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So you removed the plastic film that is on the screen?
Mike WY6K


"... somewhere in the distance, there's a tower and a light, broadcastin' the resistance, through the rain and through the night..."

My screen tab is on the protective screen.

I don't understand all the theoretical limitations associated with different FFT sizes; but from a practical use point of view if my $50 device, as a additional throw-in, can measure a 65 meter length cable accurate to 0.1 meter then I'm impressed.

The 900 MHz upper limit implies a sinc(t) width of 2.2 ns. A 256 point complex to 512 point real FFT will provide a long enough trace to reach the end of a ~50 m cable. The screen pixel count is not large enough to display all of that in time in a single screen.

The 1500 MHz upper frequency implies a 1.33 ns sinc(t). In that case the distance will be limited to ~30 m by only having 256 complex points. While more points would interpolate the sinc(t) and make it easier to pick the correct time looking at the TDR response, calculating the shift in the frequency domain will be *much* more accurate.

The screen resolution makes more than 256 samples not especially useful. Better to display the time to the cursor with picosecond resolution calculated in the frequency domain by a linear fit to the phase.

To reiterate. The sampling in time determines the maximum frequency. The sampling in frequency determines the maximum time. The maximum frequency and the numerical precision govern the achievable resolution picking a TDR trace.

The initial spike is a bug. It is a real constant added at all frequencies to the correct magnitude, but with no change in the phase angle. One can determine what that is by zeroing out everything after that initial spike and transforming back to frequency. I suspect that it is the result of a naive attempt to set the DC value in the frequency domain. If the frequency sampling were were infinitesimally fine (and the series length infinitely long), then setting that would be easy, but because it is discrete, that DC value is smeared out across the low frequency components. I did the smearing in frequency as a result of limited series length in grad school. I'll leave the problem one faces going the other way to the reader, at least for now. If you've taken a course in integral transforms or at least the Fourier transform, it's a very valuable exercise. The transform is symmetric, so once you've done one you know the other.

I haven't looked at the code, but I do know the mathematics well enough to be very sure of the preceding statement. "The Fourier Transform and Its Applications" by Ronald Bracewell will fill in all the details anyone might desire. At least until you want all the important theorems about the Fourier transform. For that, "A Handbook of Fourier Theorems" by D.C. Champeney is what you want. However, the latter is only really of use if you're concerned about whether doing the transform is valid.

Have Fun!
Reg

Erik
Congrats with your Call

73's
Erik ON8DC

Additional tidbits of information.

1. The V2 PCB is 4 layers.
2. The schematics will be posted soon, at the same time as the launch on taobao,
3. Design objectives include keeping BOM cost below $100 and compatibility with current software base.

Rather than larger, I was hoping for wireless (Bluetooth), perhaps by ESP32,
using a smartphone or tablet for display and complex calculations
without USB and ground loop concerns.

Hello,

Allow us, please, to inform you that, after:

[15] : /g/nanovna-users/message/3147

we just uploaded our well-formed mathematical expressions
of full-one port vna measurements [*] :



which--due to their apparent symmetries, as well as to their
arrangement--we consider them as the most economical and
appropriate ones for programming them in any computer
language - either having the complex data type internally or not.

Sincerely,

yin@pez@arg

[*]

16

On Mon, Sep 23, 2019 at 08:04 PM, <erik@...> wrote:

Can you show the measurement? preferably the smith chart of another VNA doing
the CH0 measurement.

I'm talking about OUTPUT impedance of CH0 (don't confuse with INPUT impedance). I didn't hear that it is possible to measure output impendance with another VNA :)

You're needs to measure amplitude on different loads and then calculate source impedance:

Z = (R1 - R1*(U1/U2)) / (U1/U2 - R1/R2)

My measurements for CH0 in attachment. R1 = 1 MOhm, R2 = 50 Ohm. Of course this is just a draft measurement for overage amplitude. I'm too lazy to perform measurement for specific frequencies. :)

I understand and thank you for answering.

On Tue, Sep 3, 2019 at 11:36 AM, RFy wrote:

Any genius/proper methods of measuring over 30dB powered source impedance?

