Re: [Discuss-gnuradio] Modify the Basic RX Daughterboard

[Marcus D. Leech]

On 09/08/2011 01:08 PM, valentac wrote:
> I have my own RF front end i'm using with the USRP, but it requires a High
> impedance load to operate correctly. Since the basic RX daughterboard has a
> 50 ohm impedance, I have to modify it or add a separate voltage buffer
> before the input. However, since the basic RX daughter board in the USRP has
> an op-amp in its first stage, perhaps I could modify this.
>
> I've tried removing R3 and R9
> (http://code.ettus.com/redmine/ettus/documents/4), but this did not seem to
> load the RF front end sufficiently. Would changing the 348 ohm resistors to
> a higher value help increase the input impedance into the basic RX
> daughterboard? If so, how would this affect the output going into the A/D
> (besides the obvious filtering effects).
I'd say consult the data-sheet for the AD8132.  But just to clear things 
up, you're discussing
  the *LF_RX* daughtercard, *NOT* the BASIC_RX, which has no op-amp on 
it at all, just a
  transformer.

If your input impedance isn't insanely high, you could use a 
transformer.  4:1 broadband impedance
  transformers are available fairly cheaply from Mini-circuits, etc.




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