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On Sun, 22 Sep 2019 at 13:42, Larry Rothman <ac293@...> wrote:

David, antistatic mats are typically constructed of 3 layers: the outside
being a work surface of some plastics composition and the middle is the
conductive portion. What was done in the video was not a valid test. He
should have used a surface charge meter to measure.
The whole idea is to bleed off any charges, not to 'short' them at the
work surface.

The middle layer of the work mats is made up of a carbon weave and yes, it
conducts. Folks also need to know that these mats need to be grounded or
they're useless. Many times in dry weather, I've gotten up from my chair
and placed my hand at the edge of the mat to be greeted with a nice zap
every so often.

Shocking!

Larry

I believe the point that Dave Jones was making, is that the surface
resistance of the mats does not have any adverse effects on 99.99% of the
measurements one makes. I don't think his aim was to show how good a
static dissipative mat was - only that it does not have any adverse
reactions.

It's generally considered unsafe to ground the mats - instead they should
be connected to ground by via a high-value resistor - typically 10 M ohm.

I personally don't think people need to worry about static on the test
ports of the NanoVNA. However, since the sides of the NanoVNA are open in
the standard unit, then there's a possibility of static doing damage there.

Dave

Carlos
The nanoVNA has about 5 to 10 dB headroom in S21 in alle ranges. This is done by adjusting the gain off the amplifier before the adc to the frequency range
Biggest concern in overloading are the mixers when SI5351 outputpower is boosted in overtone mode. But this does not happen. A filter before the amp in the adc keeps near harmonics out.
So when measuring amplifiers you always should use an attenuator
The design of the nanoVNA is not bad.

What is the difference between traces and tracks? Some firmware are said to be for 2 tracks or 4 tracks. In the Nano you can choose to show different traces. Is it two words for the same thing?

I'm playing with rather more exotic HW, a Tek 11801 & SD-24 TDR head which is a ~26 ps rise time TDR evaluating the quality of Chinese RF connectors. But the principles are the same and I'm very interested in using the nanoVNA for such things. I'm a retired seismic research guy, so I've been having a blast. Most fun I've ever had with a piece of test gear and all I'm doing is playing.

I look at the time domain S11 and the time domain S21 response. The S11 TDR shows any discontinuities and the rise time of the S21 TDR gives me an estimate of the frequency limit. In my case I'm putting the connectors between a couple of pieces of RG402 terminated with SMA-M connectors.

If you do an SOLT on a short known high quality cable, then substitute the CUT and repeat the SOLT at the end of the CUT you should get a very accurate picture of cable and connector quality. In the frequency domain there should be a linear phase shift difference between the reference cable and the CUT. The ratio of the S21 values should give you the loss.

I can't keep up with who's done what on this topic in the nanoVNA FW & SW, but the MATLAB clone, Octave is an ideal tool for taking data via the nanoVNA console and analyzing it. The frequency spacing determines how long a cable you can test. 1 MHz spacing will give you the ability to test to 100 m. The resolution is dependent upon the maximum frequency. So 901 points from 1 to 900 MHz is probably a good choice of sweep parameters. You'll need to use the reference cable results to compensate for amplitude errors in the nanoVNA. If you pad lots of zeros on to the end of the frequency domain measurements you will get very fine sampling in the time domain.

I recommend doing this with bare uncorrected data at least once using Octave just for the educational experience. It's actually *very* simple. And there are plenty of people here able to help out.

I used the 11801/SD-24 to test another APC-7/N-F adapter yesterday from the same seller as in the photos here:



Comments on second sample:



I've still got tests of BNCs and other stuff to do with the 11801 and then I'll be duplicating some of the tests on the nanoVNA and my 8753B.

Have Fun!
Reg

On Sun, Sep 22, 2019 at 06:09 AM, Mario Vano wrote:

CORRECTION: (I mistyped)

- The multi-band dipole is my 20ft high 40/20 meter trap dipole that has been augmented with parallel dipoles for 15 and 10. The feedline has a toroid common mode filter.

I've been happily using the NanoVNA and various versions of "Saver" for a lot
of projects lately, but some were just motivated by curiosity about the
device.

