Re: Calculatating RC parallel and series impedance networks.



"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> wrote in message
news:483609D9.B882B33F@xxxxxxxxxxxxxxxxxxxxx


John Byrns wrote:

In article <48358728.A5D06FE0@xxxxxxxxxxxxxxxxxx>,
Patrick Turner <info@xxxxxxxxxxxxxxxxxx> wrote:

Iain Churches wrote:

"Ian Thompson-Bell" <ruffrecords@xxxxxxxxxxx> wrote in message
news:g11vci$tj5$1@xxxxxxxxxxxxxxxxxxxx
Iain Churches wrote:
There has been some discussion on RAT recently
about amplifier stability and RC networks.

Most of us find the values for these by empirical methods,
using a rule-of-thumb or two and a couple of decade boxes.

I thought it might be useful to have these values calculated
from 10Hz to 100kHz in an Excel spreadsheet which would
also plot the impedance, Z, the phase, and the dB changes
for both parallel and series combinations.

http://www.kolumbus.fi/iain.churches/ThermionicThoughts/ImpedanceCalculat
or.html

It is still under development. Please have a look, and
download the spreadsheet (Excel 2003 or later) to put it
through its paces.

Any (constructive) comments will be greatly appreciated.

Just a couple of comments as Patrick seems to have done a
comprehensive
job as usual.

Yes. I was grateful for his comments although it was not obvious
to him what was the intention of this page. Also, I have a feeling
that
he did not actually download the spreadsheet to view and print some
graphs (I seem to remember him saying that he does not have or use
Excel)

What you are saying in the introduction now looks better, but technical
mumbo jumbo
nobody here understands is still there to confuse.

What "mumbo jumbo" are you talking about? I haven't read the entire
page carefully enough to say that I have proofread it, but it looks
straight forward and easy enough to understand to me. The math is quite
simple and is not rocket science.

Reactive network math is anything but easily understandable.

Iain has....

"The voltage is constant and the current is the sum of the vectors."


Morning Patrick. Please correct me if the above statement is not true.

Nobody I know ever works out vectors. Nobody I know has a clue....
I don't, as a result, and its because I don't have to.

You don't need to. The spreadsheet does all the work for you.
But it is important to convey the idea that the value of a CR
combination cannot be calculated as if it were a pair of resistors.

"The impedance Z of the network can be calculated by (R x
Xc)/(R^2+Xc^2)^0,5"


This is written to appear confusing to 99% of any beginner.

This is precisely the notation that Excel uses.
As I mentioned earlier, the page is probably not very useful
to beginners anyway:-) I notice also, that there seem to be
no beginners any more asking interesting questions on RAT.
They seem to have been driven away:-(


"The phase angle of the impedance is given by: Theta = arctan (Xc/R)
=-90+DEGREES(ATAN(Xc/R))"

Another completely incomprehensible formula that nobody uses, or can
understand.

Again, this is the Excel notation. Anyone familiar with spreadsheets
will recognise it. It's a tool, a means to an end, you do not need to
know the details of how it works to use it, in the same way as one does
not need to understand the workings of the internbal combustion engine
to drive a car. If one did, the roads would be pretty empty:-)

What ppl CAN understand is simpler.

You always must have a SIMPLE REAL WORLD MODEL and application of a
circuit before understanding is at all possible
or useful.

1. So, there MUST be a broad bandwidth signal source, so assume a
voltage source with 0.0 ohms output resistance.

2. Assume this signal source does in fact have some source resistance,
so add a series R to represent it.

3. The output of the source resistance is the input to whatever network
you have.

4. The source resistance becomes part of the network whether you like it
or not
as the input terminal resistance and must be allowed for in
calculations.

5. There is an output terminal after the network from which a frequency
response is measured
always by a CRO probe or voltmeter such high input resistabce or
impedance that
the response of the network isn't affected.

6. There will be some real value of terminating resistance and or
reactance between output and 0V
after the network output.

7. The terminating impedance also becomes part of the network and
affects the way the network operates.

8. To establish what the response is of a network its teribly difficult
to calculate correctly
taking all the above into consideration.

9. Just build the damn network, knowing what the Rout of your sig gene
is, and measure it.

10. Adjust R&C values until you can plot the response you want.

11. Prove to yourself that only a guess can be made about real responses
easily.

12. Learn that hands on real world work in the darn workshop solves
problems.

13. And don't be afraid to simulate so that the risk of making a mistake
in a calculation is eliminated.

14. This leaves the possibility of making a mistake due to non inclusion
of some
factor.

15. Something is dead right when what you measure is exactly the same as
what you correctly calculate
and what you correctly simulate.

All agreed. But further simplification does not have the slightest effect
on the use of the sheet. You just plug in two values and Excel does the
rest. You can then see the impedance of any series or parallel combination
at any F between 10Hz and 100kHz. This of course does not dispense
with the need for "final fettling" to get the precise values required in a
particular circuit.


I take your point about the source impedance and the impedance
which the network feeds. I would like to have the option to include
these in the calculations. Any ideas, Patrick.

When a textbook is attempted to be written, the author does well to
assume his readers have low IQ,
and are insufferably dumb, and need everything all spelled out in easily
applicable layman's terms.

RDH4 is halfway there in many respects.

RDH4 is far to complex for most hobbyists. That's why Morgan
Jones' books are so popular, they are written from a practical as
opposed to a theoretical point of view. He gives just enough theory
for the reader to understand the principle being discussed.

But thanks, Patrick for your comments. I will, when time permits
add diagrems to show the phase differences in the networks, if
that will make things a little clearer.

Best regards
Iain




.

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