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Image Rejection of IQ mixer

itos

What is the easiest way to simulate image rejection of an I/Q downconversion mixer - if possible with pss/pac?

Suppose the mixer is a black box with a PORT at the LO input (which generates 0 and 90 degree versions), a PORT at the RF input and two baseband outputs.

I can now use pss with the LO as beat frequency and set pac=1 for the RF input port.

With PAC I am sweeping from LO to LO+someOffset .... which makes a tone at the upper sideband. After downconversion, I should get signal energy in both I and Q channels.

In MATLAB (or a measurement) I would look at DFT(bb_i + j*bb_q) and read off the difference between the upper and lower sideband peaks. But in Spectre I am confused: I can use dB20(leafValue(mag(v("/vodi" ?result "pac")) "harmonic" -1)) and dB20(leafValue(mag(v("/vodq" ?result "pac")) "harmonic" -1)) to get the pac outputs but they are somewhat already in frequency domain. How do I combine them properly to read off the image rejection?

Or is there an easier way altogether?

  • Normally you'd use pxf to investigate image rejection in a downconversion mixer. You can specify the output frequency range in the pxf sweep and then you can plot the transfer function from different sidebands of each source to the output. Not sure whether it's what you want in this case, but that's how I'd normally do it rather than using pac.

    Regards,

    Andrew

  • in reply to Andrew Beckett

    Thank you - that looked promising!

    However, this is my point: with pxf analysis you select one output. However, for an I/Q downconversion mixer there are two outputs (I and Q). The image rejection depends on the matching between these two outputs.

    Maybe I am understanding that wrong - but how would you set up pxf? Would the positive/negative output node be "OUT_I_P"/"OUT_I_N"? What happens with "OUT_Q_P"/"OUT_I_N" then? Or am I supposed to combine them somehow?

  • in reply to itos

    I'm not sure you can in this case, because it's not differential - they're 90 degrees apart.

    Probably best to contact customer support to see if we can come up with a good solution to this (maybe with a bit more thought it can be done, or maybe it needs an enhancement).

    Regards,

    Andrew.

  • Hi itos,

    Did you  get any luch with simulation set up of Image rejection of Down converter IQ Mixer ?  Could please share steps to set up test bench for it ?

    Warm Regards,

    Girish W

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