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Help repairing vector signal generator with low output

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NY2KW

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I acquired an R&S SM300 signal generator has some output issue only between certain level ranges independent of selected frequency. The self-test program passes all internal testing. The following is a description of the symptoms and perhaps someone can advise if I should be focusing on the attenuator or power amplifier? On my recently calibrated HP 438A, the SM300 output seems linearly accurate from -100dBm up to -26dBM but at all levels tested it is exactly 10dBm lower than as set on the front panel controls, i.e., at -26 dBM my power meter reads -36dBM. As soon as I set the front panel controls up 1 dBm to -25dBm, all signal is lost (or it is less than I can measure ~-120 dBM). Signal remains lost until I reach a setting of 0 dBM where the signal returns but my power meter only shows -36 dBM and increasing higher than 0 dBm doesn’t do anything – signal still there but at -36dBM. The SM300 should go up to +13 dBm.

The R&S service manual is unhelpful other than saying replace the board… no schematics and nothing on component level. I would like to get some idea of how to trace through the problem and where to start - attenuator or the wide-band amplifier section. I have contacted several repair services – most say they don’t have enough technical info from R&S to repair – while 2 others want 2x the current asking price for a functional used SM300.

Any advice appreciated.

Jerry NY2KW
 

It sounds like a problem due to an attenuator in the output chain. Since you said that in a given power range, exactly 10 dB are lost, in another signal is lost, then it appears again but at a fixed level, seems to me that the attenuator control fails.

I think you should investigate first the control logic that drives the attenuator (or attenuators, if they are two or more).

Just my feeling: I don't know the block diagram of the instrument, so I cannot be precise.
 

There are four 30 dB attenuator sections in the overall attenuator. One is electronically variable in 2 dB steps. The rest are either switched in or bypassed. It appears that at some level, you have one of the fixed value pads going open. This can be due to the pad being blown open due to someone applying excessive RF power into the output. Normally the pads closer to the output is blown up. If that is the case, it may be a real problem, because these pads can normally not be repaired, being precision trimmed parts. You should be able to open the unit and determine in conjunction with the attenuator control signals which pad is being activated and whether it is blown open.

One other possibility is that the protection contact that should prevent reverse power damage, that is in series with the output activates due to some fault or logic condition. That should not be difficult to determine.
 

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To me, the logical approach is to follow the logic. :D

The outputs of the Controller vs analog error measurements then correlate which physical attenuator parts corresponds to the logic transitions of the unexpected results.

Then compare similar working replicating attenuators to the component level
 

Thank you SunnySkyguy and E-design !! I work on ham radio repairs... it would be so much easier if I had a schematic or even a block diagram with some idea of where to find some of the components. I guess I have to start at the output and work my way backwards. I'm not sure what 'controller' or uprocessor is even controlling the attenuator sections but I will open it up and take some pictures. I would appreciate your help and advice if you have the time.

Thanks again.,.. Jerry
 

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