Author Topic: Pocket VNA  (Read 3244 times)

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Offline yogee58

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Pocket VNA
« on: April 18, 2018, 10:45:47 am »
I got one of these last year for tuning Bluetooth antennas. Getting one was a bit of a struggle due to a long lead time. At the time I presumed it was due to popularity.
The device and the software all looks impressive and appears to be amazing value for £400. However.........

Post-sales support is totally non-existent. Once Martin Nirschl has your cash, that's the end of it.

Since the device uses broadband detectors, it is incredibly thermally sensitive, even though it claims to be thermally compensated. You need to put it in a thermally controlled chamber if you want any chance of re-using an old calibration file. It also takes a very long time to stabilise thermally. I insulate the case for a while when it is first turned on then don't bother to do any measurements until it is near the temperature when I made the cal file. If it's a hotter day than when you took the cal file, you'll have to cal it again.

Although it claims to have a wide bandwidth, when you run a plot or a calibration, there are big holes in the frequency plot where the results are just rubbish. It is also very slow. Too slow for live updates while you wiggle, move or change bits of your circuit.

Of course, you get what you pay for. It does sort-of work for basic measurements but you have to be very careful, particularly when working near the circumference of the Smith Chart and trying to do impedance matching, because of the limited dynamic range and the thermal sensitivity. If you are doing anything serious and you can complete your job in a reasonable time frame, you'd be better off hiring a proper VNA for the same money.
 
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Offline Wirehead

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Re: Pocket VNA
« Reply #1 on: April 19, 2018, 05:45:11 am »
While it's true it's finicky with temperature, I keep my old calibrations and verify it with my standards before taking a measurement. If it's off - I recalibrate. Not much hassle to do so. I also don't get any sudden jumps.

I have reported an issue with the latest Beta version and Martin agreed that there was a bug, namely at the start and end of the calibration it can have a spike. Workaround is to cal over a wider range and then narrow it down in the actual measurement. But - he's fixing it.

In any case, if your job depends on a 400 EUR VNA - you're doing something wrong. Get an Anritsu Sitemaster or something, even a second hand "big iron" VNA for all that matters. It'll be much faster and thermal stabilisation will be better. The pocketVNA is intended for the hobbyist.

Oh - I am working on a heater pad for the pocketVNA with an analog control loop. This should bring the pocketVNA over the ambient temperature; and at a fixed one at that. Should fix that issue :)
"to remain static is to lose ground"
 

Offline rfspezi

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Re: Pocket VNA
« Reply #2 on: April 19, 2018, 07:25:20 am »
Oh - I am working on a heater pad for the pocketVNA with an analog control loop. This should bring the pocketVNA over the ambient temperature; and at a fixed one at that. Should fix that issue :)
I recommend heating it up to over 1000°C and all your problems with it are gone.  ;)
 

Offline Wirehead

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Re: Pocket VNA
« Reply #3 on: April 19, 2018, 10:26:29 am »
Oh - I am working on a heater pad for the pocketVNA with an analog control loop. This should bring the pocketVNA over the ambient temperature; and at a fixed one at that. Should fix that issue :)
I recommend heating it up to over 1000°C and all your problems with it are gone.  ;)
:-DD nah - to be honest, once you know the ins and outs.. You know how to work around them.. :)
"to remain static is to lose ground"
 

Offline ogden

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Re: Pocket VNA
« Reply #4 on: April 19, 2018, 10:39:56 am »
Since the device uses broadband detectors, it is incredibly thermally sensitive, even though it claims to be thermally compensated. You need to put it in a thermally controlled chamber if you want any chance of re-using an old calibration file.

Pocket VNA costs 100 times less than "proper" VNA for a reason. There is no such thing as a free lunch.

