NanoVNA Custom Software

[joeqsmith]

Sigilent's website shows them taking orders for their new VNA.   $3400 or so USD, without the cal kit, no TDR and no VNA software included.  Look like you more into the $5K when it's all said and done.    Plus you still have the problem of it not being a full 2-port system.

You get what you pay for. Anyway standalone SA with S11+S21 3GHz VNA for $4000 is hellova good offer.

Hoping this time around that they give the unit to people who can actually show the practical justification of the higher cost when compared with this $50 unit.

Perhaps manufacturers which are looking for dependable instrument having better than 60 70dB dynamic range? Not even mentioning freq range. Of course - many hobbyists are fine with nanoVNA, but not every buyer of VNA is hobbyist.

[edit] Comparison of $50 vs $50000 VNA: https://nuclearrambo.com/wordpress/comparing-nanovna-with-the-keysight-fieldfox-n9952a/

For my hobby, I would most likely buy a used 8753.  Siglent isn't on the radar yet.     

I would have expected the people making reviews for the Siglent products to compare them against these low cost analyzers.   Instead, all the videos I saw for Siglent were fairly basic.   I am suggesting they up their game. 

[ogden]

For my hobby, I would most likely buy a used 8753. Siglent isn't on the radar yet.     

Oh.. Fine choice indeed. I believe that you have to get extremely lucky to find such in working condition for a "hobbyist-friendly price".

I would have expected the people making reviews for the Siglent products to compare them against these low cost analyzers. Instead, all the videos I saw for Siglent were fairly basic.

Considering that Siglent is new kid in the VNA block, perhaps their reviewers are such as well

[joeqsmith]

3.2 GHz is good. But 1.5 GHz for 50 USD is good enough for a radio amateurs

It does seem that most people who have written me are indeed amateur radio operators.   I would have guessed 300MHz would have covered most of their needs.  No doubt that the Nano does seem to fill some void for that group.   When Dave made his Siglent review, he makes a point about the amateurs being a potential market.   

If Siglent wants to gain some of that market, they need to make videos showing practical reasons why an amateur would buy their product over a $50 Nano. 

Sigilent's website shows them taking orders for their new VNA.   $3400 or so USD, without the cal kit, no TDR and no VNA software included.  Look like you more into the $5K when it's all said and done.    Plus you still have the problem of it not being a full 2-port system.   Hoping this time around that they give the unit to people who can actually show the practical justification of the higher cost when compared with this $50 unit.    

https://siglentna.com/spectrum-analyzers/sva1000x-spectrum-vector-analyzer/

You are an optimistic man if you think the Nano will get to 3.2 GHz
The nano is a nice 300MHz device.

Also you might have missed this:
"Vector Network Analysis from 10 MHz – 1.5 GHz / 100 kHz – 3.2 GHz (Now included as standard)"

I didn't notice that they now include the VNA software.   

I'm not sure where they are heading with the Nano or next generation.  My friend bought these just for learning.  From my own home use, I would more than likely pull the trigger on a used HP to replace my 1970s one.   3GHz would not be a problem.   

However wider spans with the same number of points take larger frequency steps. I wonder if the Si chip needs a longer time to settle when making larger frequency jumps? Maybe simply increasing the wait time prior to sampling will improve things in wide spans?

I wonder if this is part of the problem I saw with that last test.  I would assume most noticed that that data was always corrupt in the area leading up to 900MHz.  Once it reached 900, the data was stable.   Hopefully the people putting the firmware together are aware of these problems and are able to solve them.       

[joeqsmith]

For my hobby, I would most likely buy a used 8753. Siglent isn't on the radar yet.     

Oh.. Fine choice indeed. I believe that you have to get extremely lucky to find such in working condition for a "hobbyist-friendly price".

I would have expected the people making reviews for the Siglent products to compare them against these low cost analyzers. Instead, all the videos I saw for Siglent were fairly basic.

