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Re: Phase noise/drift and the accuracy of measurements

Yury Kuchura EU1KY
 

Hi Dan,

Interesting idea! I will try to model this approach in Python + numpy. But maybe not for this particular project:

This device is not a lab grade VNA. First of all it is just a good antenna analyzer used to finetune very unstable system which actual antenna is, considering that in real life it is exposed to extremely varying outside weather conditions which affect its parameters. I believe the provided precision is quite enough for such a system. Also, I don't think LO phase noise makes any significant influence while tuning real antennas, especially on short waves where the incoming signals are sometimes very strong comparing to this noise.

Let me clarify what is the math is under the hood.

Single precision floating point RFFT is used for phase and magnitude calculations. 512 samples used, with 48 kHz sampling rate, IF is 10031 Hz (bin 107), then Blackman windowing. I've found no improvements in precision when I used more samples, but the sampling and calculation time grows significantly.

Then the voltage magnitude ratio between channels is calculated as a root of the sum of powers in five bins where the most of the signal power is concentrated. Even if the IF is not exactly in the center of bin, this approach makes scalloping loss negligible. But even this is an overhead I think because I use the ratio of magtitudes for further calculations, it remains the same when both coherent signals are offset from the center. The phase difference is taken only from the central bins, modelling showed that it remains constant when frequency offsets up and down within the bin. With known magnitude ratio and phase difference, it is then easy to calculate the raw impedance of the DUT connected to the bridge. It is then easy to convert it to complex reflection coefficient for chosen Z0, and to apply OSL calibration.

73!
Yury


Phase noise/drift and the accuracy of measurements

Daniel Marks
 

I was going to throw this out there to see if there are any comments or ideas.

The EU1KY analyzer uses the STM32F7 discovery board with an SAI audio codec.  The audio codec provides 16 bits ADC sampling.  To get the benefit of this sampling, however, one needs to know the phase of the intermediate frequency (approximately 10 kHz) to 2^-16 radians of accuracy.   If one has a IF bandwidth say, of 50 Hz, that corresponds to 20 ms sampling time or (20 ms) X 2^-16 radians X 10 kHz = 0.03 radians or 1.7 degrees of tolerable phase drift of the intermediate frequency before the phase drift limits the accuracy of the demodulated signal rather than the bit sampling depth.

Now because one samples two signals, and one is only really interested in the difference in phase between those two signals, one should be able to use the phase drift of one to cancel the other.  If one uses the FFT to demodulate of the signal, then one may be depending on the intermediate frequency to always land in a particular frequency bin.  So if you have two signals you are trying to measure the phase difference between, you take the FFT of both of the signals, take the complex-valued amplitude and phase of each signal in the frequency bin you know where the demodulated signal is, and then multiple one signal's sample by the complex conjugate of the other, and you get the relative phase of the two.  However, if there is phase noise, the signal's FFT is split "between"  adjacent frequency bins, and does not land perfectly on the expected frequency bin, and so one does not perfectly cancel this phase noise.   The nonlinear demodulation step here is the multiplication of the two frequency samples from the two FFT's.

Another possible approach is to do this demodulation step in the time domain rather than in the frequency domain.  One can form the "complex analytic signal" versions of each of the two signals captured by the audio codec.  This is done by using the Hilbert transform to infer the imaginary part of each signal from each real-valued signal.  In practice, this may also be done by using the FFT of a real-valued signal.  The real-valued signal has a FFT that is Hermitian conjugate, that is, the negative frequencies are the complex conjugate of the positive frequencies.  If you take the FFT of a signal, zero out the negative frequencies, and take the inverse FFT, you get a complex signal that is the complex analytic signal version of the real signal.   So the idea is to form the complex analytic signal version of each of the two signals, and in the time domain multiply the complex sample in one signal by the complex conjugate of the sample in the other signal, and then sum over all of the samples.  By doing the multiplication in the time domain rather that in the frequency domain, any phase drift between the two signals is cancelled.  One can get better noise immunity by, rather than just zeroing out negative frequencies, use a passband filter in the frequency domain only around those frequencies one knows the intermediate frequency should be in, but it is a larger bandwidth than just using one frequency bin.

