Author Topic: PocketVNA Any idea what its like?  (Read 31557 times)

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Offline Wirehead

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Re: PocketVNA Any idea what its like?
« Reply #25 on: March 03, 2017, 01:32:30 pm »
Yep, works great. Initially made a "general" calibration file to do a quick check over a broad range; and then a couple precision, application specifics.

Here's an ADS-B antenna before and after tuning (1090MHz). 1/4wave GP antenne. It was a bit too long initially. Now well tuned.
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Offline julianhigginson

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Re: PocketVNA Any idea what its like?
« Reply #26 on: March 08, 2017, 06:31:38 am »
I had some interesting introduction to my pocketVNA, but quite happy with how it's going now.
Here's the calibration phase of 2-3G, and then a measurement of a detachable wifi antenna (plus 1.5cm or so of RP-SMA adapter.. maybe I should have compensated that??)


 
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Offline rfspezi

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Re: PocketVNA Any idea what its like?
« Reply #27 on: March 16, 2017, 10:04:44 am »
I have analyzed the miniVNA in this thread here: https://www.mikrocontroller.net/topic/351273
The implementation of the pocketVNA is almost identical.
In the block-diagram you can see how the the resistive broadband bridge works.

« Last Edit: March 16, 2017, 10:26:57 am by rfspezi »
 

Offline cncjerry

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Offline michael2

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Re: PocketVNA Any idea what its like?
« Reply #29 on: March 23, 2017, 10:36:06 pm »
I'm owning a 2.0 pocketVNA and I'm disappointed.

On its web site is claimed:
Quote
fully functional 2-port 4 GHz VNA
 

This is overdrawn to the limits in my opinion. The problems are significant temperature drift and insufficient calibration options, especially the S21 calibration is useless in my opinion.

Up to 100 MHz this VNA works good. Up to 1 GHz you have to control the ambient temperature to say 2°C to get repeatable readings.

For the range up to 2 Ghz you have to be very nitpicking. Additionally to control the ambient temperature very close you have to set up your desired measuring parameters (which should be equal to the calibration procedure) and make continuous measurements for about 20 minutes. Then you can use the accumulating function to get some nice readings.

But the range starting at about 2 GHz and exceptionally above 3 GHz is useless in my opinion. Even about 10 minutes after a fresh calibration the readings are drifting like an off-road car in the dessert... Repeatability is not achievable.

Another thing is the software. There are a some issues. But software can be fixed and I think the software gets fixed in the future. But it is annoying to live with the issues in the meantime. Maybe I'm a little bit huffy regarding the software...

My conclusion? Compared to other products in this area the pocketVNA is currently no better. Other products claim a range to 1 GHz for example and support the claimed range very well. I think with more investments into the software pocketVNA can outperform in the range up to 1GHz in the future. But the range 2 GHZ and above will never work reliably, I think. At least with the actual hardware version.

 

Offline Wirehead

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Re: PocketVNA Any idea what its like?
« Reply #30 on: March 24, 2017, 08:04:31 am »
Did you make sure to let the device warm up before calibrating? As with any VNA, you need to let it warm up before calibrating. Otherwise it will drift...
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Offline michael2

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Re: PocketVNA Any idea what its like?
« Reply #31 on: March 24, 2017, 09:04:00 pm »
Did you make sure to let the device warm up before calibrating? As with any VNA, you need to let it warm up before calibrating. Otherwise it will drift...

Of course, without warm up for an hour even the 100 MHz range is bad. Stabilization starts about 30 minutes after power up.



 

Offline michael2

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Re: PocketVNA Any idea what its like?
« Reply #32 on: March 27, 2017, 07:42:02 pm »
1 GHz to 4 GHZ, some scans the last 24 hours. The problems are clearly visible, I think.

I wonder, if someone else have this issues too?
 

Online hendorog

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Re: PocketVNA Any idea what its like?
« Reply #33 on: March 27, 2017, 08:38:27 pm »
1 GHz to 4 GHZ, some scans the last 24 hours. The problems are clearly visible, I think.

I wonder, if someone else have this issues too?

Perhaps there is no Reference path in these instruments?  i.e. Where the Source is measured directly separately to the DUT and then the ratio calculated and put through the calibration.
That should minimise the drift.

I thought that the green switch in the block diagram posted was for that purpose but it seems not - i.e. alternating between measuring the DUT and the reflection. There is a corresponding switch on the PocketVNA PCB image posted earlier in the thread too.
 

Offline YO5EB

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Re: PocketVNA Any idea what its like?
« Reply #34 on: May 12, 2017, 08:20:28 am »
I have this device. I try to write here yesterday but my message did not reach the Forum. If I can post now, I come back with technical details.
 

Offline eb4fbz

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Re: PocketVNA Any idea what its like?
« Reply #35 on: May 29, 2017, 10:14:45 am »
It has a serious design flaw: the Port 2 return loss is horrible, so the DUT is mismatched and huge ripple can be observed when measuring a coaxial cable loss (S21). How are you supposed to be able to tune a cavity filter with one port so badly terminated?