Considering that Siglent is new kid in the VNA block, perhaps their reviewers are such as well

For my personal use,  the $5K USD spent on a Siglent would cover the cost of a used HP that would more than fit my needs.  But I am used to buying used TE and restoring it.  Actually, my antique HP still fits most of my needs which is why I have not replaced it. 

I would say that your last statement is spot on.   IMO, this is not how you want to market your new line of products. 

[ogden]

However wider spans with the same number of points take larger frequency steps. I wonder if the Si chip needs a longer time to settle when making larger frequency jumps? Maybe simply increasing the wait time prior to sampling will improve things in wide spans?

I wonder if this is part of the problem I saw with that last test.  I would assume most noticed that that data was always corrupt in the area leading up to 900MHz.  Once it reached 900, the data was stable.   Hopefully the people putting the firmware together are aware of these problems and are able to solve them.     

Possible explanation in nanovna-users@groups.io post:

hugen@...Aug 5   #719 
The si5351 manual shows that the internal VCO operates at a maximum of 900MHz and a 4-divide-frequency output with a maximum frequency of 225MHz. In order to output a frequency of 300MHz, the internal VCO needs to be overclocked to 1200MHz. Not every si5351 can be stably overclocked to 1200MHz. As the temperature increases, the internal VCO operating limit frequency of the si5351 will decrease. If you notice a significant spike(>0dB) in your nanoVNA at 300 MHz or 900 MHz, I recommend that you use the 800MHz firmware.

[OwO]

This change will reduce ADC dynamic range from 100 dB to 70 dB. So, it seems that the new NanoVNA will be worse...

No, the VGA will extend dynamic range to 100dB, plus ADC dynamic range is nowhere near being the bottleneck anyway.

Where exactly you can get (genuine & new) ADF4350 for 0.4$? AD web: $6.05 @1000+

Not my decision. I just heard this from the internal chat group.

Right, MiniVNA Tiny. Existence of such design does not mean it is good. Problem with such approach - leakage through switches. Search this forum to see how bad this VNA actually is.

Please do not jump to conclusions before you've seen the design. There are 3 stages of switches between the reflection path and the receiver, for a total switch isolation of 90dB.

The bottleneck of the dynamic range is actually the common mode inductance of the two edge mount SMA connectors, and this can't be improved much other than separating the two ports as far as possible. All low cost USB VNAs on the market currently suffers from this problem and is why they are all limited to a dynamic range of around 70dB < 1GHz and 50dB at 3GHz.

Again - do not agree to such design decision. ADC of stm32 have barely 11 bit ENOB and 69dB SNR. Such VGA-augmented ADC will have worse linearity and temperature stability comparing to literally any generic audio ADC. Not to mention that VGA acting as part of ADC may slow sampling speed down because some/many points needs to be sampled at least 2 times while correct VGA gain is found.

See above - ADC dynamic range is far from being the bottleneck. If the ADC is sampling at 1Msps and the VBW is 1kHz, a 60dB ADC dynamic range leads to a 90dB measurement dynamic range, which is more than sufficient. What makes you think autoranging is slow? A typical measured transfer function is fairly smooth and in practice you likely won't ever see more than 5 to 10 autoranging events per sweep, which is equivalent to adding 10 points to a 100 point sweep.

[OwO]

And before you jump in and say RF switches are expensive - https://item.szlcsc.com/270817.html

[radiolistener]

If the ADC is sampling at 1Msps and the VBW is 1kHz, a 60dB ADC dynamic range leads to a 90dB measurement dynamic range, which is more than sufficient.

Oversampling cannot eliminate non-linear distortions of a cheap ADC. If you will be able to get good results with integrated ADC it will be nice win, none can do it. This is why all using external codec.

[radiolistener]

Where exactly you can get (genuine & new) ADF4350 for 0.4$? AD web: $6.05 @1000+

Chinese clones are much cheaper

[OwO]

I have a prototype of a NanoVNA variant using the built in ADC on the stm32. The measurements using it are nearly identical to the audio codec variant. The reason all NanoVNAs on the market do not use this version is because of software compatibility.

