Re: test fixture - how bad can it be? A non-rhetorical question
For flying wires and alligator clips to work, everything must be in the *exact
same position* as when the calibration was done. I use a couple of BNC
female connectors mounted on a piece of double sided FT-4 board. I've then
soldered alligator clips to the BNC teat which I use to mount the component
under test. This keeps everything in the same position as the cal.
Dave - W?LEV
toggle quoted message
Show quoted text
On Tue, Jan 7, 2020 at 11:43 PM Brian <vk4bap@...> wrote: I have also been looking to answer how accurate should the NanoVNA be at
HF. Most components I want to measure are wire ended so my test jig is a
60cm RG316 with SMA on each end cut in half. Each cut end has 1cm of inner
exposed and an alligator clip put on. This is calibrated with open, short
and a 50 ohm made from 2 100ohm resistors with short wires soldered on. A
100pF silver mica then gives the attached which indicates 105pF at 7MHz,
109pF at 14MHz and 126pF at 28MHz. I remove the alligator clips and repeat
soldering the connections (inconvenient but worth trying). This gives the
same results. Should the NanoVNA do better than this? Are there firmware
versions which will produce more accurate results? Is it my VNA
hardware(made in China somewhere) or test jig or test method?
I have found the VNA gives good results using S21 on filters but using a
single cable and S11, capacitors and inductors are difficult to measure. I
expect ferrite cored inductors to show a frequency variation but a 100pF
capacitor should still be 100pF at 30MHz. I've repeated calibration, tried
different leads but I don't have any confidence in the accuracy of
reactance measurement. From what I have read, I could manually add "fudge
factors" to the calibration and remove what looks like a systematic error
but that is not so easy for the mathematically challenged (like me).
My conclusion is that the accuracy depends on the accuracy of the
standards you have and the amount of effort you put into "improving" the
calibration. Some test jigs might be easier to get a good calibration but
up to 30MHz the test jig is not the most important factor.
I have done what Erik suggests and measured a 100 ohm resistor. It shows
no significant reactance at 30MHz. I don't have a record of it but
certainly not 26pF.
73 Brian.
--
*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*
|
Re: errors of "error" models
Hi Gary,
Great questions. I have no idea what the answers are. But Erik¡¯s and John¡¯s replies seem reasonable.
Perhaps Dr. Kirby might know.
Best regards,
Jeff
|
Re: test fixture - how bad can it be? A non-rhetorical question
I have also been looking to answer how accurate should the NanoVNA be at HF. Most components I want to measure are wire ended so my test jig is a 60cm RG316 with SMA on each end cut in half. Each cut end has 1cm of inner exposed and an alligator clip put on. This is calibrated with open, short and a 50 ohm made from 2 100ohm resistors with short wires soldered on. A 100pF silver mica then gives the attached which indicates 105pF at 7MHz, 109pF at 14MHz and 126pF at 28MHz. I remove the alligator clips and repeat soldering the connections (inconvenient but worth trying). This gives the same results. Should the NanoVNA do better than this? Are there firmware versions which will produce more accurate results? Is it my VNA hardware(made in China somewhere) or test jig or test method?
I have found the VNA gives good results using S21 on filters but using a single cable and S11, capacitors and inductors are difficult to measure. I expect ferrite cored inductors to show a frequency variation but a 100pF capacitor should still be 100pF at 30MHz. I've repeated calibration, tried different leads but I don't have any confidence in the accuracy of reactance measurement. From what I have read, I could manually add "fudge factors" to the calibration and remove what looks like a systematic error but that is not so easy for the mathematically challenged (like me).
My conclusion is that the accuracy depends on the accuracy of the standards you have and the amount of effort you put into "improving" the calibration. Some test jigs might be easier to get a good calibration but up to 30MHz the test jig is not the most important factor.
I have done what Erik suggests and measured a 100 ohm resistor. It shows no significant reactance at 30MHz. I don't have a record of it but certainly not 26pF.
