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Rigol DSA815 dead LO for the 900-1500MHz range

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wkb:
Sorry, life got in the way so I was offline for a bit.

Thank you all for the schematics and offered insights! I am totally out of my comfort zone with this, as I mentioned earlier.

I checked my SMD capacitor stock and I do have some really small sized caps. Whether they are the exactly same series/size as Rigol uses I have not yet checked.

So, that said, what would a first guess value be for the two missing caps?

TurboTom:
I would buy a few 402 microwave capacitors in the range 0.5pF~5pF and as a "shot from the hips" start with C245=2.2pF and C246=1pF. If it actually oscillates, I'ld check the voltage V1 with the scope if the amplitude of the third sweep equals approximately the two previous ones. If it reaches higher values or loses closed loop control, decrease C245 and vice versa. If it doesn't oscillate, increase C246. If the phase noise appears to be high, C246 may be too high or slightly too low. I guess the whole thing just needs some tweaking and common sense.

G0HZU:
I'd also suggest taking great care not to thermally stress the PCB when removing the caps if you need to try several values. SMD tweezers are ideal but using two soldering irons to lift the cap is the next best thing.

Don't try and use one iron unless you are very skilled at this stuff. Otherwise you can easily damage the PCB.

With two irons you can focus the heat onto the metal end caps of the capacitor you are removing and very quickly lift it off the PCB without adding too much heat to the PCB. Each time you swap out for another cap you run the risk of lifting the connection pad on the PCB.

wkb:
Absolutely... my standard technique for SMD is using 2 soldering irons. Metcal units in my case.

Soldering quality (and, it so appears?) the actual solder is not fun at all. The actual 'wetting' is problematic so I will have to remove the original solder. Which is not a lot of fun with a multi-layer PCB with massive ground planes.

Found some satellite LNB with truly minute caps in their SHF circuits. I guess these should do.

Anyway, we'll see..

TurboTom:
Yes, that's an important point. These multilayer controlled impedance PCBs with a lot of copper inside tend to conduct the heat away from the pads that reworking with soldering irons alone is quite a challenge, especially if lead free solder is used. It helps a lot to pre-heat the PCB with hot air to slightly above 100°C in the area to be re-worked. For re-work I always use leaded solder due to its considerably lower melting point.

If you will salvage the capacitors from an LNB (which is a good idea by the way), you will need a very decent set of SMD "smart" tweezers in order to measure the capacitance. The resolution of a standard multimeter's capacity function is by far not accurate enough. Or, if you have got a nano VNA (or a big one...  :D), it may also suit to characterize the capacitors.

Good luck! Fingers crossed  ;)

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