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Good morning guys,

I'm new to the group, and I got my nano last week.
The measurements on antennas are plausible and I was able to follow them with my "miniVNA", the situation is different with filter measurements.
Here for example on a LPF for 70cm. The filter was measured on a Wiltron Network Analyser and had an attenuation of 0.15 dB in the passband. In the blocking range at 600 MHz, the attenuation is -62 dB. With the "nano" I measured an attenuation of almost -11 dB in the passband.
A measurement on a 630m LPF yielded the same result.

Does anyone have similar measurement results for filter measurements, is my "nano" defective, or is the source of the error in front of the "nano"? :o)

73 Joe
DF2JP

Maybe stupid question, did you check if without filter you get an attenuation of 0dB?

Joe,

This looks exactly like the issue I had because I did not follow calibration procedure correctly.

If you do not follow calibration procedure correctly then you always end up with about -10db as a baseline.

Please try this..

Turn on NanoVNA.
Select desired freq range or default of 50Khz - 900MHz
From the menu press CAL
Press RESET
Then press CALIBRATE
Press OPEN with nothing connected Ch0
Menu now moves to where it says SHORT (a light shade of green)
Connect the SHORT on CH0 and press SHORT
Menu moves to LOAD and turns a light shade of green
Connect your 50R load to Ch0 and press LOAD
Menu moves to ISOLN with alight shade of green.
Keep the LOAD connected on CH0 and press ISOLN
Menu moves to THRU in a light shade of green.
Connect a short lead (provide from Ch0 to Ch1
Press THRU
Menu moves to DONE in a light shade of green.
Press DONE
Then press SAVE 0



73 de Andy

Probably you have incorrect calibration for ISOLN and THRU. Did you performed RESET before calibration? If no, try to perform RESET before calibration. Without it, you may get some issues with calibration.

Also try to connect CH0 with CH1 with cables which you're using for connect the filter. It should show 0 dB. If it shows something different, then you have wrong calibration.

Andy G0FTD, there are two mistakes in your calibration procedure:

1) Press OPEN with nothing connected Ch0

this is incorrect. You're needs to place open terminator from calibration kit.

2) Keep the LOAD connected on CH0 and press ISOLN

this is incorrect. You're needs to place 50 Ω terminator on CH1 for ISOLN.

Given the assumptions of the NanoVNA, using the supplied open is *not*
necessarily better. The NanoVNA assumes that the open presents 50fF
capacitance and no frequency dependent capacitance - and you may well be
closer to that with a "true" open than with the added capacitance of the
"open terminator" from the kit.

--
Rune / 5Q5R

Rune Broberg, the problem with "nothing connected to CH0" case is that it has different geometry configuration than usual SMA connector. It leads to different capacitance and inductance than usual SMA connector and also leads to different delay line, which leads to incorrect measurement. But it depends on what you needs. If you're work in 1-30 MHz range, you may keep "nothing connected to CH0" it will not affect result much. But if you want to use frequencies above 30 MHz, you're needs to use proper open terminator.

I'm not doubting that having a 50R load on Ch1 is better, because I have never tried it.
I've never had to do it in the past on equipment in my old professional past either.
But just for fun, I've ordered some SMA 50R terminations to test the difference, I'm willing to be educated ;-)
Does anyone have data on measurements taken with or without ?

However, since MOST users will only have the basic calibration kit supplied, I write with that in mind,
and I get sensible results so far.

As for the debate regarding the OPEN standard, well the original poster is messing about with a 630m LPF.

I doubt 0.5pf across the terminals is gonna matter at 475Khz.

73 de Andy

I agree that it leads to a different measurement. My claim is that it leads
to a more accurate measurement than using the supplied "open", since the
software does not compensate for the characteristics supplied open, but
rather assumes a near-perfect open.

The fully characterized cal kit I recently received also uses an open
connector (with suitable compensation values) as open, not an SMA cap like
supplied with the NanoVNA.

--
Rune / 5Q5R

Andy G0FTD, for proper calibration, you're needs to use all terminators (open, short and load) with the same geometry and length.If you will try to use different geometry/length load it leads to calibration error.

This is why you're needs to buy calibration kit instead of separate terminators from different sources.

I'm not sure if this cal kit supplied with NanoVNA is precise, but I tested 50 Ω load from NanoVNA cal kit and it looks better than all other 50 Ω loads that I bought on aliexpress before. So, it seems that supplied cal-kit is good enough for NanoVNA. But there are a lot of sellers and no guarantee that other NanoVNA will come with the same cal kit.

