Gilbert mixer IIP3

The first thing I should point out is that you almost certainly should be using harmonic balance for this simulation, rather than shooting. There's generally few cases where shooting QPSS is advantageous - I normally only use it nowadays for distortion in a switch cap circuit where you have a single clock (possibly with multiple phases) and a large signal input that you want to measure distortion under.

For IP3 of mixers, I'd either recommend hb/hbac (you can also use the older qpss/qpac interface and pick harmonic balance, since you already have it set up), pure hb (three tones) or better still (for efficiency) hb (just LO) plus huac in RapidIP3 mode.

Anyway, that's not the issue here - it's because your frequency selection is wrong. I suspect that the real problem may be that you've got your flo and frf frequencies the wrong way around (i.e. flo should be 2.4G and frf 2.42G), but let's go with what you've picked.

Currently you have:

• flo=2.42G
• frf=2.4G
• fqpac=2.421G

This means that your IM3 is at 2*frf-1*fqpac-flo=-41MHz (so 41MHz). Your 1st order harmonic will be at fqpac-flo=1MHz. It's fairly clear from the frequency multiplier columns in the direct plot form that you have the wrong frequencies - the 3rd order harmonic is dependent on the flo being shifted by 2*flo (which makes no sense for this downconverter mixer), and the 1st order harmonic doesn't involve the LO at all - it's just modulated with the RF input signal. You'd expect to see +/-1 for the flo column, and -2 for the frf column (for the IM3 tone) and +/-1 for the flo column and 0 for the frf column (for the first order tone).

If you're trying to have a second near tone, then if you really meant flo=2.4G and frf=2.42G, then with fqpac=2.421G, the IM3 would be at 19MHz and the IF at 21MHz. Otherwise with your setup the IM3 and IF are 40MHz apart.

Because you have the wrong tones selected in the direct plot form, you're not seeing the 1dB/dB or 3dB/dB slopes for the IF and IM3 curves.

Regards,

Andrew.

What if I do like this:

frf=2.4G , fqpac=2.401G and flo=2.42G  

then I will have 21M and 19M is this correct ?

asrf said:

What if I do like this:

frf=2.4G , fqpac=2.401G and flo=2.42G  

then I will have 21M and 19M is this correct ?

Yes.

asrf said:

What if I do like this:

frf=2.4G , fqpac=2.401G and flo=2.42G  

then I will have 21M and 19M is this correct ?

Yes.