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Rigol DSA815 dead LO for the 900-1500MHz range

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G0HZU:
Because the collector is at AC ground, it's possible to use the manufacturer's s-parameter model for the BFP450 and re-arrange the circuit to make it look more familiar as a Colpitts oscillator.

In the image below I've represented the blue circle cap as C3. I've guessed the value as 1.8pF but it might be a bit different to this in reality. This is fitted across the collector and emitter pins of the BFP450 but if the collector is ground then it is more intuitive to redraw the circuit as below.

C7 is the internal base to emitter capacitance of the BFP450. Because this is already there inside the Q2 model, I've given C7 a value very close to zero in the simulation.

You can see that this now resembles a Colpitts oscillator with the capacitor based feedback. If I simulate it, the simulator shows that negative resistance is generated across a huge frequency range extending to beyond 4.5 GHz.

So the BFP450 can do its job as a negative resistance generator :) I've plotted the series resistance looking into the base rather than Rp but you can see the resistance at the base is negative. Don't pay too much attention to the value of the negative resistance as the circuit really needs to be modelled in a way that includes the PCB layout.

TurboTom:
The varactor diodes are most likely Infineon BBY58-02V, assuming the package is an SC79 and the marking is "8". The PIN diodes are BAR63-03W (SOD323, marking white "G"). The SOD323F diode with the "US" marking probably is a BZX84J-C6V8 zener. Unfortunately, none of the capacitors is marked.

Edit: I found a slightly better photo of the 1st LO section of the DSA815 in my archives, this one even just barely shows the blind vias at the "T" section of the resonator striplines, confirming they are hard-linked to the underlying ground plane.

G0HZU:
Thanks for the latest image that shows the ground vias. They appeared to be missing in the other images but they must be there.

In the marked up image below, the C247 cap at the top will be chosen to give negative resistance over the required frequency range. The value might get adjusted during board testing by Rigol to adjust the output power of the oscillator.

The (missing) C246 will also have some impact on the output power of the oscillator and it will also have a small impact on the tuning gain of the VCO in terms of MHz/V.

The other missing cap C245 will probably have the biggest impact on the tuning gain of the VCO in MHz/V. It may need to be selected on test to offset any tolerance issues with the varicap diodes and also with the etching tolerance of the resonators. Adjusting this cap value may mean that the other two caps also need to be adjusted to help keep the output power and tuning range close to the design goals.

I suspect that the analyser can measure the tuning voltage (vs frequency) of each VCO during a power on self test routine.

G0HZU:
If you can find out for certain what the varactor diodes are, I can try and model the whole oscillator layout using Genesys and Sonnet EM and maybe predict the approximate values of the missing caps.

It would be nice to know what the typical tuning voltage range is at the test point V1 across the full tuning range of the VCO. I'm assuming it will be something like 2V to at least 10V, maybe much higher. Could this be documented somewhere in a service manual?

wkb:

--- Quote from: TurboTom on December 14, 2024, 08:04:07 pm ---The varactor diodes are most likely Infineon BBY58-02V, assuming the package is an SC79 and the marking is "8". The PIN diodes are BAR63-03W (SOD323, marking white "G"). The SOD323F diode with the "US" marking probably is a BZX84J-C6V8 zener. Unfortunately, none of the capacitors is marked.

Edit: I found a slightly better photo of the 1st LO section of the DSA815 in my archives, this one even just barely shows the blind vias at the "T" section of the resonator striplines, confirming they are hard-linked to the underlying ground plane.

--- End quote ---


My varactor diodes are marked BY, not 8. Whether that means anything, or just a different vendor was used, who knows.

Soldering on your PCB is a helluvalot better than on mine by the way.

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