Author Topic: Portable, 2-port 1 MHz to 6 GHz VNA  (Read 1519 times)

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Offline yl3akb

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Portable, 2-port 1 MHz to 6 GHz VNA
« on: September 30, 2018, 07:25:47 pm »
Hi all!

There have been very impressive DIY VNA projects around lately, like hforsten, loxodes and xavna which motivated me a lot to do my own little project. Here is the description and first results of my portable 2-Port VNA. Let me know what you think! ;)
 

Offline Wolfgang

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #1 on: September 30, 2018, 07:39:51 pm »
Interesting project, congratulations ! What I like is the usable range that includes all radio amateur bands from 160m to 13cm.
Have you tried mini circuits for a better transformer ?
 

Offline yl3akb

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #2 on: September 30, 2018, 07:50:39 pm »
Thanks! Actually trying MiniCircuits parts like TCM1-63AX+ is on my TODO list. Issue is that I need to order quantity of 20 with 7$ a peace. Maybe they offer the samples for that part if I ask.

 

Online Mechatrommer

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #3 on: September 30, 2018, 08:55:22 pm »
very good project. can you estimate the device cost? your device has familiar looking font similar to my KC901V http://www.deepace.net/kc901v-6-8ghz-handheld-network-analyzer-rf-multimeter/
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline yl3akb

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #4 on: September 30, 2018, 09:24:28 pm »
Thanks.  I actually kept track of cost - it is about 260 EUR just for all the parts, materials, PCBs, etc., without any other expenses. I must note it is not designed for manufacturing in mind, so actual figure may be very different. I attached the BOM table, if You are interested.

Yes, yellow on black is very good combination. I inspired from our R&S spectrum analyzer screens. :)

 

Online Mechatrommer

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #5 on: September 30, 2018, 09:55:19 pm »
Thanks.  I actually kept track of cost - it is about 260 EUR just for all the parts, materials, PCBs, etc., without any other expenses. I must note it is not designed for manufacturing in mind, so actual figure may be very different. I attached the BOM table, if You are interested.
thanks for the info its great. but some people might not be able to order all of them one by one, order the pcb and assemble by hand. can i ask again, what is your estimation if you can sell this assembled board without casing and with casing? the other projects lack built in monitor/GUI/SW so they are not standalone without a laptop. you have monitor i think this is a big plus. another big plus is your VNA goes up to 6GHz. this is my spec ("really" standalone + 6GHz) when i was looking for a VNA and wait for years for hobby grade/diy/enthusiast to come up with something with no luck, in the end i bought the $2000+ KC901V, too sad. even the cheaper KC901S was not in my radar due to 3GHz limitation. if you can come up with some competitive price, i think you can screw up R&S, Siglent or Deepace.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline Wolfgang

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #6 on: September 30, 2018, 10:45:42 pm »
The mini circuits is specified from 10MHz up. You will need to try yourself what happens below that, maybe it works just fine.
 

Offline yl3akb

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #7 on: September 30, 2018, 11:25:06 pm »
Thanks.  I actually kept track of cost - it is about 260 EUR just for all the parts, materials, PCBs, etc., without any other expenses. I must note it is not designed for manufacturing in mind, so actual figure may be very different. I attached the BOM table, if You are interested.
thanks for the info its great. but some people might not be able to order all of them one by one, order the pcb and assemble by hand. can i ask again, what is your estimation if you can sell this assembled board without casing and with casing? the other projects lack built in monitor/GUI/SW so they are not standalone without a laptop. you have monitor i think this is a big plus. another big plus is your VNA goes up to 6GHz. this is my spec ("really" standalone + 6GHz) when i was looking for a VNA and wait for years for hobby grade/diy/enthusiast to come up with something with no luck, in the end i bought the $2000+ KC901V, too sad. even the cheaper KC901S was not in my radar due to 3GHz limitation. if you can come up with some competitive price, i think you can screw up R&S, Siglent or Deepace.

That was exactly my specification also at the beginning! ;)
I understand that this project is not the most friendly for assembling by hand - there are just too many parts and places where something can go wrong during the process. Unfortunately I am not planning to offer any kits or assembled boards (yet) - it is just my free-time project. But I am happy to provide any information, design files, etc. But really, I believe it should not cost more than 500 EUR for 1+1 assembled boards (rf module+control board) knowing that full parts-list is <300EUR. All the parts are easy to get - actually currently all the parts, expect LCD (tons of them at ebay), battery, enclosure are available at Mouser. I would need to work out assembling part and solve issues which may come out when more than one board is made - currently just 3 iterations/prototypes are made. But its easier to say than do - All I am planning to at best is to slowly improve the project when I have some spare time and energy. I will keep the 'manufacturability' in mind when/if I do next iteration.
 