Unfortunately I have no other analyzer to compare to. In any case here are
some sweeps from earlier tests that might be useful for reference or
amusement. In most cases, they were done with my very early attempts at full
bandwidth calibration.

- The fm filter is a commercial product from the rtl-sdr.com people.

- The LW filter is a homebrewed design I use to augment VLF beacon hunting.

- The multiband dipole is my 20ft high 80/40 meter trap dipole that has been
augmented with parallel dipoles for 15 and 10. The feedline has a toroid
common mode filer.

- The 6 meter antenna is an attic mounted dipole with a toroid common mode
filter.

I've also used happily used the device and "saver" to design and build a 6
meter "squalo", but forgot to save the files from the latest testing run. I'm
currently working on a 400-700 mhz indor LPA design and plan to try to use the
device for relative pattern and gain testing.

Earlier tests with several types of 500-900mhz 1/4 wave mag-mount antennas and
TV rabbit ears showed useful relative gain results at 2-3 wavelength spacings.

73, AE0GL

Very interesting site, much to study in depth to learn more about VNA and CALKIT.
I suggest inserting the link on the wiki page
Maurizio

Carlos,
Rune's next release of nanoVNA Saver will introduce "Averaging, including truncated mean. First version. " I am interested in seeing how users who have been requesting averaging for the nanoVNA will respond.

On professional VNAs, stimulus source is calibrated for flatness on factory with a bolometer. On nanoVNA you can't control stimulus output power, so we should probably calibrate the audio codec gain to get most of it's dynamic range at each frequency, or at least for each band. However multiple mixing products, others than the desired one, could be present at audio frequencies, so this must be done with care.

This could also lead to a debate about needed headroom to measure DUTs with gain. IMHO, it would be better to sacrifice having a lot of headroom and just insert an attenuator if an amplifier S21 is to be measured.

I have not taken a look to the code in much detail yet, maybe this is being done, but probably not. I have read somewhere that people have measured their LNA's gain, so current code must be setting the codec gain too low if it has so much headroom and the S21 70-80dB noise floor we are seeing could be in fact being limited by the codec dynamic range.

I also miss an averaging mode, as some measurements are quite noisy, and it would help for duplexer adjustments with notches near the VNA noise floor. Maybe just a moving average low pass filter with just 2 taps to avoid increasing memory requirements (new=0.9*last+0.1*meas). David mentions using averaged measurements for isolation, and in fact most professional VNAs use averaging when performing calibration.

Carlos

It's been my experience what I've had problems with a vendor that they beg you to
give them a positive report if they fix the problem. And generally they will jump
through hoops to do it. Have a conversation with the vendor and take your choice,
a replacement or a refund. But DON'T give feedback until the issue is resolved.

Bruce, K4TQL

David,

For your reference here are the FFT's of the audio signal from CH1 while connected to CH0 at various frequencies. 50 MHz, 299MHz, 1200MHz and 2100MHz, all with offset of 4kHz
Next the isolation measurements for 50MHz and 2100MHz.
This is done in my own HW so there are no changes in ADC settings, apart from the SI5351 everything else remains the same.
With strong signals the noise floor increases max 10dB but the SNR differences are as expected due to decrease in the 4kHz harmonic mixing signal.
As you can see FFT'2 are rather clean. Only the 4kHz and its harmonics.
Close observation of the 4kHz shows its starts to wobble more at higher frequencies explaining why it does not fit perfectly in the 480 samples you use.
Hope you will be able to see the same once you have done the windowing of the FFT input signal.

I use this SW for many tasks and interface via the HPIB bus to load calibration tables for my VNA(s).
There are tips and application notes and excellent information at this site.
The SW is free down load.

An arrangement of this type SW with the NanoVNA added would be a significant assist. Up loading via USB.
My search on this site shows no prior mention of this tool, so find it added below:



Alan

I've been happily using the NanoVNA and various versions of "Saver" for a lot of projects lately, but some were just motivated by curiosity about the device.

Unfortunately I have no other analyzer to compare to. In any case here are some sweeps from earlier tests that might be useful for reference or amusement. In most cases, they were done with my very early attempts at full bandwidth calibration.

