On Sat, 28 Sep 2019 at 16:26, Jeff Anderson <jca1955@...> wrote:
The NanoVNA firmware (as I understand it) assumes that the SMA Short,
Open, and Load standards are ideal, with the single exception of defining
the Open to have a C0 50 femtofarads.
The definition of calibration kits in the NanoVNA seems woefully
inadequate. I don¡¯t know what irritates me the most
* Screen too small
* No proper support for calibration kits.
I was wondering -- is this a sufficient definition for these standards,
given the NanoVNA's frequency limit of 900 MHz? Or should we also be
including values for C1-C3, L0-L3, Offset Loss and Offset Zo?
I don¡¯t think your simple model is really suitable for the following
reasons
1) The variation of C with homemade standards is likely to exceed that of
commercial standards - this is from experience measuring them.
2) The inductance of shorts is likely to be more with homemade standards
than commercial ones - again this is based on experience measuring them.
3) People may well want to make measurements in a 75 ohm system.
4) it is possible to improve upon the accuracy of loads at low frequencies
by using a DC resistance measurement.
5) In the case of a female N, a simple standard can be made by just leaving
the connector open. This will create a higher impedance transmission line
than 50 ohms as the centre conductor sits in a cylindrical section with a
greater diameter than when its mated.
6) The loss of homemade standards is likely to be greater than commercial
ones from Keysight - again this is based on actual measurements I have
performed.
So I wrote some Matlab code to calculate a standard's Reflection
Coefficient (Gamma) based on Keysight's "Full" model (using C0-C3, L0-L3,
Offset Loss, Offset Delay and Offset Zo), and compared this to a simplified
model in which all of these values were set to 0 except for C0 and Offset
Delay.
I've attached a table of results.
My conclusion is that the simplified model (C1-C3, L0-L3, and Loss are set
to 0, and Zo is set to 50) is perfectly adequate for use with the NanoVNA.
I disagree with that, for the reasons stated above.
I have seen something written by Dr. Joel Dunsmore saying that depending on
the accuracy requirements, one should consider the variation of capacitance
with frequency above about 200 MHz. However, I see your notes that the
phase variation up to 1500 MHz is smaller than the uncertainty in the
calibration standards. I can¡¯t square that circle.
But my math could be wrong. Do my results seem correct?
I have not checked your maths, but based on my experience using commercial
calibration kits from Keysight, I would accept what you are saying is
probably reasonable up to 900 MHz. I have here
85032B - 6 GHz N
85038A - 7.5 GHz 7-16
85939B - 3 GHz, 75 ohm F
85050B - 18 GHz APC7
85052B - 26.5 GHz 3.5 mm
85054B - 18 GHz N
plus an Ecal and any I have forgotten!
If you want an even simpler model for commercial calibration kits you could
do without C0, *as long as the offset delay of the open is suitably
increased to compensate for this*. The problem would be people would not
know how much to increase the delay, although it can be worked out. it
would be difficult for each calibration kit.
(A detailed explanation of what I've done is in the top post of this blog:
)
Thanks,
- Jeff, k6jca
P.S. However, note that zeroing out the two delays could become an issue
if the Short's delay is NOT equal the Open's delay
The short delay should *always* be longer than the open delay, which makes
a mockery of all these web pages that go to great lengths to tell you to
make the delay of the short as little as possible. People dreaming about
zero offset shorts don¡¯t understand the subtleties of calibration kit
design. In the calibration kits my company produces, the delay of the short
is chosen for optimal performance, which is *never* zero. If you look at
the Keysight kits you will find that *most* of the kits have a longer delay
on the short than the open. You will see this more with the high-end kits,
such as the 85052B & 85054B kits I mentioned earlier.
Dave
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...
Telephone 01621-680100./ +44 1621 680100
Registered in England & Wales, company number 08914892.
Registered office:
Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United
Kingdom
