Author Topic: Agilent E4406A Repair  (Read 7054 times)

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Offline Zucca

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Re: Agilent E4406A Repair
« Reply #25 on: October 17, 2017, 10:10:55 pm »
Nice TonyG, keep on going!!!!  :popcorn:
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Offline siggi

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Re: Agilent E4406A Repair
« Reply #26 on: November 15, 2017, 11:31:15 pm »
The datasheet for the MGS-71018 is very concerned with the relative DC levels of inputs and control lines. Is there any DC on the RF pins 2, 5 & 8? If so, that might explain why the switch isn't working to spec.
 

Offline Tony_G

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Re: Agilent E4406A Repair
« Reply #27 on: November 16, 2017, 04:55:53 am »
I finally got some time to poke around again this morning and here is the result:

https://youtu.be/8qBS9zWCaJ0

At the time I thought I was no better off than I was earlier but on the way home from work I was having a chat with my Dad back home in Australia and we got around to talking about the DC Offset, control voltages and what might be happening. When I got home I grabbed the evaluation board and tried some more tests:

https://youtu.be/cJAjUFYTRgA

Now I'm pretty sure that the issue is with the DC value upsetting the bias in the mixer - Next step is to work out how best to address. I'm leaning towards lifting the VCC pin on U26 (the Op Amp) and grounding it just to test whether that can solve the issue.

TonyG

Offline Tony_G

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Re: Agilent E4406A Repair
« Reply #28 on: November 16, 2017, 05:00:44 am »
The datasheet for the MGS-71018 is very concerned with the relative DC levels of inputs and control lines. Is there any DC on the RF pins 2, 5 & 8? If so, that might explain why the switch isn't working to spec.

I think this is what is happening - While there is no DC on the RF input pins (5 & 8 in this configuration) there is once you drive the control pins into +ve voltages. I think this must have been different on the 1GG7-4218 switch that the schematic originally refers to. I believe that this is impacting the mixer bias - You can see this in the Part 6 video I just posted.

TonyG

Offline technogeeky

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Re: Agilent E4406A Repair
« Reply #29 on: November 16, 2017, 09:29:42 am »
The datasheet for the MGS-71018 is very concerned with the relative DC levels of inputs and control lines. Is there any DC on the RF pins 2, 5 & 8? If so, that might explain why the switch isn't working to spec.

I think this is what is happening - While there is no DC on the RF input pins (5 & 8 in this configuration) there is once you drive the control pins into +ve voltages. I think this must have been different on the 1GG7-4218 switch that the schematic originally refers to. I believe that this is impacting the mixer bias - You can see this in the Part 6 video I just posted.

TonyG

Tony:

I watched part 5 of the video, and looked at the available schematic and the MMIC switch specification (why does it not list the truth table for the configuration we're using????  |O ). I am not really sure what's going on, but I figured the overall circuit (up to the point things go into the mixer) is probably simple enough to simulate in the famous in-browser simulator.

In any case, I had a go at it (warning: the URL link is very long so make sure you use a browser which can handle this). I think this may be the first time I'm more confused after simulating something than before (mostly I'm confused as to why in the hell they used this +10/-10 configuration to drive this thing).

For the part of the circuit that we have a schematic for and measurements for, I put them in exactly (the +8.93 and -9.36 volt rails, the resistors, the capacitors). Obviously while there is a simplified schematic for the RF absorptive switch, I don't know any of the values of anything inside there (not that this simulator is really good enough for that anyway). One thing I do know, is that whatever the values are, they should present as a 50 ohm termination to the unused input -- a feat that I didn't really attempt to accomplish (though I included switches to try and look at this).

I also included switches to ground out the control voltages.

Someone earlier asked about "specific" values of resistors. It looks like the resistors with values 21.5k and 6.2k are chosen to provide a a 2.5V CMOS crossover point. I really don't understand why 34.8 ohm resistors are used on the control lines, but it is worth nothing that 34.8 ohm resistors (along with 1n caps) are used on all control lines for everything: every single attenuator switch control, the switch cal control, etc.

I assume you've already noticed everything I said, but since I made the simulation I figured I'd share it.



 
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Offline Tony_G

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Re: Agilent E4406A Repair
« Reply #30 on: November 16, 2017, 11:00:07 pm »
Nice - Like the simulation.

I think the next step is for me to lift the Vcc pin and see if I can make the Op Amp work from -9 to 0v (will need to check the datasheet on that to confirm). The alternate would be to lift the output pin on the switch and put a series capacitor in to block the DC as a test.

