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Modeling >> Semiconductor Devices >> itzf contribution to vco phase noise
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Message started by Ryan on Nov 28^th, 2002, 4:13am

Title: itzf contribution to vco phase noise
Post by Ryan on Nov 28^th, 2002, 4:13am

I am having difficulties trying to understand what itzf exactly means. According to the VBIC model equation, itzf is the forward transport current. So my understanding is that it's basically Ic. Then the squared noise voltage of the collector shot noise should simply be : Vn(itzf)^2=2qIc*Rp^2, where Rp is the parallel resistance of the tank and should stay approximatly constant for different Ic. I then changed Ic trying to confirm my understanding, but failed to see a proportional relationship between Vn(itzf)^2 and Ic. There must be something wrong. Please could anybody help me out of this puzzle? Is my understanding wrong or it's just computation error?

Thanks ever so much!

Title: Re: itzf contribution to vco phase noise
Post by Colin McAndrew on Dec 2^nd, 2002, 10:35am

Ryan,
the transport current in VBIC is broken into
two parts, the forward current and the reverse
current. This is done because the excess phase
shift is only applied to the forward current and not
the reverse current. Practically it should not matter
much as the reverse current is negligible under normal
operation (except when you saturate the device).
Itzf is the un-phase shifted value, when the
phase shift is applied it becomes Itxf, the It means
transport current, the final "f" means forward (the
part controlled mainly by Vbe, not Vbc), and the
"z" means zero-phase and "x" means with the excess
phase shift applied.
The phase shift is not done as an exact linearly
frequency dependent shift to gm, as is sometimes
done, as that is only implementable in the frequency
domain. Rather, a 2nd order "filter" is used to
implement the shift, and for small phase shifts it
approximates a linear phase shift, with a small change
in magnitude of gm.
For the shot noise, for Ic it is calculated from Itzf.
This will pass through the excess-phase filter
and appear in the collector current.
I am not sure what simulator you are using and
what it is printing for noise. What you see at the
terminals is not just what a simple analysis gives,
the noise sources are placed in parallel with the
elements they generate noise for, so what you see
at the output is a function of the complete small-signal
model of the device and the circuitry it is connected to,
and of all the noise generators in the model. The shot
noise is often an insignificant contributor, the base
resistance is often the dominant noise source
(assuming you are above the 1/f noise corner).
So internally to the model the noise source associated
with Itzf is the normal shot noise, however if you are
looking at the output noise it is affected by more
than just this noise term. If the simulator prints
out the individual noise sources, as some do, then
you should see the Itxf component track with Ic
(provided you are out of saturation), but if you are
looking at the noise voltage across a load the
device is driving it will not simply scale with Itzf
as (a) other noise sources contribute, and (b) it
depends on the whole small-signal network.
Hope this helps.
Colin

Title: Re: itzf contribution to vco phase noise
Post by Ryan on Dec 3^rd, 2002, 3:36am

Hi Colin, thank you very much. Your answer is extremetly helpful.

There is only a small point I'd like to double-check with you. Regarding to your comment:

(If the simulator prints out the individual noise sources, as some do, then you should see the Itxf component track with Ic.)

I was printing noise contributions of different noise sources using Cadence Spectre. Unfortunately it doesn't print Itxf, but only Itzf. Actually these two are related to each other, so there is only one source behind them. So assumably I'll never see a linear track between Itzf noise and Ic. Is that right?

Thanks again for your kindness.

Ryan