Offline yl3akb

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #8 on: September 30, 2018, 11:31:42 pm »
The mini circuits is specified from 10MHz up. You will need to try yourself what happens below that, maybe it works just fine.

Yes, it should be tested. If I keep my current front-end, where differential input of mixers are employed, all I need from transformer is to behave as direct connection at low frequencies. When I tested version with no choke, just direct differential connection, I did not see any issues at low frequencies.
 

Offline ogden

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #9 on: October 01, 2018, 04:52:13 am »
Very nice project! I like UI design very much (used to R&S screens as well). Agree that touch interface on such a small display could possibly make things only worse. I am not sure about SI5351 use for LO - because of it's jitter. 70 ps pp jitter will introduce phase noise into 1st 40MHz IF. This can degrade phase resolution and not only. VNA shall have low phase noise LO/clock sources through all it's paths, down to final ADC clock. Just checked that PLL of stm32 you use have 60 ps pp jitter. Not so critical as for 40MHz IF, but anyway worth to check using online ADC aperture jitter calc.

https://www.maximintegrated.com/en/app-notes/index.mvp/id/3359
https://www.maximintegrated.com/en/app-notes/index.mvp/id/4466
https://www.maximintegrated.com/en/design/tools/calculators/general-engineering/jitter.cfm
http://n2pk.com/VNA/JitoPN.html

You could possibly do quick & dirty tests by running LO of 1st IF @ 27MHz - from TCXO. Most likely MCU will not mind throttling and will perform OK'-ish at 27MHz clock (w/o any internal PLL). Also two cents: you could consider to use lower (audible) frequency for 2nd IF and sample it using external 24bit (audio) ADC, thus with comparably small effort getting significant dynamic range improvement.

Your thoughts?
 

Offline xaxaxa

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #10 on: October 01, 2018, 05:17:23 am »
Nice design and clever way to support <24MHz. I might try out some of your ideas (especially the way the coupler-receiver is done) and incorporate it into my next xaVNA two port design and hopefully make it commercially viable. The coupler in my full two port vna has been a pain point and is the reason it isn't mass producible yet.

I would make a hforsten cal kit and try the smith chart measurements again. In my tests I've found the DIY 50ohm standard using two 100ohm resistors performs far better than the off-the-shelf SMA terminations (no kit parameters are needed up to 3GHz). The ripples in the S11 measurements indicates either (1) bad cable or (2) nonlinearity error. It can't simply be explained by bad cal kit, because the dimensions and delays of the cal kit is very small meaning coherent frequency is very large (ripples caused by bad cal kit repeats maybe every 10GHz). For the cable I use 415-0033-018 from cinch; it has fairly good phase stability and isn't too rigid.
 

Offline yl3akb

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #11 on: October 01, 2018, 05:52:16 am »
Very nice project! I like UI design very much (used to R&S screens as well). Agree that touch interface on such a small display could possibly make things only worse. I am not sure about SI5351 use for LO - because of it's jitter. 70 ps pp jitter will introduce phase noise into 1st 40MHz IF. This can degrade phase resolution and not only. VNA shall have low phase noise LO/clock sources through all it's paths, down to final ADC clock. Just checked that PLL of stm32 you use have 60 ps pp jitter. Not so critical as for 40MHz IF, but anyway worth to check using online ADC aperture jitter calc.

https://www.maximintegrated.com/en/app-notes/index.mvp/id/3359
https://www.maximintegrated.com/en/app-notes/index.mvp/id/4466
https://www.maximintegrated.com/en/design/tools/calculators/general-engineering/jitter.cfm
http://n2pk.com/VNA/JitoPN.html

You could possibly do quick & dirty tests by running LO of 1st IF @ 27MHz - from TCXO. Most likely MCU will not mind throttling and will perform OK'-ish at 27MHz clock (w/o any internal PLL). Also two cents: you could consider to use lower (audible) frequency for 2nd IF and sample it using external 24bit (audio) ADC, thus with comparably small effort getting significant dynamic range improvement.

Your thoughts?

Thanks for the comment!
You are right, SI5351 is infamous as being not the most low noise synthesizer. I admit, I have not paid much attention to noise of oscillators, as currently dynamic range is mostly limited by leakage anyway. But probably if leakage terms could be measured with less noise it would help to improve the calibration. Thanks for the links, I will take that into a account. Also maybe trace noise in low amplitude measurements could be improved. But these are the things to improve when my systematic error issues will be perfectly solved.