You can find there are two rf paths in parallel, so 25ohm at low frequency and whatever above 1GHz where transmission lines matters. Those RFSW6024 SPDT switches are non-reflective, so ports allways present 50ohm. Look at C128, C118, U2 and U36.

Mismatch could be alleviated placing an attenuator at port 2, but it will reduce the measurement dynamic range.

« Last Edit: May 29, 2017, 10:16:16 am by eb4fbz »
 
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Offline michael2

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Re: PocketVNA Any idea what its like?
« Reply #36 on: May 29, 2017, 03:11:23 pm »
It has a serious design flaw: the Port 2 return loss is horrible, so the DUT is mismatched and huge ripple can be observed when measuring a coaxial cable loss (S21). How are you supposed to be able to tune a cavity filter with one port so badly terminated?

You can find there are two rf paths in parallel, so 25ohm at low frequency and whatever above 1GHz where transmission lines matters.

I can confirm this. I contacted the Pocket-VNA owner early this year and provided him an measurement which I now attached to this posting. The answer was disappointing and the problem not understood. I refrained from further discussion because I lost the confidence in Pocket VNA.

 

Online hendorog

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Re: PocketVNA Any idea what its like?
« Reply #37 on: May 29, 2017, 06:22:07 pm »
You can find there are two rf paths in parallel, so 25ohm at low frequency and whatever above 1GHz where transmission lines matters. Those RFSW6024 SPDT switches are non-reflective, so ports allways present 50ohm. Look at C128, C118, U2 and U36.


Yeah I noticed that in the earlier image too. You can see in the diagram of the miniVNA posted by rfspezi how it should be connected.
https://www.eevblog.com/forum/rf-microwave/pocketvna-any-idea-what-its-like/?action=dlattach;attach=299771;image

Maybe worth trying to mod it, it should work better as long as that U36 switch is actually being flipped.

Edited to fix the link to the image
« Last Edit: May 30, 2017, 07:20:06 am by hendorog »
 

Offline Wirehead

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Re: PocketVNA Any idea what its like?
« Reply #38 on: May 31, 2017, 05:11:17 am »
Curious as to what hardware version you guys have - I have a "version 2" (February 2017)

edit: here's what I get when verifying a 3 months old calibration. No issues like the above where values wildly drift.
« Last Edit: May 31, 2017, 05:19:21 am by Wirehead »
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Offline michael2

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Re: PocketVNA Any idea what its like?
« Reply #39 on: May 31, 2017, 03:03:29 pm »
Curious as to what hardware version you guys have - I have a "version 2" (February 2017)

edit: here's what I get when verifying a 3 months old calibration. No issues like the above where values wildly drift.

This looks like a totally different product!

What was the frequency span of your measurement?
 

Offline Wirehead

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Re: PocketVNA Any idea what its like?
« Reply #40 on: May 31, 2017, 05:13:57 pm »
Ok, here's some more.. First of all, some HF bands (80meter, 40meter, 20meter). Then VHF (2meter) and UHF (70cm) and finally ADS-B (+- 1090MHz) and 2.4GHz Wifi.

When calibrating, I always do 10 average, 10 adc average, no smoothing (smoothing is cheating  ;) )

When measuring: 10ADC average.
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Offline michael2

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Re: PocketVNA Any idea what its like?
« Reply #41 on: June 01, 2017, 08:08:37 pm »
This is the type label of my PocketVNA. Please could you provide your label for comparison?

 

Offline Wirehead

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Re: PocketVNA Any idea what its like?
« Reply #42 on: June 01, 2017, 08:16:16 pm »
Yep, same version. Do remember, that whenever you want to do a representative measurement on a vna, you have to calibrate it, with test leads and such. Every time. No exceptions. Also variation in lab temp has a great impact. Such is life with any vna. My calibration checks above are done in semi stable conditions, but above 2 GHz, things get critical.
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Offline eb4fbz

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Re: PocketVNA Any idea what its like?
« Reply #43 on: June 01, 2017, 09:07:46 pm »
Curious as to what hardware version you guys have - I have a "version 2" (February 2017)

edit: here's what I get when verifying a 3 months old calibration. No issues like the above where values wildly drift.

That's not the issue: you have just checked the calibration accuracy by placing the OSL kit at the Port 1 after calibration, that should be fine even with a 25ohm source impedance if the maths are done properly, that's what the calibration is for (+/- temperature drift). The problem is that having a ~25ohm port 1 impedance will affect the DUT behaviour because of the mismatch, and this can't be corrected by calibration (these things are what makes the professional VNAs expensive). Even measuring a coax cable loss will be error prone because of the S21 ripple generated by the reflections.

Unfortunatelly, you can't check the port 1 impedance without another VNA, because port 2 doesn't have reflection measurement capability (it's not a bidirectional VNA, so only S11 and S21 measurements). But it's clear by looking at the PCB that the design arround port 1 (CN2 connector!) is far from ideal.

You can check the effect of port 1 matching by measuring a couple of meters long coaxial cable. First calibrate the VNA and measure the coax S21 as usual, save the trace. Then place a 10dB attenuator at port 1, calibrate the VNA again and remeasure the same coax. You will see that measurements doesn't match, and the first one will show a huge ripple. You can use the 10dB pad technique to improve this behaviour, but you will reduce the VNA dynamic range by 10dB.

