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Thread: 8561E Reference Level cannot be adjusted


Permlink Replies: 5 - Pages: 1 - Last Post: Jun 19, 2010 1:42 PM Last Post By: vjsilva1
vjsilva1

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Registered: 02/27/10
8561E Reference Level cannot be adjusted
Posted: Jun 7, 2010 12:40 PM
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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?

Thanks for any input.
Victor
Mike M


Posts: 232
Registered: 04/10/07
Re: 8561E Reference Level cannot be adjusted
Posted: Jun 8, 2010 12:46 PM   in response to: vjsilva1 in response to: vjsilva1
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There is no Reference Level Offset turned on is there? If so, you will see an "R" on the left-center of the graticules.
vjsilva1

Posts: 22
Registered: 02/27/10
Re: 8561E Reference Level cannot be adjusted
Posted: Jun 8, 2010 2:11 PM   in response to: vjsilva1 in response to: vjsilva1
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Thanks Mike. I believe I checked that because I did put in an offset of -11dB to get the Ref Level to the right level. After that I deleted the offset. I'll recheck again tonight.

How much of an adjustment should the Ref Level adjustment have from -528 to +528?

--Victor
Mike M


Posts: 232
Registered: 04/10/07
Re: 8561E Reference Level cannot be adjusted
Posted: Jun 8, 2010 2:32 PM   in response to: vjsilva1 in response to: vjsilva1
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I get a 10 dB range on mine...
vjsilva1

Posts: 22
Registered: 02/27/10
Re: 8561E Reference Level cannot be adjusted
Posted: Jun 8, 2010 3:02 PM   in response to: vjsilva1 in response to: vjsilva1
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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?

--Victor
vjsilva1

Posts: 22
Registered: 02/27/10
Re: 8561E Reference Level cannot be adjusted
Posted: Jun 19, 2010 1:42 PM   in response to: vjsilva1 in response to: vjsilva1
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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.

--Victor

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