About 1st LO @ 27 MHz: If I assume my block diagram, the 1st LO tracks the RF signal with difference == IF1. I could try IF1 at 27.1625 MHz for example, which would allow to use 27 MHz OCXO for 2nd LO to obtain IF2 of 0.1625 MHz.

I have tried clocking MCU from RF OCXO before, but I did not see any difference/improvement. Currently MCU (and its ADC) is free running/not sync with rest of the system, REF and RX signals are sampled simultaneously and amplitude&phase is obtained relatively. It should be only important that MCU clock does not drift during acquisition of current data chunk which is less than few 10s of ms anyway. But probably I did not understand Your note about MCU throttling :)

About more ADC resolution - currently I think 12 bits are plenty, again due to mostly leakage/systematic error not noise limited. But lower IF2 and external ADC would be easily possible.
 

Offline yl3akb

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #12 on: October 01, 2018, 06:14:01 am »
Nice design and clever way to support <24MHz. I might try out some of your ideas (especially the way the coupler-receiver is done) and incorporate it into my next xaVNA two port design and hopefully make it commercially viable. The coupler in my full two port vna has been a pain point and is the reason it isn't mass producible yet.

I would make a hforsten cal kit and try the smith chart measurements again. In my tests I've found the DIY 50ohm standard using two 100ohm resistors performs far better than the off-the-shelf SMA terminations (no kit parameters are needed up to 3GHz). The ripples in the S11 measurements indicates either (1) bad cable or (2) nonlinearity error. It can't simply be explained by bad cal kit, because the dimensions and delays of the cal kit is very small meaning coherent frequency is very large (ripples caused by bad cal kit repeats maybe every 10GHz). For the cable I use 415-0033-018 from cinch; it has fairly good phase stability and isn't too rigid.

Thanks! Well, coupler type You use has much better performance, but then again, vector calibration can do miracles if measurements are repeatable. Good luck with getting Your full 2-port device ready :)

Thanks for the suggestions, I will try different things like decreasing amplitude of generator and different calkit or calkit definitions. At first, I believed that ripples are caused by inaccurate calkit def., because frequency spacing of the ripples are in all cases about 150 MHz which translates to lambda/2 ~ 1 m. If I assume that VF of cable is 0.7, then I got length about 70 cm which is almost exactly the length of my cable. That is why I think it is linear, systematic error.
 

Offline ogden

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #13 on: October 01, 2018, 06:16:56 am »
I could try IF1 at 27.1625 MHz for example, which would allow to use 27 MHz OCXO for 2nd LO to obtain IF2 of 0.1625 MHz.

Right. I said *LO* of 1st IF at 27MHz. Anyway you got it right :)

Quote
I have tried clocking MCU from RF OCXO before, but I did not see any difference/improvement.

Most likely you don't see ADC aperture jitter effects at 162KHz and 12bits. I recall that 300ps jitter start to trash stm32 ADC resolution for signals 200KHz or so - not only in theory but also in real world. Just wanted to tell so you don't fall into this pitfall making next revision of your device :)
 
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Offline yl3akb

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #14 on: October 01, 2018, 05:19:26 pm »
Here is a quick comparison  with open 6 dB attenuator  with calibration at the end of ~70cm cable and same calibration directly at VNA port.
No cable:


With cable:


Here is overlaid return loss of both cases:


I hope that better calibration will solve this issue.

 
 

Offline KE5FX

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #15 on: October 01, 2018, 06:12:37 pm »
Very cool.  :-+  I used the MAX2871A in a similar project a while back.  It'll be interesting to hear what kind of performance you can get from the TCM-1-83AX, if you try that in your bridge.
 
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Online Mechatrommer

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #16 on: October 01, 2018, 06:15:27 pm »
Here is a quick comparison  with open 6 dB attenuator...
someone adviced me to characterize attenuator's input with its output terminated with 50 ohm. not sure why your measurement is way off at -12dB for 6dB attenuator.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline yl3akb

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #17 on: October 01, 2018, 06:50:04 pm »
Well, I would expect close to -12 dB  - signal is traveling though attenuation twice, because it has open end. Or did You meant deviation from -12 above ~4 GHz?
 
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Offline yl3akb

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #18 on: October 01, 2018, 07:03:30 pm »
This is simulation of VAT-6+ with open end using MiniCircuits S-params: https://www.minicircuits.com/WebStore/dashboard.html?model=VAT-6%2B

 

Offline clockspectrum

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #19 on: October 04, 2018, 01:19:17 am »
Great project!
 
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Offline G0HZU

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Re: Portable, 2-port 1 MHz to 6 GHz VNA
« Reply #20 on: October 07, 2018, 09:17:58 am »
Yes, this looks very impressive! Keep posting updates!
 


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