I am not sure if this is already done in the EU1KY analyzer but I think that phase noise is a potential problem to obtain the highest accuracy, and there are separate crystals on the codec and the SI5351A and so some drift will occur between these.  In the analyzer I designed, I actually generate a trigger signal using a comparator from the intermediate frequency which I use to sample the ADC which eliminates that kind of phase noise, but it may be possible to eliminate it through software as well.  My design uses a comparatively low power processor without floating point and therefore is too slow to do real-time floating-point demodulation of the intermediate frequency signal.

73,
Dan
KW4TI


Re: Releasing the Gerbers and Sprint File for Updated EU1KY VNA PCBse

Ujjwal John
 

Hi Gyula,

Thanks for the appreciation. Just my small bit to return back to the community. I came back to the Group after a while and your NanoVNA really interested me. I will give it a try soon as every VNA design has something different to offer. Thanks for sharing the project. 

73's

Ujjwal, VU3ECN


Re: Releasing the Gerbers and Sprint File for Updated EU1KY VNA PCBse

Gyula Molnar
 

Hi Ujjwal John

Nice job, I hope you have more success because I have been doing this for a month now and no one was interested in pcb.
Maybe when the summer vacation ends, there will be someone who wants to build it - although nanoVNA is ready to go,
so no one wants to work but just want to buy one.
Anyway, this is best - and it knows more.

Gyula, HA3HZ


Releasing the Gerbers and Sprint File for Updated EU1KY VNA PCBse

Ujjwal John
 

Hi Everyone,

First apologies for taking so long releasing the final PCB design for the new features added in the Yuri's original Design.  Please find the the Sprint Layout file and the gerbers for  the updated PCB for the EU1KV VNA. The first run of the PCB had 3 cosmetic errors, all fixable, which our friend Wolfgang had to deal with , in the PCBs I sent him. I made 2 working VNAs for my club members and then made corrections to come up with this design. The additions  / modifications in the current PCB over the original PCB released on Yuri's blog page are :

1. Header for Bluetooth 

2. Header for RTC Clock ( 5/3.3 Volt select by solder jumpers)

3. S2 Port 

4. Provision for 3.3v TCXO - both 5x3.2mm and 3.2x2.5mm size solderable. 

5. GPIO Header for future expansion (Wolfgang's idea - he was working on programmable Attenuator for S21 measurements)

6. Buzzer with buffer Amp using MMBT3904 

7. More exposed / Tinned ground plane for putting a Metal Box for shielding on the Top - For this reason , all Connectors / Coax have Legends in the bottom 

8. LiPo Voltage Header (vBat)

You may find the files in the below location :

https://groups.io/g/Analyzer-EU1KY/files/PCB%20Modified%20by%20VU3ECN/EU1KY_VNA_RFFE_v3.2.zip

Hope the modified design helps others in building their own VNA on Yuri's amazing design. 

Best Wishes and 73's ,

VU3ECN, Ujjwal John.


Re: Protective diodes across input connector

Robert DL5ROB
 

Hi OM's,
I am not sure the diodes are not damaged by static electricity that maybe in the Kilovolt range at some special wheather conditions at shortwave dipols..
For prevention on static electricity  at non grounded shortwave-antennas i use a T-Adaptor at the Analyzers input with a shorted-plug that is pulled of before and connected again direct after every measurement.
That avoids recharging of the antenna before further measurements. A 1Megaohm-resistor permanently parallel to the input during a measurement would also avoid  the recharging.
It would be a feature to have an automatic shorting of the input by a relais and a paralell 1Megohm-resistor inside of any antenna-analyser that is active outside the measuring process, but -by my knowledge- until now nobody of the varous developers of amateur antenna-analysers realised this.
Another thougt:
The diodes should also prevent from strong rf-voltage reaching the analyser from the connected antenna.
In a controlled rf-environment the diodes may be not needed, but think on fieldday-events where you can not control that nobady is transmitting in the antenna besides yours during your measurement.
I don't know the maximum input-voltage of the mixers, but there maybe voltages that may destroy the rf-part of your analyzer if the clamping-function of the diodes is not there.
A T- adaptor with intrgrates diodes outside the analyser would also be a solution for this problem.

73
Robert


Re: Protective diodes across input connector

 

Many thanks, Yury. My expectations were the same.

