high dynamic QSD front-end

[Marco IK1ODO]

At 23.37 14/01/2007, N4HY wrote:
>It really is time to build hardware.

So I did, spending three months to optimize it.

>WARNING:  We definitely want to steer away from the FST 
>switches.  FROM MEASUREMENT,  coupled with DDS issues,  they are the 
>source of the increasing noise figure.  Marco's plots, as well as 
>Phil C's and my measurements of various QSD's using them show this 
>inescapable fact.  I think various pieces of analysis have 
>incorrectly attributed the measurements but the switches are a major 
>contributor.
>They solved an early problem and we can thank Gerald for finding 
>this part and his very gracious publication in QEX and with the SDR-1000.
>But it is time to leave them behind FOREVER.
>
>73's
>Bob
>N4HY

Bob,

why do you say so? I think I measured and demonstrated exactly the 
opposite: the switching noise from the FST may be minimized, and the 
overall level plan of the receiver has to be considered to reduce the 
noise contribution of the opamps. I see no "inexcapable facts".
Please consider the figures that I measured - in some point perhaps 
by defect, see http://www.spin-it.com/sdr/IK1ODO_SDR1.html

Low gain audio interface setting (+4dBu f.s.)

RF input level for A/D converter saturation: -3dBm
SFDR: 90dB with -10dBm RF input
Noise floor: -123dBm / 2.4kHz
3' order dynamic range: 105 dB
IIP3: +36dBm


High gain audio interface setting (-10dBu f.s.)

RF input level for A/D converter saturation: -17dBm
SFDR: 100dB with -20dBm RF input
Noise floor: -128dBm / 2.4kHz
3' order dynamic range: 102 dB
IIP3: +25dBm

How many receivers designs get similar figures with such a simple hardware?

I think it's time to go ahead with Phoenix. It will not be the 
definitive receiver, but using the ideas of Ahti and my tested 
frontend we may get a very nice intermediate solution, at a low cost. 
The LO may be derived from I0CG design (AD9951 plus a 500MHz clock); 
possibly a PLL in 10kHz (or wider) steps may be better, but it's not 
trivial to design it with sufficient spectral purity.

73 - Marco IK1ODO
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