US$200 VNA kickstarter launched

[xaxaxa]

EDIT: The kickstarter has now been launched:
https://www.kickstarter.com/projects/1759352588/xavna-a-full-featured-low-cost-two-port-vna

--------------------------------
I've been working on a very low cost VNA project for some time and was thinking about doing a kickstarter; see original thread:
https://www.eevblog.com/forum/rf-microwave/low-cost-vector-network-analyzer/

I've already adapted the design to a 4 layer board and eliminated all the off board wiring. I also ran through DFM checks and have gotten quotes from the PCBA fab (elecrow) + component suppliers. Most of what's left is to prototype and verify the 4 layer design and work out logistics. This is for the 140MHz to 2.5GHz T/R version; all the other versions will require more R&D effort.

A few people have mentioned that HF support and full two port support would be useful, but I'm curious about how important people find these features to be vs the price, so I decided to post this poll.

A T/R VNA can measure a device in one direction at a time: reflection coefficient on port 1 and through coefficient from port 1 to 2; to measure the other direction (port 2 reflection and port 2->1 through) you need to physically reverse the DUT (swap ports 1 and 2). A full two port VNA has directional couplers on both ports and can measure both directions without having to reverse the DUT. The main difficulty with implementing a full two port VNA is that the board will need to be bigger to accommodate two directional couplers, as well as keeping good isolation. PCBs larger than 10cm x 10cm get very expensive.

Note: the upper frequency limit of 2.5GHz can be increased to 4.4GHz with slight additional cost; if this is useful please comment.

My original idea with this project is to commoditize the VNA, since everything currently on the market are expensive and proprietary; since this project is entirely open source and all designs (schematics, pcb layout, simulation files, fpga logic, software) are published, eventually people of aliexpress could clone it and maybe even offer it at a lower price. I was thinking there should be a market for this if it's inexpensive enough, since many people still use antenna analyzers and a VNA can give you much more information.

[Howardlong]

I think you’ve identified the three broad use cases, and three different markets: that of the non-ham Wifi antenna tweaker, the ham antenna tweaker, and the RF circuit builder.

I would think it would be reasonable that your average ham antenna tweaker would also want to go down to 3.5 if not 1.8MHz though.

I have an old Anritsu Sitemaster VNA that’s single port but because it’s battery powered and handheld, it’s very useful for antenna tuning and setup outside in he field, coverage is 25MHz to 3GHz.

On the bench I have an HP 8753A, 300kHz to 3GHz, which, with an accompanying TR test set, will do two port as well as one port measurements. For practical antenna measurements, except for small aperture microwave antennas it’s less than ideal because it’s hardly portable!

In my experience, don’t underestimate the size of the ham market. If you have a unique product at a reasonable price, they will come, but without HF capability it will rather limit that market. Word of mouth is strong. The non-ham market is undoubtedly much smaller. You need to make sure you extol the key differentiators between your product and any competitors, as there are already products like miniVNA and PocketVNA out there which will fragment your market.

[trevwhite]

If this would allow me to tweak antenna in the range of 433Mhz, 868Mhz, 915Mhz and 2.4Ghz then I would very much be interested.

[VK5RC]

+1 re the Amateur radio market and including HF, I would add that the software for these devices is often the deal maker (or not).
There are several others out there - Array Solutions AIM UHF , N2PKs efforts  and miniVNA to name a few.

[xaxaxa]

Thanks for all the suggestions.

The kickstarter is now live:
https://www.kickstarter.com/projects/1759352588/xavna-a-full-featured-low-cost-two-port-vna

Taking into account the poll results, I introduced a full two port version; HF support will be introduced if the stretch goal is met.

[TK]

Are you planning on adding the capability of setting your own calibration kit parameters that can be saved/recalled?

[xaxaxa]

Are you planning on adding the capability of setting your own calibration kit parameters that can be saved/recalled?

Yes; the software is under heavy development and you can expect all the standard features of a professional VNA to be implemented.

[usagi]

keep in mind you're competing with existing products like these:

http://miniradiosolutions.com/54-2/
https://sdr-kits.net/index.php?route=web/pages&page_id=68_68

[xaxaxa]

keep in mind you're competing with existing products like these:

http://miniradiosolutions.com/54-2/
https://sdr-kits.net/index.php?route=web/pages&page_id=68_68

I've seen those ones; the existing low cost VNAs are much more expensive and have accuracy and temperature drift problems;
see last page of: http://www.gsm-modem.de/M2M/wp-content/uploads/2014/04/IoT-M2M-Cookbook-Harald_excerpt.pdf
https://www.eevblog.com/forum/rf-microwave/pocketvna-any-idea-what-its-like/msg1171730/#msg1171730

[usagi]

i guess the only thing now is to decide who your primary target market is.

ham market will be considerably different than typical RF builder. ham guys dont care much for two port. and no <10mhz from the beginning (no stretch) will limit ham interest. also, most hams rarely care about >500mhz.

