Agilent MSO-X 3014A DC offset

[Phoenix6478]

Hello Everyone 

I just got a used Agilent MSO-X 3014A scope and I would like to ask a question about the DC offset. All 4 channels show a DC offset of roughly -2mV (1:1 ratio) with the inputs shorted. It has been around 8 years since the scope was user calibrated.

If I buy the needed coax cables and BNC adapters for the user calibration cable, will the offset go away after the calibration? I am familiar with the self calibration of Rigol Oscilloscopes and there the DC offset gets calibrated away. Will this also be the case for the Agilent scope? I am also planning to perform the 500Mhz frontend hardware mod, so i would need the calibration coax setup anyways.

Thank you in advance!

Best
Phoenix

[HighVoltage]

That seems like a lot of offset, especially on all 4 channels.
What are your "acquire" settings?
How big is the noise?

My oldest 3000x scope is about the same age as yours and it has not shifted the DC offset at all.

[Phoenix6478]

Thanks for your reply!

The differrent Acquire modes do not influence the offset. The noise is around 1.5mVpp on all the channels.

I did the Hardware Self-Test of the scope and it says that all self tests are passed. It also seems to behave as expected under all other situations. Also the DC accuracy is good compared to my Fluke 289 if i manually center the offset to 0v before the measurement.

[TheSteve]

Odds are very good a Usercal will fix it. Even without seeing any offset I'd perform a Usercal before performing the 500 MHz upgrade just to verify everything is working as expected.

[Phoenix6478]

Thank you for the advice, I already ordered the RG58 coax cables and the BNC adapters to build the calibration setup. I will let you know how it goes once i performed the usercal.

[Phoenix6478]

The Usercal fixed it, all the traces are bang on in the middle now 

But i noticed that the 4× 25cm RG58 BNC coax cables that i ordered vary around 1cm in length  I could not notice any phase difference between the channels (I checked it with the function gen before the cal), but could this still influence the calibration? Should I make my own RG58 cables to be more precise or does 1cm difference not really matter?

[HighVoltage]

Nice, that your calibration worked for the offset.
What cables did you buy?

The signal travels about 20 cm per nano second through the 50 Ohm coaxial cable.
Or 0.05 ns / cm
Or 50 ps/ cm

So, your error is about 50 ps
So, on your 100 MHz scope it should not make any difference.

[Phoenix6478]

Thank you for the calculation!

I also calculated it by taking t=s/v and assuming v=2/3c. I got the same result of around 50ps. I was actually able to see the 50ps of skew on the scope by measuring a fast rising edge on 2 channels with 1mv/div and averaging turned on.

The scope is 200Mhz right now with the patched firmware and I am planning to do the 500Mhz hardware mod also. The 50ps skew itself would be no problem, but i am not sure if the different runtimes might be a problem for other sections of the calibration. The Usercal seems quite sophisticated, the scope shows some very weird signals during the calibration. If i recall correctly the usercal also influences the bandwith as some people reported that the bandwith decreased after the usercal with the 500Mhz mod. So maybe a skew between the cal signals might also influence something like that.

I bought Testec 81001 coax cables with 25cm. I decided to cut the BNC connector off on one end in order to trim the cables to exactly the same length. Then I will simply crimp on some new BNC connectors.

I also checked the DC accuracy of the scope after the usercal  against my Fluke 289. The worst deviation i got was around 0.5% which seems pretty good.