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Teardown, Repair & Analysis of a Rohde & Schwarz FSH3 3.0GHz Spectrum Analyzer
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rs20:

--- Quote from: G0HZU on August 10, 2015, 12:28:22 am ---I can also describe the signal path for the 6 GHz option if anyone wants to see it?

--- End quote ---
Thanks for the picture with all the different blocks overlaid -- it's nice to make observations like seeing the DC connection on the LPFs, and the lack of a galvanic connection for the BPFs (as you would expect), and the 3GHz LPF having features roughly double the size of the 6GHz LPF, etc etc. It's nice to feel like one has a clue what's going on!

I'm always happy to see more microwave magic with schematic overlays!
Hugoneus:
G0HZU,

Thanks for the analysis. As I pointed out on the video overlay, this was a very quick overview of the board. I spent less than 30 second looking over it as I was in a rush to close it up. A thorough overview of the board was not the intention of this video.

Making YouTube videos is not my full time job, time is a major constraint. When I do a full video on microwave design I'll make sure to go through everything in detail.

By the way if you are interested in seeing VCO's with 2nd harmonic or tripplers I have built a few all the way up to 160GHz. In fact, banks of VCO's with/without multipliers to cover nearly 50GHz of tuning range.
G0HZU:
I guess it's easier for someone like me to instantly spot these issues as I have many years' experience designing converters using filters just like these and in similar frequency bands and for similar applications. But I'm still very surprised that despite 6000 views on youtube nobody else pointed out that the PCB walkthrough got the various mixers and filters mixed up. These are classic elliptic LPF and interdigital BPFs. I think the 831MHz IF2 filters will be Murata SAW filters rather than cavity filters.


--- Quote ---By the way if you are interested in seeing VCO's with 2nd harmonic or tripplers I have built a few all the way up to 160GHz. In fact, banks of VCO's with/without multipliers to cover nearly 50GHz of tuning range.
--- End quote ---

I think quite a few people would be interested in seeing info about oscillator design up at those frequencies :)

However, I don't think it would be a good idea to use a VCO + tripler to generate 4 to 7GHz for the first LO on this type of analyser as it would demand so much more PCB space for the required filtering and also it would generate a lot more internal spurious issues and also the close in phase noise of the synthesiser would be higher with a tripler approach.
Hugoneus:

--- Quote from: G0HZU on August 10, 2015, 10:00:16 pm ---I guess it's easier for someone like me to instantly spot these issues as I have many years' experience designing converters using filters just like these and in similar frequency bands and for similar applications. But I'm still very surprised that despite 6000 views on youtube nobody else pointed out that the PCB walkthrough got the various mixers and filters mixed up. These are classic elliptic LPF and interdigital BPFs. I think the 831MHz IF2 filters will be Murata SAW filters rather than cavity filters.
--- End quote ---

I agree. I am an ASIC designer myself and a quick glance at a layout gives me a lot of information.


--- Quote ---I think quite a few people would be interested in seeing info about oscillator design up at those frequencies :)
However, I don't think it would be a good idea to use a VCO + tripler to generate 4 to 7GHz for the first LO on this type of analyser as it would demand so much more PCB space for the required filtering and also it would generate a lot more internal spurious issues and also the close in phase noise of the synthesiser would be higher with a tripler approach.

--- End quote ---

I also agree that a trippler is not a good idea on this board (and was surprised myself when I thought it was implemented). The mm-wave and THz oscillators have to use multipliers as it is not possible to reliably make fundamental VCOs at those frequencies with devices which have fT/fMAX <300GHz.

PS: I am shooting a VNA repair which I think you will like. ;)
Melt-O-Tronic:
VNA repair . . .

Getting a better understanding of how VNA's function is exactly what I've been wanting to learn.  Can't wait.  Feel free to make a 12+ hour video with plenty of detail if you want.   :D
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