[OwO]

I just got more info about the baseband VGA design; a RFIC switch is used to switch the shunt resistor in the feedback path. The switch is basically "transparent" because the off state capacitance is in the femtofarad range (it is an RF switch) which is negligible at the IF frequency. The on state resistance is small compared to the resistors being switched in. Since the amplifier gain is mainly dictated by the feedback network, and the switch is "transparent", there is nothing other than the tempco of the physical resistors that can cause a temperature dependence. The RFIC used is the same as for the receiver RF switch, and it turns out all the maxscend switches do not have the shunt diode problem (most RF switch ICs have parasitic diodes from RF input to ground which will start to conduct at lower frequencies), so it has no theoretical lower frequency limit and can be applied at the IF frequency. This is a big improvement over using normal analog switch ICs which have capacitance in the pF range.

EDIT: also asked about linearity. The answer is that the code will perform a calibration of each VGA step on boot up. Since there is no temperature dependence the calibration only needs to happen once.

[OwO]

Here is a preliminary block diagram of the design:

[ogden]

The bottleneck of the dynamic range is actually the common mode inductance of the two edge mount SMA connectors, and this can't be improved much other than separating the two ports as far as possible.

I am not sure about this one, but whatever... as you they say. At least put connectors on the long edge of the device.

All low cost USB VNAs on the market currently suffers from this problem and is why they are all limited to a dynamic range of around 70dB < 1GHz and 50dB at 3GHz.

Not all. Here's one with 90dB dynamic range with close enough connectors, thou those do not seem like edge mount:  https://www.sdr-kits.net/introducing-DG8SAQ-VNWA3 95dB: https://www.megiq.com/products/vna-0440

If the ADC is sampling at 1Msps and the VBW is 1kHz, a 60dB ADC dynamic range leads to a 90dB measurement dynamic range, which is more than sufficient.

Oversampling is solution, agreed. Thou at 1Msps you run into ADC aperture jitter problems that adds phase noise to measurements unless you run all stm32 clocks from XO, let's say at 24MHz w/o any PLL.

[edit] I do not buy "common mode inductance of SMA connectors". Most likely they blame connectors while actual problem is internal leakage/reflections. Look how close are connectors for Keysight P5008A which have 140dB dynamic range and better than 140dB crosstalk figures up-to 26GHz.

Where do we sign-up for beta testing?

[OwO]

All low cost USB VNAs on the market currently suffers from this problem and is why they are all limited to a dynamic range of around 70dB < 1GHz and 50dB at 3GHz.

Not all. Here's one with 90dB dynamic range with close enough connectors, thou those do not seem like edge mount:  https://www.sdr-kits.net/introducing-DG8SAQ-VNWA3

Covering 1 kHz to 1.3 GHz and powered from a PC USB-bus, the VNWA 3 offers a dynamic range of 90dB up to 500 MHz and better than 50dB above 500 MHz.

That sounds about right. It's either common mode inductance or radiated leakage from the exposed center pin depending on your point of view (they are two sides of the same phenomenon which is that E fields wander to the outer face of the coaxial shield). It's pretty well known to anyone designing network analyzers. IIRC the xaVNA full two port version had some solder between the SMA connector body and a shield can to reduce this leakage path, but even then it only got to something like 60-70dB dynamic range at 3GHz. Below 1GHz it's easy to hit 90-100dB dynamic range. The solution is a specific kind of surface mount SMA connector where the center conductor is fully enclosed.

[OwO]

I had a play with a certain full two port VNA prototype once, and was surprised how much of the leakage I can remove just by grabbing the outer body of the SMA connector. The leakage floor can go down by 20dB (to 90dB dynamic range) at >3GHz if you grab both connectors. OTOH if you touch both connectors to a piece of metal (just the outer body - the center pin is unconnected) you can see the leakage shoot up 10-20dB. From that it's pretty obvious the leakage path has to do with currents travelling on the outer face of the coax structure.

