Teardown, Repair & Analysis of a Rohde & Schwarz FSL 9kHz - 6GHz Spectrum Anal

[Hugoneus]

You can watch the video here: [25 Minutes]
youtu.be/60hasxvl5KU


* SPOILER *

In this episode Shahriar investigates a failed R&S FSL Spectrum Analyzer. The instrument shows an unusual spectrum trace and fails a wide range of self-test functions. The LO Unlock failure is likely the most important item to address which can cause a cascade failure of other functions.

The teardown of the instrument shows a single-board RF design. The system architecture is analyzed along with an X-Ray of the main board showing various RF/LO routes and interconnects. The core synthesizer is examined and measured along the signal path to verify the operation of each part. The failure is traced to an open 1-Ohm resistor which feeds the power supply of the divider IC feeding the PFD of the PLL. A ceramic capacitor failure is also observed which may explain the damage to the 1-Ohm resistor. After these components are replaced the instrument passes all self-tests. The functionality of the instrument is verified by measuring tones from 9KHz - 6GHz using a Keysight MXG.

[Hugoneus]

Ugh... Obviously didn't mean Anal...

[eldden28]

I have the 18GHz FSL that has recently stopped working right. It passes all self tests but acts like the analog input port is disconnected. The signal amplitude is small for all setting except for the 40db attenuation setting. There are two input ports to the spectrum analyzer board that gets the signal from an RF SPDT relay that seems to switch in and out a 10dB attenuator. I believe the attenuator is functional as it measure 60ohms on the input and 16 ohms on the output when out of circuit. The switch has a DC blocker on the input and it measures 0.47uf from input to output so I believe that is functional. I would appreciate any tips where to look. It seems to be an issue very early in the signal path. I have not verified that the RF relay is functional but in operation I here relays clicking when sweeping through attenuator settings.

Thanks,

Dennis Eldred