Siglent SSA 3021X... Ooops

[Joel_l]

Hi,

I think I accidentally overloaded the input to my SA one too many times. My input impedance is no longer 50 Ohms. It still functions sortof.

I took the input board out and started looking around. I saw a 49R9 resistor which I assume is the termination resistor, it actually measures fine. I it looks like there are some clamping diodes on the input path and they seem fine. the input cap measures at 1uF, seems OK. Nothing looks like it got toasty.

The first IC in the input path is U1 a 956p, the 49R9 resistor is after that component in the path. Makes me wonder if that is the part I damaged.

I can't find any info on what that part is, anyone here know? Has anyone found schematics for this SA?

After some research, I believe the part is a GaAs MMIC. Still can't find a datasheet for the 956P to be sure.

Thanks
Joel

[tautech]

Hi,

I think I accidentally overloaded the input to my SA one too many times. My input impedance is no longer 50 Ohms. It still functions sortof.

I took the input board out and started looking around. I saw a 49R9 resistor which I assume is the termination resistor, it actually measures fine. I it looks like there are some clamping diodes on the input path and they seem fine. the input cap measures at 1uF, seems OK. Nothing looks like it got toasty.

The first IC in the input path is U1 a 956p, the 49R9 resistor is after that component in the path. Makes me wonder if that is the part I damaged.

I can't find any info on what that part is, anyone here know? Has anyone found schematics for this SA?

After some research, I believe the part is a GaAs MMIC. Still can't find a datasheet for the 956P to be sure.

Thanks
Joel

Sanity check:
Preset.
Connect TG to RF IN and apply -20dB TG signal.
Sweep should be at a -20dB level.

Post screenshot.

[Joel_l]

Its already in pieces. I was doing some tx measurements and left my attenuator out when making a change. The SA got 6 Watts pumped into it. It is not working properly now, levels are wrong, frequency is also off a little, I worry that might be a bigger issue. Measurements were fine until my screwup.

[Joel_l]

Looking at it more, I believe the parts 956P are switches. Looking at the path, it looks like the first switch switches in the 49R9 resistor, going farther down the path it looks like the other switches control some attenuators. Not sure why the 50 Ohm termination would ever be switched out.

[tautech]

Its already in pieces. I was doing some tx measurements and left my attenuator out when making a change. The SA got 6 Watts pumped into it. It is not working properly now, levels are wrong, frequency is also off a little, I worry that might be a bigger issue. Measurements were fine until my screwup.

Shame. 

Really we need screenshots to advise.
Normally damage to the front end results in 20dB error however I have one here that won't show any RF In levels. 

Here's one of a few mistake/damage threads:
https://www.eevblog.com/forum/testgear/what-happens-when-you-blow-the-input-of-a-6500-siglent-ssa-3075x-plus!/

[wofritz]

Here is my repair:

https://www.eevblog.com/forum/repair/siglent-ssa3021x-hw-07-03-00-frontend-repair/msg4812686/#msg4812686

[Joel_l]

This is helpful, thank you! Guess I'm not the only one.

Thanks
Joel

[Joel_l]

I put things back together enough to turn it on. what I found this morning was slight different that what I found ( thought I found ) last night.

Frequency is OK. I checked several points  and all were good ( Agilent E4432B )

Amplitude seems OK, worst I saw was .6 dBm low. TG trace was not horrible but I don't know what it looked like before. ( pic attached )

Input impedance is still off, the only way I have to gauge this is using one of my transmitters and TX'ing into the SA. Before the over power, I was getting a near perfect SWR, now I get high SWR on all 3 TXs I tried.

When I measure the switches, U1 is different than the others.

I went back and rechecked all the diodes in the path at each stage, all were OK.

I will replace U1 ( maybe all just cause )

Thanks

[Joel_l]

Something very strange happened.

It occurred to me that I do have a better way to check the input impedance of my SA, I can use my antenna analyzer ( at least for a limited range ).

I hooked it up and did a sweep around 20MHz, it showed me an SWR off the scale. I wanted to see what the impedance looked like so I switched it to do a Smith chart. When I did the sweep, the SA barked at me with an overload. I went and changed the attenuation did a few more sweeps and things looked good. Went back to do a SWR sweep and it looked good 1.2:1.

I disconnected the antenna analyzer and hooked my TX back up and it worked just fine, no more high SWR warning.

I need to look at this more and make sure I'm not chasing bad data.

[wofritz]

I didn't check the schematic fragments I "re-engineered" during my repair attempts, but depending on the attenuator settings the signal may or may not go through the faulty path / switch.

Maybe you can experiment with manual attenuator settings.

I attached my schematics fragment.

[Joel_l]

Mine is a little different than that, I have 4 more diodes where the signal enters the first switch.

So I think I found two things going on, one really muddied the other. Looks like my attenuator is bad and is causing the high SWR on my TX. This does not explain the initial high SWR with my antenna analyzer, I did not use my attenuator with that.

I went back and looked with my signal generator again. I fed a -20dBm signal in and went through the SA attenuator settings. The reading never changed from -20dBm only the noise floor moved up and down, the SA always measured -20dBm.

Those switches are not very expensive. I think I will just replace those. I hope the programable attenuator at the end is OK, not as easy to find.

[tautech]

Something very strange happened.

When I did the sweep, the SA barked at me with an overload.

I need to look at this more and make sure I'm not chasing bad data.

Good.
After a Preset all previous settings are set to factory and the analyzer attempts to protect itself.

Some wrongly use Manual attenuation to correct for external attenuation when instead they should shift the Ref Offset for Markers to display the correct level.
When set to Manual the analyzer is locked out from automatically managing its internal attenuation that varies with settings used.

[Joel_l]

Except that when using the antenna analyzer, I made no changes to any SA setting.

As far as what the SA is reading, thinking about it, it's all good. The SA knows what the attenuation is set to, so can always display the power being applied to the input port.

I did test my external attenuator and it is indeed bad.

The only potential issue left is why when using the antenna analyzer did I initially get several high SWR readings, when I tried the Smith chart I got the overload warning, then things worked fine. I did not change any SA settings during these tests with the antenna analyzer.

The SA still acts odd sometimes, so I think I will still replace those switches.

[tautech]

Have you considered working through some of the performance verification steps in the service manual ?

[Joel_l]

Looking at it, there are a couple I think I can do, though I have always wanted a power meter....

Problem is still the flakey nature of it, might pass this time and not the next.

I will do the ones I can for a baseline. I always thought the noise floor on mine seemed high, so that will be a good one to go through.

Thanks