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
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