[ibis-interconn] Re: Why is not the contents of Touchstone file not Transfer Functions
- From: Bob Ross <bob@xxxxxxxxxxxxx>
- To: Walter Katz <wkatz@xxxxxxxxxx>
- Date: Wed, 15 Jul 2009 13:09:51 -0700
Walter:
From a strict reading of the definition, you could draw the
conclusion that a particular Xij parameter over a frequency
range defines an input/output relationship, and therefore
could be designated as a "transfer function."
However, I am still not comfortable with "transfer function"
in the context of N-port parameter descriptions for many
reasons.
1. Common practice - I have not seen anyone else refer to the
N-port parameter Xij terms as "transfer functions," either at
a single frequency or as the collection of terms for that
particular Xij entry. We should stick with commonly used
terminology.
2. Single function - While the definition could be expanded to
a matrix input/output relationship, the language implies a single
output produced by a single input and seems to imply the functional
mathematical relationship. The Xij entry at one frequency is
the transfer function of j to i at that frequency only when
the other inputs of the matrix - all other ports are terminated.
From the matrix perpective, the output is a function of multiple
inputs. Also the functional relationship might imply a
collection of such Xij(f) for each frequency individually or
for the whole set of frequencies for the Xij term.
3. Transfer Function - this seems as a mathematical expression (referred
to as complex exponentials) as opposed to the specific data entries or
quanties on a per-frequency basis in the definition. I read that
to mean a mathematical function relatinship as opposed to
a collection of terms.
4. Filter design practice - I recall a distinction in (driving
point) impedance/admittances for the same terminal versus
a transfer function gain/(trans)impedance/(trans) admittance for
different physical terminals. That is the terminology conflict
in a related field.
------
Anyway, I think finding terms that are used in common practice for
S-parameters and N-port parameters entries and distinguishing whether
we are describing a specific entry at one frequency or a set of Xij
entries for all frequencies is better practice.
Bob
Walter Katz wrote:
All,
See http://cnx.org/content/m0028/latest/ for better graphics.
Is not the Touchstone file exactly this?
The ratio of the output and input amplitudes for Figure 1
<http://cnx.org/content/m0028/latest/#rc#rc>, known as the transfer
function or the frequency response, is given by
Vout
Vin
=
H(
f)
=
1
ⅈ2πfRC
+1
(1)
Implicit in using the transfer function is that the input is a complex
exponential, and the output is also a complex exponential having the
same frequency. The transfer function reveals how the circuit modifies
the input amplitude in creating the output amplitude. Thus, the transfer
function completely describes how the circuit processes the input
complex exponential to produce the output complex exponential. The
circuit's function is thus summarized by the transfer function. In fact,
circuits are often designed to meet transfer function specifications.
Because transfer functions are complex-valued, frequency-dependent
quantities, we can better appreciate a circuit's function by examining
the magnitude and phase of its transfer function (Figure 2
<http://cnx.org/content/m0028/latest/#magphase#magphase>).
Walter
Walter Katz
Chief Scientist
Signal Integrity Software, Inc.
wkatz@xxxxxxxxxx
303.449-2308
--
Bob Ross
Teraspeed Consulting Group LLC Teraspeed Labs
121 North River Drive 13610 SW Harness Lane
Narragansett, RI 02882 Beaverton, OR 97008
401-284-1827 503-430-1065
http://www.teraspeed.com 503-246-8048 Direct
bob@xxxxxxxxxxxxx
Teraspeed is a registered service mark of Teraspeed Consulting Group LLC
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