[ogden]

All low cost USB VNAs on the market currently suffers from this problem and is why they are all limited to a dynamic range of around 70dB < 1GHz and 50dB at 3GHz.

Not all. Here's one with 90dB dynamic range with close enough connectors, thou those do not seem like edge mount:  https://www.sdr-kits.net/introducing-DG8SAQ-VNWA3

Covering 1 kHz to 1.3 GHz and powered from a PC USB-bus, the VNWA 3 offers a dynamic range of 90dB up to 500 MHz and better than 50dB above 500 MHz.

That sounds about right. It's either common mode inductance or radiated leakage from the exposed center pin depending on your point of view (they are two sides of the same phenomenon which is that E fields wander to the outer face of the coaxial shield).

No. Mainly it's because of harmonics mode above 500Mhz because signal source is DDS. Of course leakage could be contributor as well. Main problem of those "DIY" or "commercial low cost" VNA's that can't reach 90dB isolation - insufficient shielding and failure to follow even basic RF design/layout rules. Screaming example of how not to design VNA is already mentioned MiniVNA Tiny. I really hope design of next gen nanoVNA will not fall into same traps.

[ogden]

I had a play with a certain full two port VNA prototype once, and was surprised how much of the leakage I can remove just by grabbing the outer body of the SMA connector. The leakage floor can go down by 20dB (to 90dB dynamic range) at >3GHz if you grab both connectors. OTOH if you touch both connectors to a piece of metal (just the outer body - the center pin is unconnected) you can see the leakage shoot up 10-20dB. From that it's pretty obvious the leakage path has to do with currents travelling on the outer face of the coax structure.

 That's PCB design (layout/shielding), not connector "common mode" problem. Get access to proper VNA like Keysight or R&S to see that touching connectors do not change ANYTHING. Also you are advised to check  Signal Path youtube episodes of VNA teardowns - those instruments are riddled with connectors and cables, yet still reach significant dynamic range.

[OwO]

No, I do not need to spend $$$ to see what is easily shown with a simple test board. The professional test equipment do not use this style of edge mount SMA connector, and that is exactly why low cost network analyzers have trouble reaching high dynamic range. I have prototypes and test boards that show plain evidence of connector leakage, and I can assure you the NanoVNA team know what they are doing.
EDIT: V2 has far better PCB layout design compared to V1, and I can guarantee you the bottleneck is not going to be internal leakage.

[OwO]

See here:
Notice the mounting of the connector. Details like this do matter and with just one bad leakage path you are not going to get >50dB dynamic range at 3GHz no matter how good your PCB design is. The xaVNA full two port is a testament to this as it has very little internal leakage (proven by removing all connectors) but the dynamic range with connectors and inside its enclosure is not that great. Ironically the dynamic range is improved when you remove the board from the aluminum enclosure (the creators even admit this); do you know why this is?

[ogden]

The professional test equipment do not use this style of edge mount SMA connector, and that is exactly why low cost network analyzers have trouble reaching high dynamic range.

They mostly use flange mount connectors for internal modules, yet performance of those do not differ from edge-mount/edge-launch SMA connectors when latter are properly shielded. Manufacturers obviously just pick right connector and do not invent shielding transition for connector which is not made for such. Whatever. - Why don't use angled trough hole mount connectors then?

I have prototypes and test boards that show plain evidence of connector leakage

Nah. I see plain evidence that horizon is flat, so what? [kidding] - Unlikely you have chamber and equipment for leakage measurements.

EDIT: V2 has far better PCB layout design compared to V1, and I can guarantee you the bottleneck is not going to be internal leakage.

Good to hear that. How many PCB layers? Where do we sign for beta testing?

[ogden]

The xaVNA full two port is a testament to this as it has very little internal leakage (proven by removing all connectors) but the dynamic range with connectors and inside its enclosure is not that great.

