what is a port? why do we need it in noise analysis for NF?

Hi, all, what exactly is a port in analoglib and why do we need it
(consider there are already plenty of sources in the lib)? Seems to me
it's for power related simulation. Should we always set the resistance
to 50 ohm (is this a tradition for impedance matching purpose)?

Also, I know for spectre noise/pnoise analysis, we need to pick an
input port and probe the output port (btw, how to set the output port?
dc, still 50 ohm resistance?) for the NF calculation. But what is the
point of using port instead of regular vsource? Isn't the port going to
load the circiut? Sorry for so many questions... Any comments are
appreciated!

 

Ok, let me try to answer some of my questions.
1. For regular sources such as vsin, usually they are noiseless. They
can be used for "noise" simulation, i.e., input/output noise, but not
for NF calculation 'coz there's no SNRin if we have a noiseless source.
2. Seems to me the "port" component is coined more or less for RF
simulation. The default resistance value in "port" is 50, which is just
the in/out impedance for most RF circuit (50 or 75).

 

Richard,
Thanks for the reply. I certainly understand noise is measured by
power, and the measured load noise would vary depending on the load.
But I don't agree that "noise does not make sense without a load
impedance". What I was trying to do is to simulate the circuit (a base
band amp) inherent noise, input referred noise. This should be
independent of the load.
warm regards,
Jay

 

OK, let me clarify a bit.

If you're measuring noise figure, you are measuring the ratio of the signal to
noise at the input to the signal to noise at the output (excluding the noise
in the load). Consequently you need to have noise at the input.

The way to generate noise at the input for the purposes of noise figure
calculations is to use the port component - the internal resistor is an
implicit noise generator.

As for the load, provided that you tell spectre to probe the output rather
than specifying it as a voltage pair, you're OK. If the load is a resistor or
a port, and you probe that, spectre will exclude the noise in the load from
the noise figure calculation. Of course, if there is no load, then a voltage
pair is OK, and also it's OK if you specify a resistor as the load with
"isnoisy=no" set on the resistor.

If you're not measuring noise figure, but output noise, or input referred
noise, then generally you wouldn't use ports.

Regards,

Andrew.