[Skimmertalk] Red Pitaya board with front-end improvements / transformer update

[Jeffrey Pawlan]

On 1/5/2018 7:34 PM, Bob Wilson, N6TV wrote:
> A German team has described a way to greatly improve the front-end
> characteristics of the Red Pitaya board to make it into a much more
> effective HF Receiver (improved sensitivity and improved strong signal
> handling).
> In the mean time, he's told me that it will be about another two or three
> weeks before I will receive more Red Pitaya 14:1 transformers.  I don't
> think they'll be needed with the new board, but I'm trying to find out, and
> I'm also trying to find out when the new Red Pitaya board might be
> available, and if all the old software will work on it unmodified.

It was I who told you last Fall that the transformer on the input is a 
poor bandage at best.  I already have experience building and modifying 
SDR receivers to match to 50 ohms and also utilize the full dynamic 
range of the A/D converter.  The Germans are doing the same thing. It is 
published in all of the Linear Technology A/D converter datasheets.

Once the R.P. input circuit is replaced with the correct circuit, you 
will not need any of the 14:1 transformers.  If the board designer is 
willing to make a proper PCB layout, it will save you considerable time 
and effort. Make sure he uses the correct transformers shown in the 
datasheet. You can wind your own on a core but it will not be as small.

This circuit change has no important effect on the software or firmware. 
The displayed gain or signal level will be different but I doubt that 
you are using this receiver as a calibrated signal level measurement 
tool.  I do not have a R.P. nor have used one so I do not know if there 
was a way in software to change the gain of the input amplifier. If 
there was, then that is the only change to the software as the input 
amplifier will be removed so that function would do nothing.

The improvement will be between 20 and 30dB more dynamic range.

73,

Jeffrey Pawlan  WA6KBL