problems of RF passive mixer

[bigworm]

dear all,
if the pos/neg input of RF signals to passive mixer are with different amplitude,
what will happen to the IF signals?
since the double balance mixer could balance the output signals, so we can get two IF signals with identical amp but -180 degree, right?
then will this impact the IP2 and IP3 of mixer?
if so, how would this effect happen?
could you provide some reference?
thank you

 

[RF-OM]

But if your input signals are not balanced your mixer may be not balanced and in this case definitely all mixer's parameters can degrade. There usually much easier requirements for phase imbalance than for amplitude.

 

[bigworm]

hi,
I am not quite clear about "easier requirements for phase..."
do you mean the phase imbalance of LNA will have less impact on the performance than that of amplitude?

then extremely, there are some cases in IEEE paper, that the the positive output of passive mixer is fed the rf signal and the negative output is directly connected to gnd. will that apparently degrade mixer's performance?
It seems that the pos/neg input of mixer allow a larger tolerance than expected.
thank you.

But if your input signals are not balanced your mixer may be not balanced and in this case definitely all mixer's parameters can degrade. There usually much easier requirements for phase imbalance than for amplitude.

 

[RF-OM]

To bigworm,
I do not mean any particular mixer schematic, there may be a lot of them. I mean that for general differential structure differential voltage will depend on amplitude and phase imbalances of two vectors. The amplitude imbalance will affect resulting vector more than phase imbalance. For illustration I include the picture from one of my articles about differential circuits. One can clearly see from this plots that phase imbalance of about 50 degrees will results in less that dB of differential voltage degradation whereas amplitude imbalance affects it much higher.

 

[yendori]

A passive double balanced does not actually produce two IF signal 180deg out of phase. If this were the case the signals would cancel under normal operation.

If you notice that most IF's are taken from the center tap of a transformer, so obviously if the were two signal 180 of of phase they would cancel.

Yendori

 

[RF-OM]

The plots what I posted will work for any balanced structure. When you use center-tap transformer to convert balanced signal to common mode you get single-ended signal. Balanced structures always consist from two symmetrical (balanced) parts and each corresponds to its own vector V+ and V-. The sum is double amplitude signal, not zero.

 

[yendori]

His question was, does a p-asive double balanced mixer produce an IF 180 deg out of phase, the answer is no.

Of course you can use a balun to create a 180deg phase, but the mixer doesn't produce it.

IF currents flow the only 1/2 of the RF balun at any given time.

Good luck getting double the amplitude.

 

[RF-OM]

Well. It looks like you need to think again. There are a lot of signals into mixer.