- The fm filter is a commercial product from the rtl-sdr.com people.

- The LW filter is a homebrewed design I use to augment VLF beacon hunting.

- The multiband dipole is my 20ft high 80/40 meter trap dipole that has been augmented with parallel dipoles for 15 and 10. The feedline has a toroid common mode filer.

- The 6 meter antenna is an attic mounted dipole with a toroid common mode filter.

I've also used happily used the device and "saver" to design and build a 6 meter "squalo", but forgot to save the files from the latest testing run. I'm currently working on a 400-700 mhz indor LPA design and plan to try to use the device for relative pattern and gain testing.

Earlier tests with several types of 500-900mhz 1/4 wave mag-mount antennas and TV rabbit ears showed useful relative gain results at 2-3 wavelength spacings.

73, AE0GL

Ideal resistive bridge response:

Hans,
The tdr function, available standalone in the recent firmware upgrades and connected to a pc via software program, is good for evaluating cables. It can give you info about discontinuities, cable length and impedance bumps. I measured a 25 ft length of RG58 cable using the tdr option of Rune's nanoVNA Saver program and it found a short at 5 ft from the end of one of the connectors. Saved me the grief of installing a defective cable in my ham shack.

On Sun, Sep 22, 2019 at 03:15 AM, Dr. David Kirkby from Kirkby Microwave Ltd wrote:

On Sun, 22 Sep 2019 at 08:10, Dr. David Kirkby <
drkirkby@...> wrote:

I fail to see the need to worry if the active devices is connected across
a 50 ohm resistor. 50 ohms is effectively a short circuit as far as static
is concerned.

Here’s a video from dave Jones of EEVBLOG measuring the DC resistance of
antistatic mats.



Two DVM probes are put next to each other, the meter, which can read upto
300 M ohm can measure anything.

I should have said that the meter can not measure the resistance - it is
over 300 M ohm.

You are not going to zap the NanoVNA with static on its RF input terminals.
--
Dr. David Kirkby,

David, antistatic mats are typically constructed of 3 layers: the outside being a work surface of some plastics composition and the middle is the conductive portion. What was done in the video was not a valid test. He should have used a surface charge meter to measure.
The whole idea is to bleed off any charges, not to 'short' them at the work surface.

The middle layer of the work mats is made up of a carbon weave and yes, it conducts. Folks also need to know that these mats need to be grounded or they're useless. Many times in dry weather, I've gotten up from my chair and placed my hand at the edge of the mat to be greeted with a nice zap every so often.

Shocking!

Larry

tuckvk3cca,
Thanks for the feedback. I finally was able to find a directivity specification of 20 dB minimum for the Narda 3060-20 on the web, which is in agreement with my measurement made using the nanoVNA. My best directional couplers have a 40 dB directivity. The ones I own with a directivity below 20 dB I only use for indicative purpose.

For evaluating cables I normally just do a S21 log measurement, which is somewhat of a "catch all" test for losses due to impedance problems, cable defects etc. I only calibrate with a "through" measurement (with a union connector in place where the CUT will eventually be placed). Then I make the S21 log measurement with the cable being tested in place (where the union connector was during the through calibration). The frequency range depends on the needs...but higher frequencies near the nanoVNA upper limit will certainly be more sensitive in terms of detecting differences in cable performance.

Further investigation did show the dsp algorithm fails to detect the intended signal but locks on a different harmonic so only thru measurement with a filter that removes unwanted fundamentals and other marhonics may ever be possible.
On similar hardware (SI5351, resistive bridge and 3*SA612, but much more selective dsp filter) I was able to have 20dB dynamix range at 2GHz but also there the bridge lost most/all directivity.
I guess for now impedance measurement at 2GHz will not be feasible with a nanoVNA

30dB is rather low for 10MHz. I can get 53dB at 10MHz and 37dB at 35 MHz but this was not measured with the nanoVNA.

At step 3 you can also use a shorted load. Then you have directivity for short. The two should be close about 0.5 dB. Also reverse IN and OUT to check symmetry.