In order to measure source impedance, you're needs to measure amplitude for different loads. Since many RF PA cannot handle open output properly you will need to use two different loads which impedance is compatible with PA output. For example 25 Ohm and 75 Ohm. If your PA has very limited range for load impedance, use something like 40 Ohm and 60 Ohm. The goal is to get more different load to get more precision.

So, you have load R1 and load R2.
Just connect R1 and measure amplitude U1.
Then connect R2 and measure amplidue U2.

Now you can calculate source impedance:

Z = (R1 - R1*(U1/U2)) / (U1/U2 - R1/R2)

The only problem here is to measure RF signal amplitude on the load properly. This is not so easy task. You can use wide bandwidth oscilloscope for that.

PS: there is no difference if your equipment measuring RMS voltage or amplitude voltage, just use the same voltage type for both measurement.

PPS: you're needs to measure voltage on the load. But note, you cannot use random wires to do that. The better way is to use high impedance RF probe from oscilloscope. The other way is to use attenuators with 50 Ohm output and RF voltmeter with 50 Ohm input. The main goal is to measure voltage on specific load.

Never mind, figured it out. THanks if you looked!

On Fri, Sep 27, 2019 at 12:04 AM, <epif18@...> wrote:

I downloaded 1.03 and everything seems to work except the smith chart won't
draw on my PC.

Try this one:

I also have more fresh with fixed some bugs and TDR screens.TDR VSWR works great, but TDR Z (step response) still not works properly

Thanks Neil. I downloaded 1.03 and everything seems to work except the smith chart won't draw on my PC. Data comes in on all modes. Is there an install process. BTW, I was in the process of designing and building a VNA but now why bother. Works great !

It is a screen protector that prevents the screen from getting scratched during assembly/storage, etc. peel it off if you want to

That green tab might be to disable the battery during shipping??

Sent using




---- On Thu, 26 Sep 2019 17:00:15 -0400 mike watts via Groups.Io <wy6k@...> wrote ----


My unit arrived with a small green plastic tab sticking out the top near the left side.? What is that all about?? Do I remove it?
Mike WY6K


"... somewhere in the distance, there's a tower and a light, broadcastin' the resistance, through the rain and through the night..."

Mike,

It's probably the cheap screen protector that's on it.

Dave-KB1PVH


Sent from my Galaxy S9

On Thu, Sep 26, 2019, 5:00 PM mike watts via Groups.Io <wy6k=
[email protected]> wrote:

My unit arrived with a small green plastic tab sticking out the top near
the left side. What is that all about? Do I remove it?
Mike WY6K


"... somewhere in the distance, there's a tower and a light, broadcastin'
the resistance, through the rain and through the night..."

My unit arrived with a small green plastic tab sticking out the top near the left side.? What is that all about?? Do I remove it?
Mike WY6K


"... somewhere in the distance, there's a tower and a light, broadcastin' the resistance, through the rain and through the night..."

On Thu, Sep 26, 2019 at 09:08 PM, Palo wrote:

Would it be possible to implement some of the features of this project into
NanoVNA?

No, it is impossible, because NanoVNA uses much more slow CPU with much less memory.

EU1KY uses hardware with powerful STM32F746G processor working at 216 MHz with high performance DSP core, 340 kB internal and 8 MB external RAM, 1 MB flash and many other features.

NanoVNA uses very cheap STM32F072CB processor working at 8 MHz with 16 kB RAM and 128 kB flash. There is no resources to implement all these features.

But some of these features may be implemented as separate firmware with dedicated mode for specific feature.

Current NanoVNA TDR implementation is almost useless, because it shows time with 1 nanosecond resolution.

I experimented with different FFT sizes and it looks that 65536 has the best resolution for NanoVNA. It allows to see picoseconds. But it produce a lot of data, it's better to use 32768 points. 16384 also usable, but insufficient resolution becomes noticeable.

This TDR feature just eats precious memory. F072 has too small memory. So, I think it's better to remove TDR function from firmware and replace it with more useful features

One thing I noticed about the screenshot attached to the first post, is the TDR function seems to involve some human interaction. The first peak is a false positive and the cursor at the second peak was the true cable length reflection. There is probably not enough memory to make it a hands off calculation like Rune's NanoVNA Saver performs.