combining LTC6948 and LTC5562

(I asked this also to the technical support, my apologies for double asking but I need a fast answer)

Hello,
we combine LTC6948 and LTC5562. LTC6948 creates an LO signal of 920MHz and LTC5562 mixes the baseband onto it.
Now I measured the output level of LTC5562 and was expecting to see a change when I change the setting of RFO[0:1] of LTC6948. I read back all registers, so I am sure RFO is changed. Still LTC5562 output shows the same level for all four RFO values.
Changing IN level of LTC5562, which is currently DC in our case, changes the output level accordingly ie doubling IN results in +6dB output level.
So, my question is, why doesn't RFO change the output level?
Thanks for any help
kind regards
Martin

  • +1
    •  Analog Employees 
    on Jun 19, 2019 9:12 PM

    I'm assuming you're putting the 920MHz signal from the LTC6948 into the LO port of the LTC5562. Changes in the LO input power for an active mixer have minimal effects on the output. I would expect to see a small change at the output, but not much.

    You can see this on the LTC5562 datasheet, page 7. The plot titled "Conversion Gain, OIP3 and NF vs. LO Input Power".

    Thanks,
    Weston Sapia
    Sr. RF Applications Engineer