Question Concerning Attenuators in Mixer Design

Hi , i didn't understand why atteuators are used before and after mixer like in the picture.

I'm curious: What's the context of the picture? Is it an actual implementation, or is it some kind of model for a simulator?

It's sometimes done in order to improve the S11 at the RF and IF ports.

In particular it improves the S11 for very much out of band signals, which can be important as most mixers IMD numbers are strongly influenced by termination impedance, and particularly when following the mixer with an immediate roofing filter the out of band termination can be all over the shop.

Sometimes the spurious responses are a bigger problem then system noise figure, so a couple of pads really help with broadband return loss.

Regards, Dan.

It's AWR's VSS Tool for system design , you can make simulations for parameters like NF, IP3 , P1dB etc.

Hi,

Sometimes, attenuator before mixer is used to prevent mixer being saturated (i.e. reaching P1dB point). Generally, digitally controlled attenuators (DCA) were used for this purpose in which attenuation value can be controlled with respect to the power level that the system operates.

Thank you very much for the answers. So the attenuator after the mixer improves the S11 ? What is the attenuator before the mixer for ?

Think of the attenuators as providing isolation between the devices (in both directions). The one to the left of the mixer provides a slightly better match of the mixer's input (2 additional dB of RL) to components to the left of the mixer. But it also provides a better match for the mixer looking back (left) into those same components. Same on the output of the mixer.

So the mixer's output attenuator can appear to improve the S11 of the filter (especially for its out-of-band regions) that's to the right of the mixer (which is part of what Dan was pointing out I believe), making the mixer "happier".

I asked if it was for a simulation because often times the models for devices do not have a good enough representation of the real world so sprinkling attenuators around can help with the simulations. But sometimes the attenuators are also appropriate in a real design.

It is actually a real design but not tested yet. So the value of attenuators may change. Do we have another choice instead of using attenuators ?

Maybe cascaded Mini-Circuit's reflectionless filters can be used to improve S11 performance but their insertion loss's are very low in stopband.

In engineering, such as radar, the isolator is used to replace attanuator.
Obviously, pad which is made of resistors is much cheaper.