Using a calibrated nanoVNA (0.5-900MHz) using the supplied black cables and a female-female barrel for connecting the calibration OPEN/SHORT/LOAD to the black cable connected to CH0 I tested if I could see any difference between having an OPEN connected to the female-female barrel connector to the black cable to CH0 or only the female-female connected to the black cable to CH0

I could not see the difference between with of without OPEN on the display of the nanoVNA at 900MHz.

But with or without the female-female from the black cable is very visible at 900MHz
Maybe I need better glasses, or a nanoVNA with a bigger screen.

Andy G0FTD>> Does anyone have data on measurements taken with or without ?

yes, here is measurement. First one is without load on CH1. And the second one with load on CH1. As you can see, with load on CH1 there is less noise.

This is why CH1 needs 50 Ω terminator for ISOLN calibration. The difference is not much, but it clearly visible.

erik@, the black cable supplied with NanoVNA is a crap :) I don't recommend to use it for calibration or precise measurement.

It's better to buy 10-15 cm RG405 with soldered SMA connectors:

On Wed, Sep 18, 2019 at 11:59 AM, <qrp.ddc@...> wrote:

Andy G0FTD>> Does anyone have data on measurements taken with or without ?

yes, here is measurement. First one is without load on CH1. And the second one
with load on CH1. As you can see, with load on CH1 there is less noise

Thanks.

I'd say that there's no discernible difference for S11 measurements (expected) but the lower limit of noise floor
has been extended by about 5db.

Not got my reading glasses on for my laptop though ;-)

So being able to measure a filter's attenuation at -80db rather than -75db @ 200Mhz (your marker) I'm presuming is the practical outcome ?

I won' pass comment on that, because for some users, -75db might be good enough, depends what the operator whats.

But I suppose 5db is 5db and in some cases that could be everything.

I've had another thought on this too.

Why not do a port isolation calibration using the 50R load supplied on Ch0, and the supplied SHORT on the input of Ch1 ?

Any noise drifting around entering the port would surely be eliminated too ?

73 de Andy

Andy G0FTD>> Why not do a port isolation calibration using the 50R load supplied on Ch0, and the supplied SHORT on the input of Ch1 ?

Both ways - to use short or open on CH1 for isolation calibrate are bad. The amplifiers and other circuits have best performance at limited input impedance.

So, it's better to use 50 Ω terminator to reduce noise. Because short or open terminator will leads to higher noise and as result worse calibration.

For ISOLN calibration there is almost no difference if CH0 has load or not. But it has noticeable lower noise with load on CH1.

On Wed, Sep 18, 2019 at 04:24 AM, <qrp.ddc@...> wrote:

For ISOLN calibration there is almost no difference if CH0 has load or not.
But it has noticeable lower noise with load on CH1.

That leads to the question on how to calibrate for best filter rejection measurements as many filters have impedance far from 50ohm outside passband.
Should you compare with isolation measured with 50ohm or isolation measured far from 50ohm????

Well folks, I just found a second load to put on Ch1 for a test.

Using my calibration method, with nothing on Ch1 at all, just open with a 50R load on Ch0 gives me about -80db noise floor at 250Mhz.
NOTE - That's an averaged figure in my head because there was plus/minus 5db variation on the readout anyway.
Plus/minus 5db, which could also have been advantageous at the right point if anyone was trying some comparison and not using averaging.

Then I did the same but placed a SHORT on Ch1 = no difference,

Did another re-calibration with a load on Ch1. Oh dear big surprise, about 6db WORSE.

Thermal noise in the load being introduced ?

Full reset and full calibration each time, repeated several times.

This was a genuine test, I'm not here to try prove me or anyone else right or wrong here. I just wanted some real evidence for a real education.

Well that's the test, make of it what you want folks ;-)

73 de Andy

Did you look at the phase of CH1?
If its coherent (not all over the place but a more or less straight line with frequency) then its not noise that is limiting the CH1 measurement but leakage from the SI5351.

Hi Andy,

you make my day ;o)
The error was before the "nano" :o))
I had probably callibrated correctly, but was of the opinion DONE = Done, saved. I wondered why the Save window would appear again and again.
I confess, I didn't have time to read the manual yet, or was I of the opinion, as an experienced VNA user I wouldn't have to... the next mistake ...
But I promise to read the fine manual tonight (rtfm)

73 and thanks again for your help
Joe

forgot