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Rigol DSA815 dead LO for the 900-1500MHz range
TurboTom:
If the marking of the varactors is "B", which can easily be misinterpreted as "8", considering the size of the packages, the component may be a BB555-02V. Yet, this diode requires a higher control voltage of up to 28V. On the other hand, its minimum capacitance is way lower than that of my previously assumend component, the BBY58-02V (2.1 vs. 4.7pF). The BB555-02V would also better fit the frequency range of this oscillator. It's highly likely that Rigol used an Infineon component here since the other diodes and the transistors are of this origin as well.
G0HZU's hint that these capacitors are actually used to adjust the oscillators during initial testing at the manufacturer makes good sense: On my unit, for the highest frequency, all three capacitors (series, parallel and feedback divider) are hand-soldered while the mid range oscillator only has got the feed back divider cap "patched" and the low-range none at all. The higher the frequency, the more critical parasitics and component tolerances get, so voila...
It's strange that your DSA815 main board is of such a bad soldering quality, actually all of the Rigol gear that I had a "peep" into, was of very decent build quality, I never found any reason to complain about that. Their firmware is a different story, though ;). Yet, that's not the point here... You may have received a lemon that slipped through the quality control, but obviously, that won't console you.
wkb:
Better visible under the stereo microscope but to me the varactors are marked BY. Really a B, not an 8
As for a 'lemon' Rigol, I agree. But buying from a company that has no adequate quality control is just crap shoot. My conclusion is simple "Rigol? Nie wieder!"
TurboTom:
--- Quote from: wkb on December 15, 2024, 10:38:02 am ---Better visible under the stereo microscope but to me the varactors are marked BY. Really a B, not an 8
As for a 'lemon' Rigol, I agree. But buying from a company that has no adequate quality control is just crap shoot. My conclusion is simple "Rigol? Nie wieder!"
--- End quote ---
You are right, this is clearly a "B". I had to use the old photos that I took of my own unit to identify the components since the instrument now is located at our "production site", some 400km from me, so I cannot verify with the real thing. I've got many, quite good photos, also of the other, similarly configured oscillators distributed across the PCB, and I was drawn between "B" an "8", and then I first found the (datasheet of a) varactor with the "8" marking, and I thought "that's it"... But your microscope shot clarifies the situation.
I very well understand your feelings towards Rigol. Fortunately, there are enough other sources for such instruments that may suit you better. Anyway, receiving an instrument paid for out of the "hobby wallet" or at least with personal money, that turns into a dud like that, hurts and affects future purchase decisions. I've also been through that and try to avoid certain brands (unless the deal is too good to let pass...). I hope the repair attempt will turn out successful!
G0HZU:
I think there's still a lot to be learned from studying the design challenges associated with this type of VCO.
The printed resonator probably looks like 0.8nH at 3.5 GHz. Each diode will probably have 0.5nH package inductance and the C245 cap and its associated PCB grounding might add another 0.4nH.
So that means about 2.2nH inductance in the resonator. This only requires about 1pF capacitance for resonance at 3.5 GHz. Therefore, some more study is required as below...
A big clue to the challenges involved for Rigol is that they had to use tiny 0402 parts for C245 and C246. I think this is because C245 acts as the dominant cap in series with the varactor diodes. So it probably has to be >2pF.
There only has to be 0.5nH inductance in series with 2pF to make the 2pF cap look more like 4pF at 3.7 GHz. So tiny parts are needed here or they can misbehave up at 3.7 GHz... :)
Something like the Kemet HiQ-CBR series in 0402 package size will be needed. There are 0402 caps available from ATC and PPI that would also be fine here but they will cost more.
The higher the value of C245, the higher the VCO tuning gain of the VCO in MHz/V. It's important to try and achieve roughly the same VCO tuning gain (Kvco) as the original design because this affects the loop bandwidth of the PLL used for the first LO.
That's why it would be nice to know the correct tuning voltage at 3.186 GHz and at 3.786 GHz. If this is known, then it helps a lot. However, C245 needs to have similar package inductance to the original Rigol part. This needs to be really low and this is why there isn't much point trying 0603 or 0805 sized parts in this circuit.
wkb:
Thanks for your insights with regards to the physical size of the capacitors involved. I did not realise that stray inductance is that important, on the other hand I should have known that already :-//
So, I went and soldered a strand of wire onto the V1 varactor control voltage line. The outcome of what I saw on the o'scope is in the attached screen captures.
I captured both a full 9kHz - 1.5GHz sweep as well as the sweeps per VCO range. I guess the filenames of the attached captures are self-explanatory.
That all said: I admit, I am now quite confused :-/O
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