Modify the Basic RX Daughterboard

GNU Radio
1

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).

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2

On 09/08/2011 01:08 PM, valentac wrote:

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 L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

3

My mistake, I am using the BasicRX with the transformer (I was looking
at the
wrong datasheet). I could go ahead and get a new transformer, but I
would
ideally like a higher impedance than is offered. I just tried to modify
the
transformers in the schematic
(http://code.ettus.com/redmine/ettus/documents/2) by removing R3 and R9,
and
then changing R6 and R7 to 1 MOhm, however, this didn’t seem to give me
a
large enough impedance.

Is there a better way to modify the transformer that is on the BasicRX
to
give me a high impedance?

Marcus D. Leech wrote:

the LF_RX daughtercard, NOT the BASIC_RX, which has no op-amp on

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