tektronix 491 spectrum analyzer calibration problem

[neazoi]

hello I am trying to calibrate an otherwise fully working 491 spectrum analyzer.
The analyzer seems to work ok but when I set the resolution to higher values (i.e more resolution) the signal appears like the signal B in the photo, whereas it would expected to be like A.


I am trying to focus the problem and I would like your help on this please. What could cause the problem?

*I attach the block diagram for your convenience too

 

[E-design]

Look at which RBW setting the fault start to show. Then you can work it back to the which section is giving you problems.

 

[neazoi]

Look at which RBW setting the fault start to show. Then you can work it back to the which section is giving you problems.

Ok, so you say it is a resolution problem. Not a video detector problem neither a sweep problem right?

 

[biff44]

It looks to me like you are sweeping too fast for your RBW/VBW settings. Does the waveform look right if you slow down the sweep time?

 

[neazoi]

It looks to me like you are sweeping too fast for your RBW/VBW settings. Does the waveform look right if you slow down the sweep time?

Exactly, the waveform is just like sweeping too fast for the resolution setting. But when I lower the sweeping rate (even at the lowest point) the waveform does not changing (improve) shape.

I tried to calibrate the resolution settings like the manual says, but no luck. It works ok for the lower resolution positions but not for the higher ones...

I am trying to find out what would cause the problem?

 

[E-design]

Can you upload some photo's of what it really looks like?

 

[neazoi]

The signal is just like the one in the beginning on this page. it looks like it is cut on the middle. It is just like sweeping too fast for the resolution setting. If you want real photos I can post it here.

 

[E-design]

Can't harm to post a photo. We might spot something that can give an idea to what is causing the problem.