Tektronix MDO3102 - Spectrum Analyzer - Fails SPC  - Tektronix RANT! - Page 1
Forum section: electronics / testgear

author: AJ3G
author_rank: Regular Contributor
timestamp: February 09, 2022, 02:39:53 am
content: All:I have a MD3102 with the 3GHz SA Option. The scope has less than 200 Starts, and <500 hours. For some unknown reason it began failing the RF Selftest, which I believe is a SPC Test. I wish I could explain why such a new scope would encounter such an issue (i.e. misuse), but I have never exposed the SA built into the MDO to anything over 0dBm and typically far less. The only conclusion I can come to (not that it matters) is I got a bad part, or subjected the front end to ESD somehow.Today I called Tektronix to see about a repair. Without even seeing the instrument they claim the cost to repair is $3000.00, which I find absurd. I took a look at Dave's teardown of the scope, and I too have taken apart to have a peak of what is going on inside. I figured if it cost $3,000.00 to repair what is likely a fault in the Acquisition Board, then I certainly could not make it any worse by having a peak/poke inside. To add insult to injury, I calculated every part in the Spectrum Analyzer RF Chain, and if I were to replace all the parts, it would be less than $75.00, and I am not buying in quantities.With the above in mind, I think I am going to attempt a repair myself. I was hoping someone with good SA repair knowledge can take a look at my findings below and tell me if I am completely off base in the conclusion's I am coming to based on observation of the Spectrum Analyzer Module's displayed behavior.With a reference level set to 0dBm, and injecting a calibrated -30dBm source into the front end of the analyzer I have an IL of about 12dB. Where it gets interesting is if I change the Reference Level to anything below -10dBm the signal vanishes from the display which indicates the insertion loss is getting worse. Now, if I set the Reference Level to +10 through +20dBm I am only losing about 3dB so the symptom seems to improve.I am not 100% sure, but I believe what I am doing by changing the reference level is simply changing the front end step digital attenuator settings. The digital attenuator is a AD/Hittite HMC624, which is widely available. Would it be safe to say, that the issue I observe would be caused more likely than not from a blown input attenuator, or a part in that section? For a while I was toying with the idea I may have a blown LNA, but that seems less plausible. The LNA in this case is not adjustable, and I can see no way this device would have varying gain with change of reference level. From what I can see it simply runs all the time, and should provide the gain specified. I suspect if it was damaged, I would see nothing in the diplay as there would a massive loss from the input to the output.Does my approach seem logical, or am I not understanding the mechanics of how this specific Spectrum Analyzer functions?Rich

author: 1Ghz
author_rank: Unknown
timestamp: February 09, 2022, 04:37:15 am
content: I think your approach seems logical.Check the attenuator in/output level first.And "Follow the signal path".https://youtu.be/VFX47ZGOn_o?t=2669

author: AJ3G
author_rank: Regular Contributor
timestamp: February 09, 2022, 12:06:32 pm
content: Hi 1GHz:I am going to sit down with it today and dig a bit deeper. I recall there being some attenuation in the input/output with the instrument set to off. A look at the logic table for the chip attenuator claims the attenuation should be zero when all bits are set to low. With the above said, I have seen some components like this enter a weird transitory state when not powered.I wonder, do you recognize the circuit leading up to the attenuator? I see the DC Block, and Coupling input capacitor, but those components leading from the N Connector to the input look more like PI Attenuator networks to me. On my board, at least two of the components from the RF Path to Ground are in fact resistors (marked that way).Rich

author: AJ3G
author_rank: Regular Contributor
timestamp: February 09, 2022, 01:30:14 pm
content: I apologize - The Truth Table is opposite for the attenuator chip - It should be 31.5dB of IL with D0 - D5 set to Low. The opposite is true when all digital outputs are set to High, then one should see no more than 2.4dB IL up to 3GHz.Of course, I will be signal tracing with a probe which will likely not have a perfect 50 Ohms impedance so I am sure I will see something greater with D0 - D5 set to High.Rich