Even high-end VNA better be heated-up for at least 30 minutes before use and calibrated for freq range of measurement. Buy paying just 400EUR for VNA you clearly valued your money more than your time, so spend some of your time to calibrate your ultra low cost VNA before each measurement (session) :)
 

Offline ogden

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Re: Pocket VNA
« Reply #5 on: April 19, 2018, 10:55:43 am »
Oh - I am working on a heater pad for the pocketVNA with an analog control loop. This should bring the pocketVNA over the ambient temperature; and at a fixed one at that. Should fix that issue :)

That thing have low thermal mass - even slight breeze can change it's temperature. First thing to try - put device into insulated (with bubble-wrap/whatever) shoe box and measure temperature of device, do some tests. Maybe you can get away w/o heater, just additional insulation+thermal_mass.

 

Offline Wirehead

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Re: Pocket VNA
« Reply #6 on: April 19, 2018, 11:51:38 am »
Oh - I am working on a heater pad for the pocketVNA with an analog control loop. This should bring the pocketVNA over the ambient temperature; and at a fixed one at that. Should fix that issue :)

That thing have low thermal mass - even slight breeze can change it's temperature. First thing to try - put device into insulated (with bubble-wrap/whatever) shoe box and measure temperature of device, do some tests. Maybe you can get away w/o heater, just additional insulation+thermal_mass.

Correct, I have been checking that out as well. However there's some "if's and but's": if I add thermal mass, it will take much longer for the device to heat up and be stable (but it'll be rock solid) - also, how to make good contact, this might add capacitance to some traces. (solution: mill a block with "elevated pads" to make contact to the broadband detectors only). On the other hand, insulation does take care of stabilizing it a little faster. But you still have the dependency to the temperature of the environment.

That's why I was thinking to go the other route with a heater pad. No matter what the outside temperature is, the detectors would be at the exact temperature you want it to be.
"to remain static is to lose ground"
 

Offline ogden

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Re: Pocket VNA
« Reply #7 on: April 19, 2018, 04:52:04 pm »
Oh - I am working on a heater pad for the pocketVNA with an analog control loop. This should bring the pocketVNA over the ambient temperature; and at a fixed one at that. Should fix that issue :)

That thing have low thermal mass - even slight breeze can change it's temperature. First thing to try - put device into insulated (with bubble-wrap/whatever) shoe box and measure temperature of device, do some tests. Maybe you can get away w/o heater, just additional insulation+thermal_mass.

Correct, I have been checking that out as well. However there's some "if's and but's": if I add thermal mass, it will take much longer for the device to heat up and be stable (but it'll be rock solid) - also, how to make good contact, this might add capacitance to some traces. (solution: mill a block with "elevated pads" to make contact to the broadband detectors only). On the other hand, insulation does take care of stabilizing it a little faster. But you still have the dependency to the temperature of the environment.

That's why I was thinking to go the other route with a heater pad. No matter what the outside temperature is, the detectors would be at the exact temperature you want it to be.

My point is - don't overengineer. Not at the very beginning. Try simple experiments first. Maybe it will be fine in shoe box - who knows :D
 

Offline Wirehead

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Re: Pocket VNA
« Reply #8 on: April 19, 2018, 05:56:13 pm »
Well, on my desk it's fine.. I keep it plugged in all the time and temperature doesn't vary much. So at home it's not an issue. It's when I take it with me to test some things. (but then again, if it's outdoor, I make sure it's placed out of the wind and I let it settle some time.. )

I've learnt to take note of it's shortcomings and I make sure my test method works around it.. Bit of a hassle, but eh, at that price..

You're absolutely right about not over-engineering as well  :-+
"to remain static is to lose ground"
 

Offline Jaimon

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Re: Pocket VNA
« Reply #9 on: June 07, 2018, 11:24:15 am »
I got this device one year back, the software was not calibrating the transmission S12 properly.
Then I used the beta version of the software.  it was working  ....

The costomer support need to improve a lot....
No reply if we try to contact Martin
« Last Edit: June 07, 2018, 11:30:45 am by Jaimon »
 
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Offline model

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Re: Pocket VNA
« Reply #10 on: September 08, 2018, 10:21:57 am »
I wish I had read this before ordering. I paid for the fast shipping and no word from them in well over a week.
 


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