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8753E Phase Lock Issue at 3 GHz

Question asked by alex.forencich on Nov 5, 2018
Latest reply on Nov 9, 2018 by alex.forencich

I have an 8753E option 006 with a phase lock issue at 3 GHz.  I have been unable to narrow down the problem following the service manual; none of the procedures have turned up any obvious problems. 


The symptoms are as follows: on power up, the analyzer displays the "no IF found" message and displays an S11 trace that is flat from 30 kHz to 3 GHz, spiky from 3 GHz to 5 GHz, and flat from 5 GHz to 6 GHz.  If I set the stop frequency to 3 GHz, the phase lock message goes away and I get a flat trace from 30 kHz to 3 GHz.  Likewise, if I set the start the start frequency to 3.1 GHz, the error message goes away and I get a flat trace from 3.1 GHz to 6 GHz. If I set the stop any higher than 3 GHz (with the start at 30 kHz) or the start at any lower than 3.1 GHz (with the stop at 6 GHz), I get various phase lock messages (possible false lock; no IF found). 


If confronted with a phase lock error, the manual recommends running tests 46, 45, and 48.  Tests 46 and 45 pass, but test 48 fails.  I have run through most of the phase lock error section in the service manual but haven't been able to isolate the problem as all of the components seem to be working correctly.  I originally thought the A7 pulse generator might have had an issue as the manual states that the teeth should be about -46 dBm at 6 GHz and I was measuring much lower, but this was with the 8753E set to 16 MHz CW.  Setting the 8753E to 6 GHz brought the tooth level up to the vicinity of -46 dBm at 6 GHz.  And a bad A7 seems like it would have an effect across a much wider frequency range.  It seems like the whole loop is for the most part working correctly, there only seems to be an issue at the low end of the 3 GHz band, and the service manual doesn't seem to address such a narrow band issue. 


In source tune mode, I swept the source in open-loop through 3 GHz.  It seems to jump from 3 GHz to about 3.08 GHz on my spectrum analyzer when the 8753E switches bands.  When this happens, the DAC value shown drops to 398.  I presume this is because of the default pretune parameters loaded from test 45.  But perhaps 3.08 GHz with a pretune DAC value of 398 is too high.  Has my YTO drifted too high in frequency and therefore needs to be replaced?  Or could this be a problem with the YIG driver?  Or is the calibration routine in test 48 simply not driving the DAC low enough?