author: AJ3G
author_rank: Regular Contributor
timestamp: February 09, 2022, 07:23:08 pm
content: I had a chance to look at the parts leading from the RF Connector (N Input to the Attenuator). In this example I will start from the connector and work m way towards the digital AttenuatorFirst Series Capacitor - .4uF - Blocking (measured)First Series Resistor - 11.3 Ohms (Measured 11.3 Ohms here)Second Series Resistor - 18.2 Ohms (Measured 16.8 Ohms here)Third Series Resistor - 30.1 Ohms (Measured 26.9 Ohms Here)Second Series Capacitor - 10pf Coupling (Measured)First Shunt Resistor/Shunt In - 45.3 Ohms (Package Read no Measurement)Second shunt resistor - 17.4 Ohms (Package Read, no measurement)Third Shunt Resistor - 110 Ohms (Package Read, No Measurement)Fourth Shunt Resistor/Shunt out - 180 Ohms (Package Read, No Measurement)I figured this would help someone that has to look at these components in the future.

author: AJ3G
author_rank: Regular Contributor
timestamp: February 09, 2022, 09:53:26 pm
content: Made some more measurements. I think something is very wrong with the attenuator chip in the MDO Spectrum Analyzer.I made some relative measurements using a VNA. From the VNA, I connected the source, set for 100MHz at -20dBm to the input N connector of the MDO Spectrum Analyzer module. I then used a 500MHz oscilloscope probe to the VNA receiver input. I used this probe to measure the signals at various points from the input jack of the spectrum analyzer module to well beyond the input attenuator chip.These are relative measurements, but what I have found is with the Attenuator Chip unpowered (D0-D5 LOW) I am losing about 6dB. According to the specification sheet, I should have an insertion loss of 31.5dB with D0-D5 Low. I think I will try to replace this chip, and maybe order the LNA while I am at it.Does anyone know the Part Number to the SPDT Switches which appear to bypass the the LNA? In an unpowered state this switch appears to be a short between the pole and both throws.Rich

author: TurboTom
author_rank: Super Contributor
timestamp: February 10, 2022, 12:16:26 am
content: TheQorvo RFSW8000would fit the interconnection -- unfortunately I don't find any document that specifies the package marking of that component... But I'm pretty sure it will suit just fine as a replacement.

author: AJ3G
author_rank: Regular Contributor
timestamp: February 10, 2022, 12:24:39 am
content: Hi TurboTom:That is good information. I am trying to document as much of my findings as I can so if someone else has to conduct this repair there is information available.I think my switches are fine. These are another device, which when powered off land in a very strange state, which is short on both Throws to the Pole (Bridge on Both RF Paths). I will be posting more information in a minute.Rich

author: AJ3G
author_rank: Regular Contributor
timestamp: February 10, 2022, 12:36:03 am
content: All:I have some more updates about this circuit.1. The LNA BIAS is not tuned on until the reference level is set below 0dBm. Bias Voltage at Vs is around 7.5V when you move the Reference Level Down. The series BIAS Resistors measure to be about 74R0, which means the LNA Current should is being set to about 40mA. If you measure the BIAS Voltage at Pin10 (RFOUT), its around 2.5VDC.2. The Two SPDT Switches which engage the LNA do not respond correctly until the scope is energized. Both paths will looks shorted to one and other but when you engage the scope this clears.I have both LNA and Attenuator on order from Mouser. I hope to have those by tomorrow. I am going to install them both since I am going to preheat that section of the board anyways.Another interesting observation on the Pi attenuator circuitry prior to the step attenuator. This seems to be designed to attenuate about 14 - 15dB prior to the attenuator chip. I really can’t think at the moment as to why, other than to accommodate the LNA.Rich

