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What's the mixer output if LO signal is applied but not RF signal?

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STOIKOV

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in a mixer if LO signal is applied and RF signal is absent, what shall I obtain in its output?
 

Test a mixer

ideally nothing, cause there should be isolation between IF and LO (espicially in double balanced mixer)
but if there is some signal at the IF output then u should calculate the ratio between output and LO which should give the isolation ,,,(tis is normally done by PXF)
 

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    STOIKOV

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Re: Test a mixer

I was thinking on this equation : IF=RF-LO, so if frequency of RF=0, then IF would have the frequency of LO ?? can it be possible in a double-balanced topology ?
 

Re: Test a mixer

The LO to IF isolation is frequency dependent, but may be 20-50 dB also for a double balanced topology. The balance is not perfect and stray capacitance between LO port and IF port of the mixer gives a LO leak at this port.
 

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    STOIKOV

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Re: Test a mixer

STOIKOV said:
I was thinking on this equation : IF=RF-LO, so if frequency of RF=0, then IF would have the frequency of LO ?? can it be possible in a double-balanced topology ?
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u can think of it like the input to the LO pair is DC+RF if RF is zero then input is just DC which would result in LO at the IF output but in case of the double balanced , idealy u get at one end of the differential output LO+ and LO- which eleminates each other giving good isolation , note that all this assuming ideal case but in practise u will get good isolation but not infinte
 

Re: Test a mixer

I think you will see some LO leakage, harmonics of LO and some level of DC. The amount of LO leakage depends on the mixer LO-IF isolaton as well as the IF port bandwidth. The DC level, assuming the IF port is DC coupled, depends on the LO-RF isolation.
 

Test a mixer

so is this the typical way people measure LO leakage / isolation? thanks!
 

Test a mixer

the common way is to do PSS followed by PXF
 

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