Electronics > Repair
CMU200 Rx level issue / failed loop test
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Sas:

--- Quote from: Michael-VK2BEA on January 06, 2022, 07:49:04 pm ---
Yes, the whole chain is used for the factory calibration. The unit I repaired was within 0.1dB so there was little difference in the filters I swapped.
Has anyonw cracked the code on the calibration file? (where is it stored and what is it's structure?)

Michael

--- End quote ---

The difference of 0.1 dB is acceptable in the two new filters. I was unable to decode the table structure. I stopped while trying to run VERM and CMU on a virtual machine. There are traces in the configuration files that the software can run under dos itself, without the entire device (for programmers). I think the calibration software is already installed on the device. I tried to figure it out two years ago, now I don't remember much. Re-calibration after the factory is probably done by connecting the standards and sending a few commands via GPIB. I think there is one general table for tx and one for rx that can be calibrated outside of the R&S factory e.g. in LGE Korea. It seems to me that the correction of other tables is only possible at the R&S factory (e.g. temperature corrections).

Where did you buy DFC3R836P025HHD ?
Sas:

--- Quote from: _Wim_ on January 05, 2022, 08:37:23 pm ---Then I removed the soldered copper tape again (which probably overstressed the 836 filter, see below), and solder a tiny copper plate on top  (plate pressed flat on the surface filter, only soldered the sides while pressing down). This improved the result on the 1842 filter to only -3dB (was -10dB before), my best result so far. Unfortunately no more output on the 836...

--- End quote ---

Soldering surfaces is a risky idea. The filter is not soldered from the bottom to the PCB. PCB for RF doesn't like high temperatures. After all, the result of -3.5dB is quite a good result. It is not known how linear the generator is, in my CMU200 it has a deviation of about +/- 0.5 dB for -30dBm in the entire frequency range.
Yesterday I checked the first channel of my CRTU. The worst loss on an internal loop 1->1 is -1.5dB and 2->2 is -2dB. External loop 1->4 is -2.1dB and 3->2 is -2.4dB. The loss of -2.1dB is repeated for -35dBm in loops 1->4 and 3->2, which excludes problems with RF inputs. Probably the 836 MHz filter has too much loss, there is some fault or the module requires calibration. I don't have new 836MHz filters to verify this.

I add photos of filters before and after cleaning from the first channel of my CRTU. I cleaned the upper surface of the filters with a cotton swab and isopropylene alcohol, then a mixture of methyl alcohol, water and detergent (dishwashing concentrate), neutralized with isopropylene alcohol, then cleaned with ketchup (it cleans copper perfectly) and neutralized again with isopropylene alcohol. I isolated the filters from the RF absorber with laminating pouches (0.15mm). I did not want to stick anything to the surface of the filters so as not to damage the copper layer.
I will check the results of the loop measurements after the initial repair in the CMU after the return from the buyer (he replaced the rx / tx board and more :rant:).
Sas:
FreRes CMU loop 3-> 2 after replacement of BPF 1842 MHz with a new one and BPF 836 MHz tightly covered with copper tape. The tape is soldered on the corners of the pcb filter. The 836 MHz filter repaired in this way has too much loss of about -4.4dB. If the filter factory has a loss of -2.6dB, it is about -7dB after repair. I note that this filter was in a very bad condition (photo 31_CMU_B_836_return_buyer _cheat.jpg).
Internal loop 1-> 1 and 2-> 2 test in the range of 10-1200MHz are from + 0.3dB to + 3.1dB and in the range of 2205-2700MHz they are from -1.4dB to + 1dB. I guess it should be like that, in the internal loop of the same input there is too little isolation in the signal paths. Someone has already written about it.

Is there any way (apart from printing a screenshot onto a CF card) of logging these results (any text format)?
Do you switch the RF4 IN input correctly in FreRes (the analyzer remains on the RF2 input instead of switching to RF4)?
_Wim_:

--- Quote from: Sas on January 07, 2022, 01:47:08 pm ---Is there any way (apart from printing a screenshot onto a CF card) of logging these results (any text format)?
Do you switch the RF4 IN input correctly in FreRes (the analyzer remains on the RF2 input instead of switching to RF4)?

--- End quote ---

The failed results are stored in a file cst.err when running in continuous loop test mode. The good results unfortunately are not. A workaround is installing an attenuator in the loop path so all results are bad...

To get the cst.err file, you have to have a keyboard attached to the CMU. When all the tests have run (in continious test mode), you have to hit alt+F4 to exit in dos modus. The cst.err file is found in the firmware folder (if I remember correctly). This file has to be renamed/copied in order to prevent it from being deleted automatically.

After renaming, you can reboot the CMU and transfer the file to your hard drive using the "remote service tool" software.

There might be an easier way, but that is the only way I know of.
Sas:
At CMU (buyer's cheat back), after several hours of warm-up, the RF3 OUT output died  :-BROKE  :wtf:. Yes, I thought the screw heads were damaged for a reason. He also replaced the RF IN / OUT module  :palm:.
And I wanted to try the log method with the cst.err file. The generator works on RF1 and RF2 (loop 1 i 2 is ok). It may RF3 OUT work when it cools down, but finding such thermal faults is not easy. We'll see tomorrow. I'm fed up, I'm going to sleep .

Edit:
The MMIC amplifier at the RF3 OUT output is damaged. The cool one worked as normal, but as it warmed up it limited the gain to the band above 2.2 GHz, and then the full band. Cool it worked, warm no, and over and over again. After disassembling the in / out module and heating the amplifier with hot air, it got a short circuit on the output (and power supply) of 60 ohms. It is now completely dead.
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