Products > Test Equipment
Teardown, Repair & Analysis of a Rohde & Schwarz FSH3 3.0GHz Spectrum Analyzer
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Dubbie:
Everything you post a new video I get a big groan from my wife because I like to watch it on the TV in the living room. It's funny though, for some of the repairs she even exclaims "yay" when the thing turns on for the first time. Don't stop making videos. I learn a lot from them.
Hugoneus:

--- Quote from: Dubbie on August 06, 2015, 08:30:48 am ---Everything you post a new video I get a big groan from my wife because I like to watch it on the TV in the living room. It's funny though, for some of the repairs she even exclaims "yay" when the thing turns on for the first time. Don't stop making videos. I learn a lot from them.

--- End quote ---

Thanks! This one had a yay moment from myself. :)
G0HZU:
Sorry to be a neghead but... I wasn't very impressed with the signal path walkthrough parts of this video... :(

I skimmed over the repair bit to get to the part I was most interested in which was the block diagram and internal view of the RF PCB. I didn't think this was presented that well because there were lots of incorrect things said. I guess none of this really matters but the PCB walkthrough was especially poor because Shahriar got the description/location/roles of the various mixers and filters mixed up.

I can glance at that PCB and see the correct signal path very easily including the unused sections for the 6GHz version on the other side of the PCB.

A couple of other things to note. At 46:36 the loop BW of the PLL will be chosen at 30kHz mainly for phase noise but also for the tuning/startup/lock speed of the PLL.

At 46:40 the 'VCO 3X' doesn't refer to a frequency tripler or overtone. To put a tripler here would not be a good idea... 'VCO 3X' refers to a bank of 3 discrete VCOs that get selected as required to cover the total 4-7GHz range of the first LO. It looks like they used printed resonator designs. They use three VCOs here because it gives the best phase noise performance compared to trying to do this with a single VCO. This is a fairly common approach. Also the design is technically easier/safer than trying to cover this range with a single VCO. The downside is the added circuit size/complexity and the associated management and startup/locktime issues as it sweeps the full range in these three chunks. It's easy to spot these three discrete VCOs in the PCB layout as they take up a lot of space. eg see the image below where they are arrowed/circled in blue.

G0HZU:
Dunno if this helps but here's a crudely marked up image showing the location of the filters and mixers in the basic RF path for the 3GHz range.

Obviously, I don't work for R&S and this is the first time I've seen inside one of these but I think my marked up image below shows the locations/descriptions better.
G0HZU:
I can also describe the signal path for the 6 GHz option if anyone wants to see it?
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