EXAMPLE from DAYS PAST: In the TTL days of logic long past to newbies, it
was common practice to place a black, CK05 (0.1 ?F) capacitor at each end
of a row of logic chips. This was placed between Vcc and return. In those
days, we seldom considered self resonance of a passive device. The
intended use of them on the boards was to keep logic switching noise off
the DC rail. Turns out those CK05 capacitors which peppered our boards
became self resonant somewhere between 1 and 2 MHz, usually around 1.4 to
1.6 MHz. Therefore, above self resonance, they became DC-blocked
inductors. Now, ask yourself: "is an inductor good at bypassing rail noise
as the capacitor was intended to accomplish?" NO!
Several decades ago, I had the privilege of issuing a couple of new-hires
from Kent State into the real world of 'parasitic component' behavior (the
real world they would have to live in). I was given the task as they would
specify totally unrealistic component values for designs and had only an
understanding of the ideal behavior of electronic components. I asked
them to check out a small handful of those CK05 capacitors from engineering
stock. We warmed up the HP impedance meter of the time (the one that had a
tunable drum as a frequency indicator and topped out at 110 MHz). Sure
enough, *EVERY* CK05 capacitor went purely resistive between 1.4 and 1.6
MHz and inductive above that. Their eyes bugged out. They could not
understand or comprehend how a capacitor could possibly become resonant
(+jX = -jX) and ultimately become an inductor. I sent them back to their
test books and pointed them in the direction of our local building
library. It took them a week of digging, but they finally came back with
the classic capacitance to resistance to inductance curve with frequency.
Dave - W?LEV
On Sun, Feb 14, 2021 at 2:26 PM Manfred Mornhinweg <manfred@...>
wrote:
Ed,
What I am trying to
understand is how one relates the marked value of a component to what
nanovna
shows.
The value marked on a component is supposed to be valid at a very low
frequency. As soon as the frequency goes up, parasitic effects become
increasingly important, so the actual value of the part varies with
frequency. Also, of course, there is always a tolerance. With good
capacitors and air-core inductors this tolerance might be only 5%, but with
ferrite-cored inductors it can easily be 40%, and with some sorts of
ceramic capacitors it can be even larger.
So, rule #1 is to measure on the lowest frequency possible, and rule #2 is
to never forget that the marked value is subject to a tolerance.
But then the characteristics of the measuring instrument come into play.
The NanoVNA is natively a 50? instrument. It should produce the best
accuracy when measuring impedances reasonably close to 50?. When the
impedance gets close to zero, or into the kiloohm range, the accuracy of
the NanoVNA drops. So, if you are measuring small values of capacitance or
inductance, measuring at the lowest frequency the NanovNA supports might
produce poor measurement accuracy.
So, rule #3 is to measure at a frequency where the impedance of your part
is at least close to the order of magnitude of 50?.
In practice that means that you should look at what frequency the part has
a reactance of 50?, and then measure at a frequency a few times lower. If
the measured value is reasonably constant over the range between those two
frequencies, then probably you have a valid measurement. If instead it
varies all over the place, it probably means that there are too high
parasitics even in that frequency range.
Of course I'm assuming that you properly calibrated the NanoVNA, putting
the short and the load exactly at the same place where you then put the
part to be measured.
In my experience it's best to use the shortest possible connection between
the NanoVNA and the part under test. It seems that correct measurement of
difficult impedances through a long piece of coax cable is harder for the
NanoVNA, even when carefully calibrated through that long cable.
Always keep in mind that if you do all this and get a consistent result,
then you are getting the low frequency value of the part you are testing,
and that at higher frequency its actual value will change. At a high enough
frequency a capacitor becomes a short circuit, further up it becomes an
inductor. And what's an inductor at low frequency will become an open
circuit at some high frequency, and a capacitor beyond that. Generally both
inductors and capacitors will rise in value, when you start going up in
frequency starting from a low frequency. But core materials tend to
decrease their permeability beyond some frequency, and this effect can win
over the other in some cases, so you might see cored inductors whose value
goes down rather than up, when increasing the frequency. These are all real
effects, not measurement errors! In RF work you often need to measure each
part at the frequency you will be using it, rather than trusting the value
printed on it, which is valid only at low frequencies.
--
*Dave - W?LEV*
*Just Let Darwin Work*
