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The Amplitude jumping issue


Kurt Poulsen 2020/09/13 18:07

Hi Gabriel and others

I can confirm the Tindie SAA2 does not have the jumping issue. It is only
the SAA-2N and I made a new input to
<https://groups.io/g/NanoVNAV2/message/1305>
https://groups.io/g/NanoVNAV2/message/1305 with a video link
I seem it is linked to the first point jump you fixed with the unofficial
2_2 fiel. I have not updated the SAA-2N yet to see if that is triggering the
problem

Kind regards

Kurt

Brian Flynn GM8BJF 2021/07/27 05:42

Hi Kurt,
I am having an similar issue with my NanoVNA SAA V2_2. Connecting a 1 dB step attenuator between the ports the attenuation values are displayed correctly as I increase the attenuation to around 15 dB where an error around 1 to 2 dB arises. The diplayed attenuation actually reduces on stepping to 16dB and is then a constnat 2 dB low all the way down to 70dB. Was the cause of this behaviour ever discovered and is there a way to remedy the problem. I am running version 20200617 of the software. This is the version that supports the 4inch screen with the ST7796S driver chip. I have changed to the larger screen size.

Regards
Brian Flynn.

Jim Lux 2021/07/27 17:56

On 7/27/21 5:42 AM, Brian Flynn GM8BJF via groups.io wrote:
> Hi Kurt,
> I am having an similar issue with my NanoVNA SAA V2_2. Connecting a 1
> dB step attenuator between the ports the attenuation values are
> displayed correctly as I increase the attenuation to around 15 dB
> where an error around 1 to 2 dB arises. The diplayed attenuation
> actually reduces on stepping to 16dB and is then a constnat 2 dB low
> all the way down to 70dB. Was the cause of this behaviour ever
> discovered and is there a way to remedy the problem. I am running
> version 20200617 of the software. This is the version that supports
> the 4inch screen with the ST7796S driver chip. I have changed to the
> larger screen size.


have you tried a fixed attenuator of, say, 30 dB? (to isolate whether
it's the step attenuator or not).

The other test would be to look at S11 of the attenuator with the other
port shorted or open.  You should see twice what the attenuator is set to.

A fixed 2dB error is kind of interesting - that sort of implies a
leakage path, but one that is proportional to the attenuator setting - a
fixed leakage between Port 0 and Port 1 of the VNA (aside from being
calibrated out on the thru and other cals) would get worse as the
attenuation in the DUT increases.

Or, of course, an arithmetic error - tough to see how that might arise,
though.

DiSlord 2021/07/27 20:41

If you have 1-2dB error, check version screen
In it see bbgain3, it should be near -20 dB

If bbgain3 near -18, you need:
Update firmware (i not remember last official firmware solve this problem or not, you can use my last on https://groups.io/g/nanovna-users/message/23608 )
Go to STIMULUS->CFG SWEEP->TX POWER and select minimal power for ADF, save this setting in 0 slot.
Power off device, and after power on (also check version and bbgain) recalibrate and save to 1 slot.

But better update fimware first.

Brian Flynn GM8BJF 2021/07/28 13:48

Hi DiSlord and Jim,

Thanks for coming back on my query.
I have already tried another attenuator as I also thought at first the attenuator was at fault. I have now tried both an HP step attenator and a Telonic-Berkley one and both show identical behaviour.

I have checked the BBGain3 setting and it shows up as -19.19 dB so I will try the reflash of the firmware. My gut feeling is that it is a firmware issue as I noticed that there was a 1dB step in the S21 trace at 140 MHz which I beleive is the frequency at which the signal source switches from the SI 53xx clock generator chip to the ADF4350. I will do that tomorrow n let you know how I get on.
Many thanks
Brian, GM8BJF.

Brian Flynn GM8BJF 2021/07/29 03:26

Hi DiSlord,

I have now flashed in your version v1.0.69 git-20210723 and that has cured the issue. The s21 trace now follows the attenuator setting correctly. Many thanks for your help and making the software available.

Brian Flynn GM8BJF.

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