author: AJ3G
author_rank: Regular Contributor
timestamp: February 14, 2022, 11:56:40 pm
content: All:Just as an update, I repaired the Analyzer portion of my MDO! It turned out to be the Digital Attenuator Chip. What I can tell you is, this is not an easy chip to replace, and neither is the LNA. You will need to preheat the board, and in my case I had one person holding a hot air nozzle at 150 degrees on the bottom of the board, while I went in on top to remove the package’s from that section of the PCB.Both packages have “belly pads”, and there is a ton of copper in the area. If you are without this type of equipment, and the knowledge on how to use it, your best bet is to buy the part, and take it to someone who can put it on, even if you must pay them. The information below will suggest even paying someone a fair rate will be far less than the ripoff artist now dominating Tek.Tektronix has pissed me off for the last time on this deal. When I called to inquire about fixing this board, I was told that all the and all the boards and parts for the MDO are supported, and there are plenty of parts. I contacted Tektronix about simply getting a new board, and their parts department responded the part was obsolete, and their was no inventory. This all feels a bit shady, and geared towards generating undeserved revenue. I strongly believe that one should pay for this type of service, but $3,110.00 for a Flat Rate Repair on parts you apparently do not have, with the caveat that if they are unable to repair the unit will be returned to me, but I get the pleasure of spending $710.00 if they cannot fix it is pure B.S.In my previous writings here on the EEVBLOG, I have complained about trying to get an Applications Engineer to speak with regarding two simple questions I had about the scope, which was not covered anywhere in their documentation. I called three times, and each time I was promised a call back, which never came.Now when I called about buying some Keithley Products that we need I can’t get them to stop calling me. Six weeks ago I did place an order for two Keithley Supplies and a SMU. Those were promised to us two weeks ago. Thanks to this experience, those orders are now canceled.After this experience - I may have been turned into a HPAIK, R&S, Siglent, RIGOL, Anritsu, Advantest,….. Fanboy. Until some serious changes are made at Tek, I will not spend a personal or company dime on their equipment.Rich

author: dew
author_rank: Contributor
timestamp: April 03, 2022, 03:13:23 am
content: Greetings,i have similar issue on a MDO4104B-6. Scope fails POST with RF error. Fails SPC.From the debug console during startup:[...]Demux initialization completedebugConsoleInitdiagRunPostDiagsStarting POST diagsDiag Failure: Mode 100 12, RF Path,Base Noise, Low Limit 4.00, value 1.00Diag Failure: Mode 100 12, RF Path,Amped Noise, Low Limit 10.00, value 8.94Diag Failure: Mode 100 12, RF Path,HighBand Path, Low Limit 10.00, value 4.56Diag Failure: Mode 100 12, RF Path,MidBand Path, Low Limit 10.00, value 6.98 Finished POST diags[...]And during SPC:[...]16:58:04 RF SPC NOISE FLATNESS16:58:13 RF Noise Cal - FAILED16:58:13 RF SPC DONE16:58:13 - FAILED16:58:13 SPC Total Time: 0:9:51Scope shows nice, clean spike precisely at the generated frequency with matching dB values in the whole first 3GHz range which is all I can get from my AeroFlex. There is very little attenuation when narrow band is selected down to only 0.1dB less than on the generator's screen. Not much noise or extra harmonics that can point to a problem.Any ideas?Thanks

author: nctnico
author_rank: Super Contributor
timestamp: April 04, 2022, 01:22:51 am
content: Did you try various signal levels as well? I guess you did but I'm asking just to be sure.

author: dew
author_rank: Contributor
timestamp: April 04, 2022, 02:41:18 am
content: I did. It measures close to the value on SigGen in all first 3GHz (half of the bandwidth) I can test. I tried various levels throughout that range. Smaller range on the SA gives more precise match on dB. As expected I guess. The weird part is that the noise is below the low limit, not above the high. I can't believe device can get orders of magnitude more sensitive than it was designed to be by some accident. I wonder if the problem is in the upper 3GHz. I can't test that with my gear yet. I only have one and a half 3GHz generators. One is missing P/S. If I can get that TDK-Lambda P/S for my second AeroFlex IFR 3413 I can try to duct tape them into one