R&S CMU200 - "Input Overload" (nothing connected!) while Sweeping 10-40 MHz

[niconiconi]

It has only been a week since I received the R&S CMU200 tester, after replacing the HDD and installing a PCMCIA to SD card interface tonight, I thought I am good to go. Unfortunately, I think I just discovered a serious problem in my analyzer: while sweeping 10 MHz - 40 MHz using the spectrum analyzer, it reports an error message: Overload at connector RF 1! The error is reported every time it completes a sweep  . But the problem is, nothing is connected to RF 1  . See the screenshot attached.

It can still detect the signal within the range, but every time it sweeps, it reports the error message. It only happens when I'm sweeping 10 MHz - 40 MHz, if I shift the sweep range a little bit, I can no longer reliably reproduce the problem. The problem doesn't seem to occur at elsewhere, it can perfectly sweep around 100 MHz a 1 GHz without reporting an overload. Also, the error only occurs at a fine resolution bandwidth, I can only reliably reproduce the problem with a RBW around 5 kHz or less. The only other abnormality is a spur at 38.8 MHz.

The spectrum analyzer is in manual mode, with a maximum power level set to 30 dBm, and a reference level of -20 dBm, there's no way it can overload the input.

Even worse, in the R&S operating and service manuals, there's no any explanation of this error message. I'm going to ask the vendor if he knows anything about it, but I don't think the answer will be positive.

What on Earth could create such a strange issue?! I suspect the problem is only affecting one signal path, that would explain the symptoms. Unfortunately, the official block diagram doesn't show any detail on the operation of the instrument besides three IFs.

[tautech]

Set defaults to full span and post a pic of what's there.

[ocw]

The spectrum analyzer is in manual mode, with a maximum power level set to 30 dBm, and a reference level of -20 dBm, there's no way it can overload the input.

With your reference level of -20 dBm, you would probably overload the SA if there was a +10 dBm signal just above swept frequency range.  It is not made for a +30 dBm signal regardless of your reference level setting.

[niconiconi]

The spectrum analyzer is in manual mode, with a maximum power level set to 30 dBm, and a reference level of -20 dBm, there's no way it can overload the input.

With your reference level of -20 dBm, you would probably overload the SA if there was a +10 dBm signal just above swept frequency range.  It is not made for a +30 dBm signal regardless of your reference level setting.

How could there be a +10 dBm signal? Nothing was connected to the spectrum analyzer.

[niconiconi]

Set defaults to full span and post a pic of what's there.

Nothing is connected, and nothing to see here.

[tautech]

Set defaults to full span and post a pic of what's there.

Nothing is connected, and nothing to see here.

Oh yes there is !
NO overload.
The plot deepens. 

[niconiconi]

Set defaults to full span and post a pic of what's there.

Nothing is connected, and nothing to see here.

Oh yes there is !
NO overload.
The plot deepens. 

So basically,

* Overall, the analyzer seems to work well. I can generate and measure signals at other frequencies.

* The overload error almost always occurs while I'm sweeping 10 MHz - 40 MHz, with a RBW of less than 5 kHz, even when nothing is connected to the analyzer and a reference level of 30 dBm. Other settings cannot reliably reproduce the problem.

* RF 1 and RF 2 can be selected. If I select RF 1, it always says RF 1 is overloaded. If I select RF2, it will say RF 2 is overloaded.

This is why I suspect there's a damaged component in one signal path, either in the RF front-end or the RXTX board. Unfortunately, the official block diagram doesn't tell much.

[niconiconi]

I did some testing again today and found if the reference level is greater than 30 dBm, the issue won't always occur, but it's still here. I have read some troubleshooting posts on the CMU200 and saw there are many RF switches and ICs in the instrument, this reinforces my suspect that an RF signal path has damaged component, which will explain why the issue doesn't exist elsewhere. But what is so special about 10 MHz - 40 MHz, and in which location does the CMU200 even detect an overload? Is it detecting it on the front-end or the RXTX board, or at the ADC?

Again, the official service manual only covers high-level repair, not low-level diagnose, and there's zero technical information on the construction of the circuitry. However, in page 157, there's an intriguing notice that says,

For advanced troubleshooting on board-level, see Service Manual Modules (1100.4903.94-01).

I don't believe it's available anywhere on the Internet. Perhaps I should try contacting R&S and attempt to purchase this manual.

Meanwhile, I guess at least I can try disconnecting the RXTX board from the front-end and see if the overload error is still here.

[richnormand]

There is a lot of switch and programmable attenuators on that board as well as on the several IF stages on the upstream board.

Just removing the TXRX module will change a lot on boot.
I would be tempted to deal with the Rx or Tx coax at the bottom.
That way you can also introduce set attenuation on each channel and find out if either Rx or Tx are causing this.

Other than that should the behavior not change are the upstream IF stages and the software that switches the modes between bands.

I'll let you know if I get more ideas.

Best of luck with it

[niconiconi]

For advanced troubleshooting on board-level, see Service Manual Modules (1100.4903.94-01).

I don't believe it's available anywhere on the Internet. Perhaps I should try contacting R&S and attempt to purchase this manual.

Update: I was told by R&S that the Service Manual Modules (1100.4903.94-01) is for internal use only.   So, yeah, perhaps we need to wait for another decade before the product line is 100% dead, and then, they'll perhaps release the document.