73,
Martin, OK1RR

Dne 04. 08. 19 v 9:19 Yury Kuchura EU1KY napsal(a):

Hi Martin,
I think the diodes are not really needed. Some people reported that they
affect precision above 200 MHz, though I never observed this effect,
probably caused by diodes quality. I don't use diodes in my devices any
more, and none were damaged by static.
73!
Yury
вс, 4 авг. 2019 г., 6:40 Martin OK1RR <martin@ok1rr.com>:

I have several schematics here, some with protective diodes across input
connector, some without them. What should be preferred? Should be the
diodes placed directly onto connector body or on the board?

73,
Martin, OK1RR




Re: Protective diodes across input connector

Yury Kuchura EU1KY
 

Hi Martin, 

I think the diodes are not really needed. Some people reported that they affect precision above 200 MHz, though I never observed this effect, probably caused by diodes quality. I don't use diodes in my devices any more, and none were damaged by static.

73!
Yury

вс, 4 авг. 2019 г., 6:40 Martin OK1RR <martin@...>:

I have several schematics here, some with protective diodes across input
connector, some without them. What should be preferred? Should be the
diodes placed directly onto connector body or on the board?

73,
Martin, OK1RR




Protective diodes across input connector

 

I have several schematics here, some with protective diodes across input connector, some without them. What should be preferred? Should be the diodes placed directly onto connector body or on the board?

73,
Martin, OK1RR


Just received my Analyzer

Larry Naumann <n0sa@...>
 

I just received my analyzer today.
I bought a fully built and tested unit from Funtronics.
The thing is built like a tank and all the adapters and OSL parts were there.
I did notice that when I tried to go to the Panoramic display I was having hard time
with the screen reacting to my finger push or touch.
It took anywhere from 1 to five tries to get it to switch modes.
On closer inspection I noticed the top cover plate was covering the top portion of the screen.
The bottom of the screen looked fine, the cutout was just a tad too small on height.
Being fortunate enough to have a small milling machine, I removed .075" from the top of the hole.
I reassembled it and now I go into Panoramic Mode with every push or touch.
I believe the metal covering the top portion of the screen was having an adverse affect on the touch screen operation.
I had read of some complaints about the touch screen not working properly, this may have been the cause as it was in my case.

Anyway, thought I would pass that along and I will be downloading the KD8CEC firmware and trying out the S21 function.
This thing looks to be a keeper.
Larry
n0sa


Re: Hardware calibration without moving the jumper

Gyula Molnar
 

Hi,
I congratulate you on DIY but you shouldn't question the original author's opinion, gentlemen. If anyone is interested in the story, read the links and you'll find it in summary.

73, Gyula


Re: Hardware calibration without moving the jumper

dl9hda@...
 

Hi, I just added a simple miniature push button switch with two changeover contacts. Above the middle of the two cinch jacks i drilled a whole and glued the switch. Pushed means HW cal.

kind regards
Holger, DL9HDA


Re: Hardware calibration without moving the jumper

Steve
 

I've been wondering whether I could bring out the HW Cal header pins to a 2 pole changeover because I have to unseat the board from its header pins each time to move the jumper. Although Ian's work is completed for now, I'd still like a tiny panel mounted slide switch on the case. Is this something that would be acceptable?


Re: Hardware calibration without moving the jumper

Yury Kuchura EU1KY
 

Hi Chris,
Yes, only this method is correct.


Re: Hardware calibration without moving the jumper

Chris
 

Hi Jury,

thanks for stepping in.

Just to be sure, tha I understood you correctly:
Does that mean, that for the HW cal,, the procedure with moving the jumper gives the better results?

Chris


Re: Hardware calibration without moving the jumper

Yury Kuchura EU1KY
 

Hi Chris,

The DUT impedance transformation caused by stray capacitances, inductances and  transmission lines (like pigtail) can be presented as a quadripole and can be effectively excluded by OSL calibration. But constant phase and magnitude offsets cannot. The main goal of HW calibration is to remove these constant amplitude and phase differences between the frontend channels before impedance measurements.


Hardware calibration without moving the jumper

Chris
 

Hi,

looking at the schematic, it would make sense, to perform the hardwarecalibration with a good 50 ohm load on the N-connector while leaving the jumper in the "work" position.