TonyG

Offline Tony_G

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Re: Agilent E4406A Repair
« Reply #31 on: November 30, 2017, 05:32:13 am »
Part 7 - Results of an eBay auction

https://youtu.be/V1CP-b-YwUY

Up next will either be a look at the 86900 VSA software ot checking out the working RF board.

TonyG

Offline Leo Bodnar

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Re: Agilent E4406A Repair
« Reply #32 on: March 01, 2018, 11:41:00 pm »
I have checked the control voltages on my E4406A unit and they are +9.24V and -9.41V.
They are swapped when you choose 50MHz as Input source.
Cheers
Leo

« Last Edit: March 02, 2018, 06:22:48 pm by Leo Bodnar »
 
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Offline Tony_G

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Re: Agilent E4406A Repair
« Reply #33 on: March 02, 2018, 05:38:42 am »
Thanks Leo, appreciate it.

I have my units on the floor by the bench because I wanted to go look at the DC great coming from the switch (I'm working on a 16902b right now). My current suspect is that the MGS and Minicircuit ones are impacting the bias in the microwave mixer Z2. If you still have it open, could you see if there is an offset on pin 2 of U38?

Thanks,

TonyG

Offline Leo Bodnar

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Re: Agilent E4406A Repair
« Reply #34 on: March 02, 2018, 06:34:22 pm »
Offset on pin 2 is 3.4mV, the same as on pin 5 (RF Input.)
If the switch is in 50MHz CAL input position the offsets are about 2.7mV on pins 2 and 8.
Leo

My current suspect is that the MGS and Minicircuit ones are impacting the bias in the microwave mixer Z2. If you still have it open, could you see if there is an offset on pin 2 of U38?

Offline Tony_G

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Re: Agilent E4406A Repair
« Reply #35 on: March 02, 2018, 07:04:48 pm »
Outstanding. Thanks.

So with the MGS and Minicircuits replacements I'm seeing between 250-500mV of DC offset when the other control line s driven positive. This is clearly a difference between those ICs and the original HP 1GG7-4218 that was on the board.

For test purposes I'm thinking of lifting the pin and dropping in a 1uF smd cap and seeing if having that as a coupling cap blocking that DC makes it at least init cal again.

Thanks once more Leo for taking a look. Will need to see if I can find a true replacement IC.

TonyG

Offline Leo Bodnar

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Re: Agilent E4406A Repair
« Reply #36 on: March 03, 2018, 11:57:19 am »
I have probably missed the answer in your videos - did you find that 500mV offset is present on the switch terminals even in the test fixture?  Unless 7dB pad is AC coupled - which is unlikely - the input to the switch will be 50R loaded and having 500mV DC implies 10mA current flowing out of the switch.  Both into 7dB pad and AT1.

Is DC offset present on both RF_IN and switch out?  Is it present on 50M_CAL?
What happens to them if you set Input to "50MHz CAL" in the menu?
What is the DC offset at AT1 input with U38 removed?

Leo

Offline Tony_G

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Re: Agilent E4406A Repair
« Reply #37 on: March 05, 2018, 05:55:37 am »
I have probably missed the answer in your videos - did you find that 500mV offset is present on the switch terminals even in the test fixture?  Unless 7dB pad is AC coupled - which is unlikely - the input to the switch will be 50R loaded and having 500mV DC implies 10mA current flowing out of the switch.  Both into 7dB pad and AT1.

I just hooked the test fixture back up again and here is what I see on the scope (with all outputs terminated in 50 ohm loads). I'm seeing less DC Offset but I can't remember what my original settings were so let's go with these numbers as I just ran the test.

Here is the first channel with -9.5V & 0V at 50MHz ~0dBm:



If I power the other control line to +9.5V I now see:



Here are the two images for the second channel:





My bench if full now so I can check this in the actual instrument but unless I've hooked something up incorrectly with the text fixture (8657B->Test Fixture->50 \$\Omega\$ Feedthorugh->Scope) I would expect to see the same thing on the actual RF board using the non 1GG7-4218 HP part.

TonyG

Offline Tony_G

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Re: Agilent E4406A Repair
« Reply #38 on: March 05, 2018, 06:14:43 am »
Is DC offset present on both RF_IN and switch out?  Is it present on 50M_CAL?
What happens to them if you set Input to "50MHz CAL" in the menu?
What is the DC offset at AT1 input with U38 removed?

U38 is supposedly either a 1GG7-4218 or a MGS-71018 (which should be the same as a Minicircuits KSWA-2-46+) but to answer the specific questions I'll need to get the unit back on the bench. I have a working RF board in the 2nd unit I bought so I can compare it to the non working one.

Thanks again for the info.

TonyG


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