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| E4407B Tracking Generator Repair |
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| smgvbest:
--- Quote from: analogRF on October 06, 2020, 12:15:03 am ---what are these numbers? --- End quote --- What I have so far The Attenuation is the 1st column, next is the dB min/max values (ie it was not fixed it varied) as measured with the power meter, the next is the average dB and last is the Peak Detector Input to the AD620 i need to complete this by getting the reference into the AD620 and the output of the AD620, just sharing the values so far. |
| analogRF:
--- Quote from: smgvbest on October 06, 2020, 12:29:44 am --- --- Quote from: analogRF on October 06, 2020, 12:15:03 am ---what are these numbers? --- End quote --- What I have so far The Attenuation is the 1st column, next is the dB min/max values (ie it was not fixed it varied) as measured with the power meter, the next is the average dB and last is the Peak Detector Input to the AD620 i need to complete this by getting the reference into the AD620 and the output of the AD620, just sharing the values so far. --- End quote --- you mean the amplitude actually fluctuates when sitting there? How? keeps oscillating up and down? That clearly means the ALC loop is not working. It is like having no closed "loop" control of the amplitude which is possible but unlikely from a high quality tracking generator. If there is indeed a control loop and the amplitude just plays around like that, then the loop is not working (not closed) It actually starts very early on even at -5dBm or -6dBm. From the schematics I know that the Unlevel error only happens when ALC_MON goes out side the +/-1V window. So the loop control (if exists) must be closed somewhere on the BITG circuit. Perhaps somewhere on the RF signal path they sample the amplitude and compare with a reference and then control the V1 to HMC346 to keep the amplitude steady. If this loop is broken then the behavior you see is explained. It is important to figure out the circuit behind generating V1. I think it originates from the PWR_LVL from DAC on A2A1 but that voltage could be just providing the reference value for the control loop. But another (unlikely) option could be that there is no control loop and they just set the V1 based on some previously calibrated values and let it go and then only check whether ALC MON is within range or not. again we need to know how V1 is produced a little bit of reverse engineering is needed I think |
| smgvbest:
--- Quote from: analogRF on October 06, 2020, 01:49:11 pm ---you mean the amplitude actually fluctuates when sitting there? How? keeps oscillating up and down? That clearly means the ALC loop is not working. It is like having no closed "loop" control of the amplitude which is possible but unlikely from a high quality tracking generator. If there is indeed a control loop and the amplitude just plays around like that, then the loop is not working (not closed) --- End quote --- So yes, reading from the power meter (U2000H) i see the levels jumping around and the numbers I show are the Min/Max for each Attenuation level. --- Quote from: analogRF on October 06, 2020, 01:49:11 pm ---It actually starts very early on even at -5dBm or -6dBm. From the schematics I know that the Unlevel error only happens when ALC_MON goes out side the +/-1V window. So the loop control (if exists) must be closed somewhere on the BITG circuit. Perhaps somewhere on the RF signal path they sample the amplitude and compare with a reference and then control the V1 to HMC346 to keep the amplitude steady. If this loop is broken then the behavior you see is explained. It is important to figure out the circuit behind generating V1. I think it originates from the PWR_LVL from DAC on A2A1 but that voltage could be just providing the reference value for the control loop. But another (unlikely) option could be that there is no control loop and they just set the V1 based on some previously calibrated values and let it go and then only check whether ALC MON is within range or not. again we need to know how V1 is produced --- End quote --- I was planning on having to look at both the V1 which feeds to the HMC346 from the interface board on the TG so may have additional circuitry there and the how the peak detector circuits are fullly implemented to see if there is more on the interface board. PWR_LVL appears to go direct to the RF board with no circuit on the interface board. --- Quote from: analogRF on October 06, 2020, 01:49:11 pm ---a little bit of reverse engineering is needed I think --- End quote --- Been figuring this was coming Lots of probing ahead I see. |
| analogRF:
--- Quote from: smgvbest on October 06, 2020, 10:11:11 pm --- So yes, reading from the power meter (U2000H) i see the levels jumping around and the numbers I show are the Min/Max for each Attenuation level. I was planning on having to look at both the V1 which feeds to the HMC346 from the interface board on the TG so may have additional circuitry there and the how the peak detector circuits are fullly implemented to see if there is more on the interface board. PWR_LVL appears to go direct to the RF board with no circuit on the interface board. --- End quote --- First thing I would check is the control voltage PWR_LVL coming from A2A1 to see if it has any instability/fluctuation in it. Also that 5V rail which was a bit low and must be checked for noise/ripple too. If those are good, then there is a ALC loop on BITG which is open thus the amplitude fluctuates. there should be a feedback mechanism from peak detector circuit to generating V1. I think that circuit is on the other TG board. At this point I think lots of things are pointing to that other board. The problem is more likely in there I think. The only components on the RF board that might be of concern now are the HMC346 itself which "most probably" is ok and the peak detector diode and cap which you can easily check. if those check out, I think something is happening on the other board |
| smgvbest:
@analogRF I'm working to trace the circuit out for the PWR_LVL and ALC_MON and others in the mean time I fired up benchview and took a look at the TG Output at a few levels, I added a strip chart which lets you see whats going on better than just looking at the MIN/MAX values I'm posting the -10dBm and 3dBm screen caps What's interesting to me is the dips in the chart are very regular. |
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