73 Brian.
|
Re: Part Number for the Demokit Cable
Hi Herb. I made a simple tool to install the board side connectors, but also doubt they will survive very long. My first thought was to solder them onto the board, but that would probably cause the small connector to lift off.
Perhaps some epoxy or super glue? I could make a 'dam' around the outside of the board and fill it 1/8" or so with clear fiberglass resin.....
ron
N4UE
|
On Tue, Jan 7, 2020 at 03:06 PM, BruceN wrote: Lithium chemistry battery charges have a setting called "Storage Charge". Interesting, I'd never seen that parameter on data sheets before. That'll give me something new to study ;-) 73 de Andy
|
Re: First PCB pictures of the V2
Mel, thanks for the great help with this project (and to Gabriel-san, of course).
You should change your username on groups.io to Vignette Tsukinose April.
Sorry about the inside joke, at least you can google it :)
73, Mike AF7KR
|
Re: Part Number for the Demokit Cable
On Tue, Jan 7, 2020 at 01:47 PM, Phil Royce wrote:
" looks like 23 more cables to order and fix them permanently on to sma patch panel, I don't want to take on and off these, knowing in the past these connectors are not continuous use parts."
====================================================================
I bought a couple of demo boards for classroom teaching and they didn't even last out the day with my students. A lifted trace on one board and intermittent cable connections on both boards after use by my better students. For a teaching environment, I would suggest something sturdier like the SDR-Kits Testboard Kit, or as you suggested permanently attached and fixed in place demo board cables.
- Herb
|
Re: errors of "error" models
Gary, just a guess (I'm not a VNA designer) but it might be because it's
easier to design and characterize an "absolute" (open or short) with
nominally infinite impedance than something that needs to match some
arbitrary value. And how would you choose the arbitrary values?
Different users have different requirements.
I think, but am not sure, that using arbitrary values also would prevent
any pretense at corrected measurements beyond those arbitrary values.
When your limits are infinity, nothing stands in your way. :-)
73,
John
----
toggle quoted message
Show quoted text
On 1/7/20 2:48 PM, Gary O'Neil wrote: Hi again Jeff;
I believe I now sufficiently understand the technical aspects of the discussions in this thread to forego the wizardry behind the pursuit of high accuracy. it appears sufficiently sound.
On that happy note¡ I will state my one remaining question succinctly. Why the obsession over accuracy at the the two most unstable phase regions of highest Q and unreachable limits of infinity and zero?
A reasonable and credible answer will be a bounded tolerance of impedance or phase in those regions, and an estimate of the consequence of exceeding the tolerance boundaries.
I will reiterate¡ There is nothing wrong with how this is treated what is being done or the rationale behind the obsession. The only question is simply... Why?
|
Re: Part Number for the Demokit Cable
|
Re: errors of "error" models
|
Re: Part Number for the Demokit Cable
Found a better price here for bulk. I'm use to lab prices so these are pretty cheap but not going to use constantly just for demo and test theories.
|
Re: Part Number for the Demokit Cable
looks like 23 more cables to order and fix them permanently on to sma patch panel, I don't want to take on and off these, knowing in the past these connectors are not continuous use parts. Thanks for the part number.
|
Measuring resonance from coax far end.
Hi folks,
This is more a generic VNA question, but thought i'd ask.
I've seen a trick to "eliminate" the coax by calibrating via the OSL on the
far end of the coax, rather than having OSL connected directly to the VNA.
Essentially it's re-calibrated with the coax factored into the equation,
virtually moving the feedpoint to inside the shack for accurate antenna
feedpoint measurement.
If i calibrate the far end, take resonance readings, then recalibrate with
the OSL connected directly to the VNA, re-take resonance readings, would it
be safe to assume if there is common-mode current interference on the coax
it would account for differing results (if there are any)?
If there's no common-mode currents, the coax-length should be invisible.
