Is ADS handles Term and a normal circuit differently?

[yolande_yj]

When an LNA or Mixer is simulated or tested individually, matching networks for input and output are needed.
**broken link removed**
In a single chip RF receiver front end (such as Low-IF or Zero-IF), LNA output signal is coupled directly into the Mixer RF input port:
**broken link removed**
In the normal design procedure, LNA and Mixer are firstly designed (simulated) separately, and then combined together later. Basically the LNA output port and Mixer RF input port do not match. They are not necessary to be match since, inside the IC, the connection between them is so short that it can be neglected comparing with the wave length of the desired RF signal (100mm for 3GHz).

My question is:

When ADS simulates, does it use different mechanism to handle the “Term” component (Terminal) and a normal circuit? Otherwise there will be a significant signal loss due to the unmatched ports of LNA output and Mixer RF input.
If yes, what kinds of mechanism are they?
If no, how does ADS handle that?

 

[yolande_yj]

I am almost lost when I think more about that, now I am quite confused about this question:
In the picture above, why do we not consider the mismatch effect between LNA and Mixer?

If the connection between LNA and Mixer is long enough to be considered as a transmission line (as show in the picture below), then we need to use matching networks. But what happens when the length of the transmission line (L) reduce to Zero? IF we remove the matching networks, I think there should be reflection loss due to the mismatch ports, am I right? Why we don't care about it?

 

[cmosbjt]

Re: Is @ds handles Term and a normal circuit differently?

I am almost lost when I think more about that, now I am quite confused about this question:

In the picture above, why do we not consider the mismatch effect between LNA and Mixer?

If the connection between LNA and Mixer is long enough to be considered as a transmission line (as show in the picture below), then we need to use matching networks. But what happens when the length of the transmission line (L) reduce to Zero? IF we remove the matching networks, I think there should be reflection loss due to the mismatch ports, am I right? Why we don't care about it?


1. Inside ICs, the connection length between LNA and Mixer is very short (in RF range), transmission line effect is not considered. Though the mismatched ports cause reflection power and thus power loss, at this time we are more concerned about the signal voltage that couples to the mixer. For a CMOS mixer, the input MOSFET convert the input voltage signal into current signal, we do not care much about the input current (very small infact, P=V*I, so input power can be small). Most active mixer such as Gilbert cell have a certain conversion gain, which relax the power delivery requirement from LNA to mixer.

2. When LNA and Mixer is connected through a transmission line, mismatched load introduces standing wave along the transmission line, the singnal voltage (amplitude=Vmix) received at the mixer input port is not always equal to the one at the LNA output port (Vlna). Depending on the length, ZL, Z0 and bla bla bla, Vmix will vary from 0 to 2*Vlna (corret me if I am wrong). That's the reason why we need to match them. If you are designing PA, the reflection power from the antenna due to mismatch causes power loss and can even damage the PA.

 

[yolande_yj]

Is @DS handles Term and a normal circuit differently?

Thanks a lot. But what is the purpose of the two matching network separately?

 

[cmosbjt]

Is @DS handles Term and a normal circuit differently?

The first matching network is to minimize the insertion loss (IL) and provide a maximum available power from LNA. The second one is to do the job as I mentioned previously (to give a flat standing wave along the TL if you don't use the TL to do the maching), of course it also reduce the power loss. These two matching networks are related.