[pigrew]

One advantage of the full two port VNA is that the full twelve-term error model can be used. With only T/R, my understanding is that the cables cannot be fully de-embedded.

I've mostly used VNAs for characterizing amplifiers.

[xaxaxa]

i guess the only thing now is to decide who your primary target market is.

ham market will be considerably different than typical RF builder. ham guys dont care much for two port. and no <10mhz from the beginning (no stretch) will limit ham interest. also, most hams rarely care about >500mhz.

The intended market is RF engineers using the ISM bands, i.e. 433MHz, 863MHz, 915MHz, 2450MHz, and as a plus it can also cover UHF ham bands plus 144MHz;

Because of the way the LO synthesizer is fed into the mixers it requires a lot more extra circuitry to insert a switch and prescaler in there. I decided that adding full two-port support is enough feature creep for now and to limit the scope to reduce risks.

I could replace the synthesizer with the adf4351 or max2871 and cover down to about 30MHz, but not sure how useful that is.

[rfeecs]

One advantage of the full two port VNA is that the full twelve-term error model can be used. With only T/R, my understanding is that the cables cannot be fully de-embedded.

I've mostly used VNAs for characterizing amplifiers.

You can use the full twelve-term error model with T/R.  Requires two measurements, one in each direction.
Some calibration methods like TRL require the full two port to have 4 detectors to be fully accurate:

"Applying Error Correction to Vector Network Analyzer Measurements"
http://literature.cdn.keysight.com/litweb/pdf/5965-7709E.pdf

[Bud]

i guess the only thing now is to decide who your primary target market is.

The intended market is RF engineers...

RF engineeres are too busy to toy with arduino-style kits. To them you have to offer a finished solution.
Also, "calibration kit requires soldering" does not add credibility to the product.

[xaxaxa]

RF engineeres are too busy to toy with arduino-style kits. To them you have to offer a finished solution.
Also, "calibration kit requires soldering" does not add credibility to the product.

It isn't a kit as the board is ready to use;

The supplied calibration kit is something like this:

(plus the thru reference)

The "requires soldering" just means adding a blob of solder for the s.c. reference and soldering two resistors (provided) to the load reference.
I will update the kickstarter to clarify this.

The provided sma connectors will be genuine ones from digikey. It's mainly just to save you from having to order it yourself and paying for shipping again.

[TK]

Mouser has some stock male SMA termination, open and short plugs.  They are male, not female like your homemade kit, but you can add a female-female SMA connector and measure the new parameters and use them as calibration kit. 

Short: http://www.mouser.com/ProductDetail/Amphenol/132331/?qs=%2fha2pyFadui%252bsOUeTj9xiXC2xh5fwe1gjFO04t3RQW9wolrRJhIfTw%3d%3d
Open: http://www.mouser.com/ProductDetail/Amphenol-RF/202112/?qs=sGAEpiMZZMvlX3nhDDO4ANxAIWTV9VDXYOGp73w%2fDZI%3d
Load: http://www.mouser.com/ProductDetail/Amphenol-RF/132360/?qs=sGAEpiMZZMvkloRJ4SuqXuLVgwQXDW1k58OMTbQUIq8%3d
Adapter: http://www.mouser.com/ProductDetail/Amphenol-RF/132169/?qs=sGAEpiMZZMs022Iw%2foIyC4%252bc6P8fzLVU

Open seems to short when closed too tight, or maybe it is just the piece I received.  I just avoid screwing it too much.

If you know how to browse mouser, you might be able to find the equivalent female standards.

[xaxaxa]

Mouser has some stock male SMA termination, open and short plugs.  They are male, not female like your homemade kit, but you can add a female-female SMA connector and measure the new parameters and use them as calibration kit. 

Short: http://www.mouser.com/ProductDetail/Amphenol/132331/?qs=%2fha2pyFadui%252bsOUeTj9xiXC2xh5fwe1gjFO04t3RQW9wolrRJhIfTw%3d%3d
Open: http://www.mouser.com/ProductDetail/Amphenol-RF/202112/?qs=sGAEpiMZZMvlX3nhDDO4ANxAIWTV9VDXYOGp73w%2fDZI%3d
Load: http://www.mouser.com/ProductDetail/Amphenol-RF/132360/?qs=sGAEpiMZZMvkloRJ4SuqXuLVgwQXDW1k58OMTbQUIq8%3d
Adapter: http://www.mouser.com/ProductDetail/Amphenol-RF/132169/?qs=sGAEpiMZZMs022Iw%2foIyC4%252bc6P8fzLVU

Open seems to short when closed too tight, or maybe it is just the piece I received.  I just avoid screwing it too much.