One benefit ist, that one does not have to open the analyzer and move the jumper, in case a HW cal is neccesary.

The other benefit is, that the pigtail to the N-connector with the choke is included in the calibration.

Any thoughts on this?

Chris


Re: Improving the hardware

Bob van Donselaar
 

Compensating for one (internal, known) effect may take up headroom in other (external, unknown) situations.

Selecting most effective materials for the basic design, will avoid to compromise elsewhere.

Bob

 

Van: Analyzer-EU1KY@groups.io [mailto:Analyzer-EU1KY@groups.io] Namens Daniel Marks
Verzonden: zondag 21 juli 2019 21:59
Aan: Analyzer-EU1KY@groups.io
Onderwerp: Re: [Analyzer-EU1KY] Improving the hardware

 

For a reflection measurement (as is done with the EU1KY analyzer) the absolute impedance of the choke is not critical as long as it is well about 5 ohms.  The calibration helps account for the variation in the choke impedance.  However, if there is not sufficient impedance, the signal across the 5 ohm resistor is insufficient to get a good measurement.  This is one of the advantages of calibration.

 

Dan

 

On Thu, Jul 18, 2019 at 9:56 AM Bob van Donselaar <bobvandonselaar@...> wrote:

I’m afraid your link is somewhat incomplete about ferrites, since also loss (as a frequency dependent parameter)

is very important for wideband chokes and common-mode transformers for total impedance is key.

The EU1KY input choke is in parallel to a 5 Ohm resistor and for a  < 5 % measurement accuracy should exhibit

total impedance above 100 Ohm.

 

 

Bob, ON9CVD.

 

Van: Analyzer-EU1KY@groups.io [mailto:Analyzer-EU1KY@groups.io] Namens Gyula Molnar
Verzonden: donderdag 18 juli 2019 13:58
Aan: Analyzer-EU1KY@groups.io
Onderwerp: Re: [Analyzer-EU1KY] Improving the hardware

 

Ferrite Mix Selection

 

Virusvrij. www.avg.com


Re: Improving the hardware

Daniel Marks
 

For a reflection measurement (as is done with the EU1KY analyzer) the absolute impedance of the choke is not critical as long as it is well about 5 ohms.  The calibration helps account for the variation in the choke impedance.  However, if there is not sufficient impedance, the signal across the 5 ohm resistor is insufficient to get a good measurement.  This is one of the advantages of calibration.

Dan


On Thu, Jul 18, 2019 at 9:56 AM Bob van Donselaar <bobvandonselaar@...> wrote:

I’m afraid your link is somewhat incomplete about ferrites, since also loss (as a frequency dependent parameter)

is very important for wideband chokes and common-mode transformers for total impedance is key.

The EU1KY input choke is in parallel to a 5 Ohm resistor and for a  < 5 % measurement accuracy should exhibit

total impedance above 100 Ohm.

 

 

Bob, ON9CVD.

 

Van: Analyzer-EU1KY@groups.io [mailto:Analyzer-EU1KY@groups.io] Namens Gyula Molnar
Verzonden: donderdag 18 juli 2019 13:58
Aan: Analyzer-EU1KY@groups.io
Onderwerp: Re: [Analyzer-EU1KY] Improving the hardware

 

Ferrite Mix Selection


Virusvrij. www.avg.com


Re: Improving the hardware

Bob van Donselaar
 

I’m afraid your link is somewhat incomplete about ferrites, since also loss (as a frequency dependent parameter)

is very important for wideband chokes and common-mode transformers for total impedance is key.

The EU1KY input choke is in parallel to a 5 Ohm resistor and for a  < 5 % measurement accuracy should exhibit

total impedance above 100 Ohm.

 

 

Bob, ON9CVD.

 

Van: Analyzer-EU1KY@groups.io [mailto:Analyzer-EU1KY@groups.io] Namens Gyula Molnar
Verzonden: donderdag 18 juli 2019 13:58
Aan: Analyzer-EU1KY@groups.io
Onderwerp: Re: [Analyzer-EU1KY] Improving the hardware

 

Ferrite Mix Selection


Virusvrij. www.avg.com

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