[Skimmertalk] The Red Pitaya 122.8-16 -- an important lesson learned.

[N4ZR]

Sure, Jeffrey -  I also have no precise idea what the insertion loss of 
the two series filters is, but they cleaned up my 15 and 12-meter 
issues.  Some rough-and-ready tests with an Elecraft XG-3 Signal Source 
and HDSDR's S-meter suggest it's a few dB, and more than made up by the 
11-dB Norton preamp.

I am confident that a high-quality set of band-pass filters would be 
preferable, but since I'm trying to skim 8 bands simultaneously, that 
would be pretty clumsy to implement - imagine an 8-way splitter, 8 
filters, an 8-way combiner and an amplifier to make up for the losses.  
Actually, that's what W3LPL uses at his station, as described on the RBN 
blog recently - reversebeacon.blogspot.com

73, Pete N4ZR
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On 8/14/2019 9:39 PM, Jeffrey Pawlan wrote:
>
> The partial schematic shows what is in the front end between the SMA 
> connector and the A/D converter.  He even removed some components that 
> should be there for optimal performance. There is definitely no filter.
>
> I modified one of my Perseus radios to have a similar unfiltered 
> input.  I carried it will me all over the US, Canada, and Israel to 
> give lectures on SDR. I took a simple signal generator with me to show 
> that the untuned SDR radio would receive very well up to almost 500MHz 
> and demonstrated how alternating Nyquist bands would change tuning 
> direction.
>
> Perhaps you might find limited performance with the purchased external 
> filters. That would come as a result of the reflections of the signals 
> and the noise from the A/D sample and hold being reflected back by the 
> filters into the A/D input. To maximize performance one would need to 
> buy or build what are called non-reflective (aka absorptive) filters 
> for your specific bands of interest.
>
> 73,
>
> Jeffrey Pawlan  WA6KBL
>
>
>
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