Indeed. It is clear that he missed to put (legs of) connectors in the shield. Port1 and port2 have connector center legs, some components and quite a transmission line length in common cavity which is main enclosure. That is "no no". I will say it last time - shielding can of particular channel shall cover everything including legs of the connector. [edit] Unfortunately xaVNA is also example of how not to design VNA.

Ironically the dynamic range is improved when you remove the board from the aluminum enclosure (the creators even admit this); do you know why this is?

Reflections. They forgot to put RF/microwave absorber foam inside aluminium enclosure.

[ogden]

Commercial VNA's do not have any TX or RX component or trace or even connector leg exposed into enclosure or outside world. RF PCB is covered by CNC-machined shield of many submodule cavities from both sides.

https://youtu.be/HxBcQDooAYs?t=1333

Relevant article:
https://www.edn.com/electronics-blogs/the-practicing-instrumentation-engineer/4418080/Shields-are-your-friend--except-when-

[Bicurico]

Hi,

I am still new to VNA measurements and started yesterday to do a calibration of my device.

I have the latest FW installed and I can understand some of the steps in calibration, but not all:

OPEN --> connect open header to CH0
SHORT --> connect storted header to CH0
LOAD --> connect 50 Ohmn terminator header to CH0
ISOLN --> what do I have to do here?
THRU --> my guess is I straight connect CH1 to CH0?
DONE --> finish!

Why do I have to do a RESET first? So that measurements are done without any previous calibration? Makes sense, I guess...

Am I correct that CAL should be performed using the same cables that will be used for measurement? Makes sense, because the length will/can determine phase shifts, right?

If these questions are too basic for this thread, I can ask them in a separate topic - I don't want to hijack this nice thread, but I thought that it does generally fit in here.

Thanks,
Vitor

[hendorog]

Hi,

I am still new to VNA measurements and started yesterday to do a calibration of my device.

I have the latest FW installed and I can understand some of the steps in calibration, but not all:

OPEN --> connect open header to CH0
SHORT --> connect storted header to CH0
LOAD --> connect 50 Ohmn terminator header to CH0
ISOLN --> what do I have to do here?
THRU --> my guess is I straight connect CH1 to CH0?
DONE --> finish!

Why do I have to do a RESET first? So that measurements are done without any previous calibration? Makes sense, I guess...

Am I correct that CAL should be performed using the same cables that will be used for measurement? Makes sense, because the length will/can determine phase shifts, right?

Yes always try and calibrate at the end of the test cable _and adapters_ you will connect to the DUT. Not always possible of course - if you don't have cal kits for Male and Female you will need to use an adapter. Then you are exposed to any phase and amplitude effects of the adapter. It is possible to correct for that and different methods can be used which have different levels of accuracy.

The Reset clears the data arrays on the device. It's generally a good idea as the firmware isn't perfect, and it also allows you to ignore the ISOLN step if you want to. Otherwise it would probably apply the ISOLN correction from the last cal you did.

For ISOLN you are supposed to put a terminator (load) on both ports. That correction is only applied to the thru measurements - S21.

For a 1 port measurement (CH0) you only need to do Short, Open and Load then Done.

If using the original NanoVNA app _on a PC_, then it is important to do it in the sequence Short Open Load for the best accuracy. Because the Short button also does a Reset. So if you do the Open first and the Short second, then the Open sweep is discarded. This results in a 0.3 dB error approximately.
Middle chart in this link shows this issue: https://github.com/hendorog/nanovna_test/blob/master/NanoVNA%20test.ipynb

[radioactive]

Ok,  dumb question (but serious).  I've never owned or used state-of-the-art equipment, but what kind of application requires the ability to measure better than 50dB of return loss?  Best I've used is probably 70dB.  If I can achieve 20 dB S11 over a wide operating frequency,  I'm pretty happy.  I can understand if you are doing some kind of advanced science experiment, that you would need >90dB of measurement range, but aside from that,  what practical use is it?  Examples?