I have an 8561E that passes all diags and can display a consistent trace when swept through its full bandwidth. The problem is when I perform the Reference Level Calibration as shown in the User's Guide, Chpt 1the peak is too high.
I set the center to 300Mhz, Span to 20Mhz but the peak that is there is not -10dB, but slightly higher than 0dB. The Ref Level Adj between -528 and +528 barely varies the peak. Also the AmpCorr entries all seems to be empty. Is this possible a Calibration issue?
I have maybe 1dB or 2dB max. Also it does not seem linear. For example if I am adjusting it up, it starts going up but then goes down again while I'm still increasing the level. Almost like a counter missing a high order bit that is being fed into a DAC. Maybe that could be the problem?
I did confirm I have no Amplitude Level Offset enable, no R on the left center.
I have checked the unit with an RF generator and it's clearly showing the input signal as too high across the whole bandwidth. I input a sine at 300MHz, 1, 2,3,4GHz and they all show up about 10dBm too high. It's starting to look like there is a problem with the Ref Level adjust circuitry. I have the CLIP, any suggestions where to look for that circuitry?
Found the problem. It was related to the reference level circuit. I traced it to the AF7837BN dual DAC not outputting the correct Ref Level. Tracing back, all the control signals seemed fine except L_IA0, which comes from U505c (74HCT02). This gate was bad and was preventing all writes to the RF Gain DACs.