question about LO leak and LO+2*IF leak in a transmit branch

[l.j.zheng]

hi,all.
Recently I designed a transmit branch,including:mixer+filter+driver 1+driver 2+
PA(use GaAS FET matched by cap and ind). Now comes a problem: when measure
the branch at the driver2's output,the LO leak and LO+2IF leak are both about -70dBc(the PA doesn't work). But when measure at PA's output the LO leak is about
55dBc and the LO+2IF leak is about 50dBc. The output of PA hasn't reached P1dB(that is: the linearity is still good),and the IM3 is also good. Then why the LO leak
and LO+2IF leak becomes so strong, it badly disturbs me for several days.

I wish to get help from you!
Best Regards
L.J.zheng

 

[vfone]

Re: question about LO leak and LO+2*IF leak in a transmit br

Try to verify the gain of the PA at LO frequency.
Second reason could be a bad layout.

 

[nxing]

What's your PA's transfer function? plotting that out maybe helpful.

 

[l.j.zheng]

Re: question about LO leak and LO+2*IF leak in a transmit br

I measured the PA, the gain of PA at RF and LO and LO+2*IF frequency are almost the same(difference<1.5dB). For the RF is in UHF band, and the IF is only 21.4Mhz.

 

[vfone]

Re: question about LO leak and LO+2*IF leak in a transmit br

In this case make sense to have higher LO level at the output of the PA.
If you fail a spec with this issue, have to filter the LO signal in the front of the PA.