LO Feedthru of LTC5588-1 EVM

Question

As for LTC5588-1 board, Since I had no single-ended to differential converter, The single ended signals I and Q connected to BBIP and BBQP ports with BBIN and BBQN 50 ohm terminated.
The component f0+fIQ is highly removed, but LO feedthrough is high. I would like to know why this feedthrough is large. Does this come from the unbalanced drive of I and Q ???

shsiao · Accepted Answer

The unused differential input pins still need to biased to 0.5V common-mode voltage and bypassed with 10nF to ground. LO feedthru is a function of DC offset voltages on I and Q inputs. It should be as small as possible.

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LO Feedthru of LTC5588-1 EVM

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