If you know how to browse mouser, you might be able to find the equivalent female standards.

I checked all the suppliers and haven't found any sma female terminations that have specified return loss/swr and is also reasonably priced;
using an adapter is possible but the S parameters could change over time because of the extra connector interface.
The homemade termination can achieve return loss >30dB up to 3GHz (according to graphs on hforsten's site).

Do you think it's worth it to switch to pre-made terminations? Most people who would buy a VNA sure have a soldering iron and don't mind soldering two resistors right? 

[Jay_Diddy_B]

Hi,

For the calibration standards you have to understand the concept of a reference plane.

The load at the reference impedance is easy, there should be no reflections, if everything is properly matched so the distance from the reference plane to the load doesn't matter.

The short will have a little inductance and this can be measured and it needs to be known. The reference plane for the short is fairly easy to establish. All the incident power should be reflected by the short.

The open is actually the most difficult standard. Because of fringing capacitance and the open is a short distance beyond the back of the open connector. The open will also reflect all the incident power.

The difference between the open and the short is phase. There should be a 180 degrees difference between the open and the short.

Distance / velocity = time

and time delay results in phase shift



One of the measures of how good the calibration standards is how much the phase, between the open and the short, differs from 180 degrees at the maximum frequency.

Jay_Diddy_B

[Jay_Diddy_B]

Hi,

I would check this link:

https://www.sdr-kits.net/index.php?route=product/product&product_id=50

These connectors have been characterized.

A m/m or f/f connector is used for the open. The open listed earlier, by TK, is really a dust cap, to keep dirt out of a connector.

Jay_Diddy_B

[xaxaxa]

Hi,

For the calibration standards you have to understand the concept of a reference plane.

The load at the reference impedance is easy, there should be no reflections, if everything is properly matched so the distance from the reference plane to the load doesn't matter.

The short will have a little inductance and this can be measured and it needs to be known. The reference plane for the short is fairly easy to establish. All the incident power should be reflected by the short.

The open is actually the most difficult standard. Because of fringing capacitance and the open is a short distance beyond the back of the open connector. The open will also reflect all the incident power.

The difference between the open and the short is phase. There should be a 180 degrees difference between the open and the short.

Distance / velocity = time

and time delay results in phase shift



One of the measures of how good the calibration standards is how much the phase, between the open and the short, differs from 180 degrees at the maximum frequency.

Jay_Diddy_B

Yes that is the other issue with using an adapter on the cal references; you'd need to measure the combined S parameters of the termination + adapter (using another vna) and input it into the calibration algorithm.

[TK]

Do you think it's worth it to switch to pre-made terminations? Most people who would buy a VNA sure have a soldering iron and don't mind soldering two resistors right? 

I think the impedance of the connector can change if heated, so even when you ship the same connectors, each user might end up with different set of calibration kit and make the standard kit parameters that you include with your instrument wrong.  They might need to enter their own parameters.

[xaxaxa]

I think the impedance of the connector can change if heated, so even when you ship the same connectors, each user might end up with different set of calibration kit and make the standard kit parameters that you include with your instrument wrong.  They might need to enter their own parameters.

It's all relative; you can make fairly good measurements with no cal kit parameters, just pretending the references are ideal; in fact all the graphs shown on my page are done this way since I don't have access to an expensive VNA or calibration kit.

The dielectric characteristics may vary after heating but that's likely less significant than the fringing field issue which can be corrected identically for all units.

I'm going to offer the option of pre-soldered cal kits (at no extra charge); just leave a remark when pledging. [updated kickstarter]

[exe]

Stupid question, why it cannot go down to, e.g., 100KHz? I'm asking because I'd like to characterize capacitors and filters, but even 10MHz is already way to much.

It looks like there are two types of VNA: designed low frequencies (say, below 0.5GHz) , and for RF work (several GHz).

[xaxaxa]

Stupid question, why it cannot go down to, e.g., 100KHz? I'm asking because I'd like to characterize capacitors and filters, but even 10MHz is already way to much.

It looks like there are two types of VNA: designed low frequencies (say, below 0.5GHz) , and for RF work (several GHz).

There are generally two architectures used in VNAs, the single conversion type where the signal gets downconverted to low frequency IF and get digitized, and the spectrum analyzer style architecture where the signal gets upconverted first (above the maximum input frequency). The former can't receive signals down to anywhere near its IF frequency because of the filtering, and the latter requires much more circuitry and much more care to preserve signal integrity.

Both the hforsten VNA and mine are single conversion because it's much easier to get good accuracy this way. The miniVNA seems to be the upconverting type and can cover down to near DC. It's mainly a tradeoff between accuracy, frequency coverage, and cost.

[exe]

The former can't receive signals down to anywhere near its IF frequency because of the filtering

Is it possible to bypass the filter?