Author Topic: PocketVNA Any idea what its like?  (Read 15708 times)

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Offline Kalvin

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Re: PocketVNA Any idea what its like?
« Reply #50 on: June 26, 2017, 10:07:31 PM »
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).....

This doesn't go down to 300 kHz but nevertheless inexpensive and nice design with detailed explanation:

http://hforsten.com/cheap-homemade-30-mhz-6-ghz-vector-network-analyzer.html

With some improvements:

http://hforsten.com/improved-homemade-vna.html

The design files can be found in the Github.
 

Offline fcb

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Re: PocketVNA Any idea what its like?
« Reply #51 on: June 27, 2017, 05:57:14 AM »
That's a really impressive project - might be very interesting if he does a third version.

300KHz is overkill - but 30MHz still very usable as the bottom end.
 

Offline Wirehead

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Re: PocketVNA Any idea what its like?
« Reply #52 on: August 10, 2017, 06:55:19 AM »
It seems Martin quietly released a beta version of the new pocketVNA software. Much improved calibration screens and methods.. I'll give it a go tomorrow :)
"to remain static is to lose ground"
 

Offline Wirehead

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Re: PocketVNA Any idea what its like?
« Reply #53 on: November 15, 2017, 06:40:47 PM »

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.

I found a good site explaining that technique with the attenuator.. Might be interesting for others:
https://www.evaluationengineering.com/features/0200loss.htm
It also mentions "The technique is not appropriate for S-parameter VNAs because a full 12-term, two-port calibration inherently will produce accurate results." So the issue could be overcome by not using a pad; but doing a 12-term 2port calibration? Maybe that's why it's in the new beta? Hopefully eb4fbz can chime in  :)
"to remain static is to lose ground"
 

Offline xaxaxa

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Re: PocketVNA Any idea what its like?
« Reply #54 on: November 15, 2017, 08:45:09 PM »
I found a good site explaining that technique with the attenuator.. Might be interesting for others:
https://www.evaluationengineering.com/features/0200loss.htm
It also mentions "The technique is not appropriate for S-parameter VNAs because a full 12-term, two-port calibration inherently will produce accurate results." So the issue could be overcome by not using a pad; but doing a 12-term 2port calibration? Maybe that's why it's in the new beta? Hopefully eb4fbz can chime in  :)

12-term calibration can't be done on a T/R vna because the second port does not have a signal source and can't measure reflection.

Both the miniVNA tiny and pocketVNA are of the T/R (transmission/reflection) type.

The full two port version of the xaVNA supports this kind of calibration.
« Last Edit: November 16, 2017, 07:54:56 PM by xaxaxa »
 
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Offline Wirehead

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Re: PocketVNA Any idea what its like?
« Reply #55 on: November 16, 2017, 07:27:10 PM »
Ok, so hardware limitation of not having a full blown VNA... the 10db pad is a good idea then.

Do drop the sales pitch please. No need for that when discussing technical issues.



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Offline xaxaxa

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Re: PocketVNA Any idea what its like?
« Reply #56 on: November 16, 2017, 08:20:55 PM »
Ok, so hardware limitation of not having a full blown VNA... the 10db pad is a good idea then.

Do drop the sales pitch please. No need for that when discussing technical issues.

Not sure why you would think that was a sales pitch; I was trying to be helpful and clear up some confusion about T/R two port vs full two port, since some manufacturers are vague about it.  :-//
 
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