On Mon, Oct 7, 2019 at 04:03 AM, Kurt Poulsen wrote:
Kurt,
Thanks for your more detailed explanation. I think I understand the "trick" you are suggesting more thoroughly. It certainly makes a difference if you want to more precisely characterize the 13 micro-Henry inductor at higher frequencies like 30 MHz to 100 MHz.
--
Bryan, WA5VAH
Hi Bryan
Thank you for your comment. I see you have not fully under my trick, so I will
take some small steps to explain what is going on.
Consider we have calibrated at the female SMA calibration plane at the end of
the female female adaptor which we do using the supplied male calibration kit
and we then add a male to female adaptor and we place a component e.g. a coil
on the far end of this added adaptor, one way or the other. The added male to
female adaptor constitutes a shunt capacitor to the coil by the delay in ps in
this adaptor multiplied by 20fF as 1ps=20fF. For such an adaptor the delay is
about 50ps so we have 100fF or 1 pF shunted to the coil and that gives a
resonance frequency of 150MHz, if the coil has no self capacitance, which Is
has. Bottom line, this 1pF changes the self resonance downwards in frequency.
The way the inductance changes you need to do some calculation, but at least
you know that the effective inductance will change and drop when we approach
towards resonance as the capacitance "steals" some inductance.
In the real life we would like to use a female SMA bulkhead/PCB edge adaptor,
for soldering the coil (in this example) to the far end, so we need after the
calibration to remove the female female adaptor, which has a delay of 82ps,
so now our calibration plane is sitting somewhere out in the blue air. It is
actually 17mm from the male calibration plane, to which we fit the
bulkhead/pcb edge adaptor, because the velocity in the female female adaptor
is 1 mm per 4.83ps and 82/4.83 is giving 17mm without decimal. So what to do
??
That is exactly what the display/scale/Electrical Delay can fix, as it simply
pulls the calibration plane backwards and 100ps is just a nice number which
pull the calibration plane 18ps into the test cable allowing to add up to
100ps again. As when we then add the bulkhead/PCB edge adaptor we the must
move the calibration plane forward again to the rearside of the added adaptor,
and a phase trace it the tool to use, as when it show 0 degree out
measurement plane is equal to the shifted calibration plane. Thus there is
nothing shunting the test object e.g. your coil. There is only a very minor
snag as such, because if you used a bulkhead/pcb edge adaptor shorted with a
cupper disk, then the delay shall be adjusted by a few ps (now with 180 degree
as reference and a hopeless case for th NanoVNA as wobbling up and down
between + and - 180 degree )because we have actually removed the fringe C from
the center pin being maybe 100fF, but a benefit as we the cane measure
capacitances down to a fraction of a pF. Measuring Q requires that if we use a
shorted bulkhead/pcb edge adaptor to check the resistance is 0 ohm or few
mohm, and that can be this moving backward/forward does not fully satisfy this
requirement, as the electrical delay take for granted the impedance of the
adaptor/test cable is pure 50 ohm.
I hope this get you on level with this trick
Kind regards
Kurt
-----Oprindelig meddelelse-----
Fra: [email protected] <[email protected]> P? vegne af bryburns
via Groups.Io
Sendt: 7. oktober 2019 03:49
Til: [email protected]
Emne: Re: [nanovna-users] Inductor S21 measurement using nanoVNA
Kurt,
Good points for more precise calibration. It would certainly be more precise
than my suggestion. I certainly agree with you that it is a good idea to
calibrate as close to the measurement plane as possible.
My basic question is fairly simple. How much change will your approach make
to the measurement of a 13 micro-Henry inductor? At 1 MHz a 13 uH inductor
has a reactance of 2*pi*13 ohms which is about 82 ohms. At 10 MHz the
reactance is about 820 ohms. As an example, how much impact does 100
pico-seconds have at 1 MHz or even 10 MHz. It seems a 100-pico-second delay
error would be a pretty small phase change at 1 MHz or even 10 MHz. Perhaps
I don't understand your procedure completely.
--
Bryan, WA5VAH
