AD9910: EVB R43 resistor value

Document created by analog-archivist Employee on Feb 23, 2016Last modified by analog-archivist Employee on Feb 23, 2016
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There is a question about the EVB of AD9910.
Would you kindly tell me the function of the resistor in the circle.


Is it used to do impedance match? If so, it should be 100ohm if the
transmission line is 50ohm. Is this correct?
I tested the VSWR of the AD9910's output according to our EVB.
The frequency is from 100MHz to 400MHz, but I found the VSWR is around 2 when
it output low frequency, but refer to high frequency, the VSWR is more than 3.5.
Would you kindly give some professional suggestions about how to reduce the
VSWR.

 

You are correct, a 100 ohm value for R43 would be the best choice for impedance
matching.
In that case, the reflected impedance from the combination of (R7, R9, R43) at
the output of the transformer would be 50 ohms.
That would then match the 50 ohm filter with 50 load.
Note, the original intention of R43 (if used) was to remove R7 and R9 and
populate R43 only.That would eliminate resistor mismatch and possibly improve
common rejection.
If R43 is not populated, R7 and R9 would need to be 25 ohms for a proper 50 ohm
match.Some of our DDS evaluation boards are not matched. Having a higher value
termination resistor can deliver more power but the trade off is VSWR.

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