Now imagine trying to adjust or optimize a high Q circuit, like a band pass cavity or duplexer: it will not be possible because it's behaviour will be very different when connected to the VNA. Resonances will change, loss, rejection... You could be able to get the desired response, but it will not work the same way when attached to a 50&50ohm system.

If your VNA behaves differently, please take a photo of the PCB to look for the differences.
« Last Edit: June 01, 2017, 09:23:50 pm by eb4fbz »
 

Offline Wirehead

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Re: PocketVNA Any idea what its like?
« Reply #44 on: June 01, 2017, 09:23:08 pm »
Curious as to what hardware version you guys have - I have a "version 2" (February 2017)

edit: here's what I get when verifying a 3 months old calibration. No issues like the above where values wildly drift.

If your VNA behaves differently, please take a photo of the PCB to look for the differences.

I'll try to take a picture tomorrow. Bit hesitant to open it up, but hey, for the good cause :)
"to remain static is to lose ground"
 
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Offline Wirehead

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Re: PocketVNA Any idea what its like?
« Reply #45 on: June 01, 2017, 09:36:28 pm »
By the way - here's the effect of the VNA warming up.

1GHz - 4GHz - 1001 step sweep - 10adc avg - 10 avg cal.

Initial calibration was made after warm-up of 4 hours. Then I've let the device settle unpowered, and then let it warm up again. Last measurement is after 4 hours again in exact conditions (yes, nitpicking. Same usb port, cable orientation, connector cleaning, ...) of the load calibration standard. You can see it slowly creeps to the correct value once the device is warmed up.

No through measurement yet, and I'll need to get that 10dB pad eb4fbz is suggesting  :-+
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Offline pete463251

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Re: PocketVNA Any idea what its like?
« Reply #46 on: June 20, 2017, 01:49:33 am »
Hi Everyone,

Great thread, first time poster here.  I wrote antennatheory.com and also launched a kickstarter this month of a 400MHz-2.7GHz 2 port VNA, I'd be thrilled if you checked it out:
https://www.kickstarter.com/projects/1913839854/chazwazza-a-low-cost-2-port-400mhz-27ghz-vna

People send me the miniVNA and pocketVNA a few times and ask why the one I developed, called the Chazwazza is better.  I really liked the comparison referenced earlier (http://www.gsm-modem.de/M2M/wp-content/uploads/2014/04/IoT-M2M-Cookbook-Harald_excerpt.pdf) on page 74, which shows the comparison of the miniVNA to a high end VNA and it doesn't stack up too well.

The reason I developed mine was to have a low cost (and accurate!) stand alone device - which means no software to install.  I also found I could do integrated automatic calibration, which was a side benefit. 

My kickstarter will probably fail, so I'm trying to decide if there is enough demand for VNAs out there to do another spin, cost down and wider frequency range but with continued solid accuracy.  Feel free to ping me on any of this stuff!

Thanks
Pete

p.s. 40 dB of range is more than enough for tuning antennas.  It is a rookie move to care if your s11 is less than -20 dB, the effects of the cable and installation in any real environment will alter the antenna.  So if your target is s11<-40 dB, your wasting your time!   source: me    :-DD
 
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Offline eb4fbz

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Re: PocketVNA Any idea what its like?
« Reply #47 on: June 26, 2017, 10:42:20 am »
p.s. 40 dB of range is more than enough for tuning antennas.  It is a rookie move to care if your s11 is less than -20 dB, the effects of the cable and installation in any real environment will alter the antenna.  So if your target is s11<-40 dB, your wasting your time!   source: me    :-DD

You are right, but i would expect much more from 2-Port VNA than just a VSWR antenna analyzer. For filters and duplexers adjustment you absolutely need >80dB S21 dynamic range, and none of these "amateur" VNAs is able to do so at 1GHz. Besides that, accurate and repeatable calibration is a must, and test ports impedance should be perfectly matched to 50ohm or performance will be severely degraded. I know this is not cheap to achieve at >500MHz, but an inaccurate VNA is a no sense.
 

Offline fcb

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Re: PocketVNA Any idea what its like?
« Reply #48 on: June 26, 2017, 10:59:01 am »
Everyone seems to be building 2 port VNA's.  My personal view is that this is just because there are now some really nice RF IC's available at low cost, and engineers are thinking "great IC, lets build a 2 port VNA).

Even Pico Technology have just built one.  They certainly have their uses, but most seem to be destined to become posh VSWR meters.

What would be really useful is a USB connected (powered via mains!) 4 port VNA that goes from 300KHz to 6GHz (to replace my aging 8753C).....
https://electron.plus Power Analysers, VI Signature Testers, Voltage References.
 

Offline fcb

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Re: PocketVNA Any idea what its like?
« Reply #49 on: June 26, 2017, 11:05:58 am »
And just to add insult to injury. I really don't see why the chazwazza has a crappy display on it, why not just USB it into a PC - better chance of accessing the pretty good performance so far, or at least a USB port.
https://electron.plus Power Analysers, VI Signature Testers, Voltage References.
 


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