Re: [Discuss-gnuradio] Building an RF Front end for DSP FPGA Kits with ADCs

[Marcus D. Leech]

> Thanks Marcus.
>
> The reason why I do not want to attenuate is because I want to receive a
> high-powered signal and low-powered signal at the same frequency.
>
> If I attenuate then the low-powered signal will be reduced and if I go
> beyond the noise floor, I might see it.  The goal was to stretch the
> difference in power between the high-powered signal and the low-power.  But
> now if I think about it no matter what I will be limited to my Dynamic
> Range.
Yes, given that perfectly-acceptable signals can arrive at the front-end 
at -90dBm or lower, and you want to be able
  to "see" +30dBm at the same time, you'll need roughly 120dB of 
dynamic range.  But +30dBm is about 7.07VRMS, or
  about 10VP.  There's no way on gods little green earth that you're 
going to find an ADC that can sample at 100Msps or
  more, and have a peak input voltage of +10VP (+7VRMS), and have a 
120dB dynamic range.  Let alone a receiver
  front-end RF chain that can tolerate anything more than about +15dBm 
or so.

For 120dB dynamic range, you'd need a 20-bit ADC, *and* it would need to 
run at a fast sample rate, *and* it would need to be
  able to handle +10VP input.  Not gonna happen.

I'm curious about why you need to inject +30dBm into a receiver.  You're 
getting no more information out of that signal than if
  it arrived at -30dBm.


--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org