High noise floor of Agilent DSO80304B

[Robert_Sky]

Recently acquired a parts only DSO80304B, It is found the noise floor is much higher than the spec. Some 250MHz clock signal shown in the noise floor measurement when port is open. Attachment is a screen of port1 open and its FFT mag. The FFT show some 250MHz and its harmonic spur.

Open the back of scope, the acquisition board does not show obvious bad parts. Indeed someone has try to repair it and failed
I guess the high speed clock drive ADC may not be good quality. Then i check the 10Mhz Ref clock out and there is nothing output at the BNC connector of 10MHz. Next step i would like to find why the Ref out is broken. Maybe i will a get a window based 200MHz bandwidth scope
Could anyone offer some hints for debugging?

Thanks.

[Jwalling]

Have you run the calibration, and does it pass?

[Robert_Sky]

@Jwalling, Thanks for the advice. I have run the calibration. Both vertical and trigger calibration failed.
After some near field probe, it is found the clock circuit is not locked to 1000MHz, but around 930MHz. More measurement will be done to the clock circuit

Thanks.

[Jwalling]

After a calibration is run, a .txt file is generated in the calibration folder. Open it with wordpad to get more details on the failure(s).

[Robert_Sky]

Hi Jwalling

Thanks for the input. After a manual search on the C:\ disk, no calibration file is found. Spent some time debugging the PLL circuit, one of ferrite bead is broken and replace it with a new one. The PLL can lock to 1GHz now, but the noise floor is still same. The noise has nothing to do with the clock.
There are not may things can be done for the solder-down BGA.

[Jwalling]

It's in a hidden folder. \scope\cal

With an explorer window open, put C:\scope\cal into the address bar at the top.
Something in these scopes doesn't allow you to show hidden folders persistently. If you change it to show them, it gets changed back.