Re: [Discuss-gnuradio] Re: Low cost hardware option

[Marcus D. Leech]

On 01/15/2011 09:15 AM, Moeller wrote:
> On 15.01.2011 13:45, Patrick Strasser wrote:
>   
>> For flexibility, being able to bypass stages or feed signals e.g. at the
>> ADC would be cheap. Preparing for different transport systems would make
>>     
> I wonder how much noise will be introduced with a switch at the ADC.
> At least I would use 2 different input connectors, because switching
> in front of the LNA would surely introduce too much noise (except
> some special shielded RF relays).

The HMC221AE that I've shown in the latest diagram has an insertion loss of
  0.4dB (or less at lower frequencies).  Further, my assumption is that the
  "direct" inputs would likely have their own gain/filter chain ahead of
them anyway.
  I built a science instrument with the BASIC_RX, and had about 40dB of
gain+filtering
  in front of the BASIC_RX.  Certainly if you wanted those external
inputs to be
  "off air" capable, you'd need a gain stage or two.

But one of the significant uses for a BASIC_RX-type of interface to the
outside world
  is for doing things like pulling in the IF of an existing receiver,
where there's already
  plenty of signal available, so the insertion loss of the switch is
negligible.

If those external inputs are used for general signal probing, a
slightly-higher noise
  figure in front of the ADC isn't going to matter much.  Actually,
looked at another way,
  the effective noise figure of the ADC is quite high anyway, so a small
amount of additional
  noise added by the RF switch just isn't going to be an issue.


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