Just want to make sure i've got my theory correct. :)
Thanks & 73s,
Chris.
|
Re: Part Number for the Demokit Cable
30 mating cycles ?? That gets you through the Demo Board only twice !! Where can one get extra cables, I think I may need them if I am to use the Demo Board to show off what the NanoVNA can do. 73, Dick, W1KSZ ________________________________ From: [email protected] < [email protected]> on behalf of reuterr@... <reuterr@...> Sent: Tuesday, January 7, 2020 12:42 PM To: [email protected] < [email protected]> Subject: Re: [nanovna-users] Part Number for the Demokit Cable ebay.com, eBay item number: 352840116109, $2.29 free shipping Please see my documentation at: /g/nanovna-users/wiki/RF-Demo-Kit-use73, Rudi DL5FA
|
Re: errors of "error" models
Hi again Jeff;
I believe I now sufficiently understand the technical aspects of the discussions in this thread to forego the wizardry behind the pursuit of high accuracy. it appears sufficiently sound.
On that happy note¡ I will state my one remaining question succinctly. Why the obsession over accuracy at the the two most unstable phase regions of highest Q and unreachable limits of infinity and zero?
A reasonable and credible answer will be a bounded tolerance of impedance or phase in those regions, and an estimate of the consequence of exceeding the tolerance boundaries.
I will reiterate¡ There is nothing wrong with how this is treated what is being done or the rationale behind the obsession. The only question is simply... Why?
--
73
Gary, N3GO
|
Re: Cancelling out coax feeder length?
As luck would have it, Times Microwave website has a coax calculator that includes the delay! My feeder is 26cm of RG316, which comes out as 1.3nS. Was also pleased to find the nanovna allows delay to be entered in pS or nS!
The resulting traces for this antenna are very complex, but it does show the VSWR is good. The complexity I think is likely to be due to the multi-element groundplane arrangement of the antenna! but I will do more testing later.
|
Re: Cancelling out coax feeder length?
How long is your feeder and what kind of, I assume, coax? It's really quite easy. Give me those parameters, and I'll show you. Dave - W?LEV On Tue, Jan 7, 2020 at 7:19 PM Martin Barfield G7MRV <marting7mrv@...> wrote: That makes sense, thanks,
however the problem I have is that I cannot do it this way, due to the
antenna being fixed and not being able to access the feeder connection, so
it will need the maths!
So, its the electrical delay parameter thats needed, and this is in
picoseconds... I also know the length and the type of feeder, so should be
able to find the necessary parameters now to calculate the delay,
It would have been so much easier of course if i'd bought two of the same
patch leads, like I usually do!
Cheers
Martin
-- *Dave - W?LEV* *Just Let Darwin Work* *Just Think*
|
Re: Part Number for the Demokit Cable
|
Re: Cancelling out coax feeder length?
That makes sense, thanks,
however the problem I have is that I cannot do it this way, due to the antenna being fixed and not being able to access the feeder connection, so it will need the maths!
So, its the electrical delay parameter thats needed, and this is in picoseconds... I also know the length and the type of feeder, so should be able to find the necessary parameters now to calculate the delay,
It would have been so much easier of course if i'd bought two of the same patch leads, like I usually do!
Cheers
Martin
|
Re: Nanovna Saver Q issue
On Tue, Jan 7, 2020 at 09:40 AM, vincent coppola wrote:
" ..The problem appears to be in determining the correct 3db points. Please note the band stop filter is the only filter that I have tried to measure Q .."
============================================================
Vince,
You might include a screen capture of showing the analysis data and the bandpass filter. If your filter is not flat in the bandpass, has large ripple in the bandpass, or there are not enough measurement points then an accurate automated determination of the 3 dB points may not be possible.
The program asks the user to place a marker in the flat area of the bandpass and then computes the lower and upper 3 dB frequency points from that marker value. The computed points may not correspond to any of your actual measurement points and the program will choose the closest point. For better accuracy, choose a higher number of sweep segments which will give more measurement points over the frequency range.
- Herb
|
Phil Royce
Dick