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Rigol DSA815 dead LO for the 900-1500MHz range

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wkb:
hi there,

Has anyone ever reverse-engineered the three LO VCO's in the Rigol DSA815?

A couple of years back my 815 failed for the 900-1500 MHz range. That corresponds to the #3 LO VCO. eevBlog forum member ted572 has nicely documented this as the LO range 3,1857 MHz - 3,7857 MHz.

In recently acquired a real SA, an HP 6.5 GHz unit, which finally allows me to check on the LO signals. Some measurements proved my assumption to be correct: the two lower frequency LO VCO's produce signal as they should, the top-band VCO does not produce any signal.

<RANT>My 815 is suffering from absolutely abysmal soldering quality. And, believe it or not, C245 and C246 (hope I read the silkscreen correctly) were simply gone... The caps managed to hold on for the 24 months warranty period + 1 month and then Poofff, spontaneous failure.

Of course our beloved Dutch importer did not want to address the issue at the time: "sorry, out of warranty, and we only repair at module level. That would be EUR 1000+ please". I will spare the forum my response. Let it suffice that I will never again buy any Rigol stuff.</RANT>

So, any chance of repairing would be based on someone who has put effort in re-creating the schematics for the VCOs. And even more importantly: knows the component values used?

Any help much appreciated!
Wilko

G0HZU:
I don't have a Rigol DSA815 (never used one) but can you put up a closeup image of the VCO?

I might be able to help a bit if I can see an image of the VCO, especially if you can provide a measuring scale alongside it or at least identify if the SMD caps are 0603 package as this will help me scale it all.

TurboTom:
I can contribute a photo of the 1st LO section of mine but the component values are unknown... Peculiarily, both the mentioned caps and a few others as well had been reworked right from the factory on this specimen. Is the LED of the 1st LO branch in use lighting up?

Edit: Transistors are BFP450 (Marking "AN") and BC817 (Marking "6C").
Edit2: Added a photo of the rear side of the 1st LO section, containing all the power supply and PLL circuitry. The PLL (ADF4169) is controlled by a Spartan FPGA.

wkb:
Yes, all 3 VCO range LEDs light in sequence for a full sweep to 1.5GHz.

And some pictures with scale.

G0HZU:
Thanks for the images and the scaling info :)

Can you confirm that there is actually a row of ground vias at the end of each resonator as shown in yellow in the image below? I'd expect to see those there. It may be that they are filled vias so difficult to see them. Don't worry if you can't see them under the solder resist.

Also can you read the part number code on the two varactor diodes shown by the green arrows?

I assume the missing caps are the ones circled in red in TurboTom's image.


You will need to use caps that are well behaved up at 3 GHz as the package inductance will begin to affect the capacitance up at 3 GHz.

Is there a tiny part code stamped on the red circled caps in TurboTom's image? ATC parts often have a part code on them.

The VCO looks like a negative resistance design. To aid in reverse engineering it I can explain how I think it works, but this might take several posts. Ultimately you just want the values for those two caps. I suspect that they could be select on test parts and the same goes for the cap just above the BFP450 BJT (circled in blue). This cap helps define the frequency range over which the BFP450 generates negative resistance. The red circled caps help define how the resonator is tapped into by the BFP450 BJT.

They are probably selected to give the most linear tuning response for the VCO and they also help define the oscillator power in the resonator. I don't know if the Rigol analyser is clever enough to self calibrate its VCOs to linearise them using some form of lookup table that helps provide a linear sweep on wide spans (eg across the full range of each VCO). This would help a lot in terms of achieving close to factory performance when you fit new caps here.

I can explain how it all works but maybe this will just bury the real goal which is for someone to tell you what the actual cap values are?



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