Why strong LO might desensitize IF stage in mixer block?

[super]

Dear all :
In Mixer block, we all know LO will feedthrough to IF.
Why strong LO might desensitize IF stage ?
LO freq(2.4G) and IF freq(2Mhz) is very difference , why we will
care about this issue ?

thanks.

 

[nxing]

about Mixer LO-IF

Although this LO is far alway from you IF requency, if it's strong enough, it can cause your IF circuit misfunction. For example, say you have a filter following the mixer, the attenuation of this filter for the LO may be not enough if it's too strong so that the following circuit will be affected.
Hope this will help!

 

[super]

about Mixer LO-IF

I try to fed 2.4G (20mv) & 2M (1mv) into amplifier,
and see the waveform like mixing in the time domain , I think that is you told, right ?

thanks .

 

[safwatonline]

about Mixer LO-IF

think of it like a differential pair the current will fully stear into one direction according to the LO signal , then even if there is an RF signal the current is already fully switched so the output is not affected by the RF signal.
i.e. lets say the LO-IF gain is 5dB , if the LO signal is large enough the output will be a rail-rail square wave at the LO frequency and hence the RF signal being small cannot affect the square wave swing so u will only see the LO signal at the IF port

 

[tangqin55]

Re: about Mixer LO-IF

LO-IF feedthrough must be limited to avoid the desensitization problem in the stage following the mixer, because the amplitude of LO is much bigger than RF signal. If there are a interferential signal with frequence 2MHz higher or lower than f_LO, the 2nd harmonic component will appear in f_IF.

 

[super]

about Mixer LO-IF

thanks safwatonline and tangqin55
I have get my answer.