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Can I repalace varactor with capacitor to check if other parts of VCO are working?

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If it is possible, wich kind of capacitor is applicable, wich capacity of mF?
Thanks!
 

depending on the freq of the oscillator, nost varactors would be only a few pF say as a ballpark figure 2 - 15pF
so, without knowing more about your particular circuit, it would be worth trying a disc creamic of ~ 10pF

cheers
Dave
 
Thank you!
Here is schematic:

Frequency maybe 5..10Ghz, i guess it from lambda/4 size of band-pass filters.
 

greetings

Tho I have built up a bit of 5 and 10 gig gear for amateur radio use. Im not a designer at those freqs
but now knowing the freq, I would be considering quite a small capacitance of the varactor diode, maybe ~ 1 - 5pF

Hoping one or 2 of the real designers that on this forum may be able to get a bit more specific for you :)

best experimenting

Dave
 
Thanks again! Maybe someone know which kind of feedback this scheme is? Is it series feedback with microstrip of special length on base of transistor? But i thought it must also deal with emitter?

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Thanks again! Maybe someone know which kind of feedback this scheme is? Is it series feedback with microstrip of special length on base of transistor? But i thought it must also deal with emitter?
 

chuckey said:
I am not sure how the transistor gets its base current?
Click to expand...
I am even not sure if it is base (NPN) or gate (FET)...
As i can see from schematic, base is connected to DAC, where small voltage 0v to maybe +2v goes.
I've found something similar here: https://www.adv-radio-sci.net/4/21/2006/ars-4-21-2006.html
feedback2.gif

"harmonic oscillator". Maybe it is almost the same, but common-source FET harmonic(?) oscillator.

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feedback.gif
from here: https://www.ask.com/web?q=amw9908+dro&search=&qsrc=0&o=0&l=dir
another Similar shceme BUT with DRO.


I am comfused about difference between this two schemes. I think the first one
(feedback2.gif) harmonic choosen by microstrip length and band-pass filter, and the second one (feedback.gif) is coupled to DRO, so frequency is defined by DRO =)
 
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yes, you are right. The 1st is by BPF and microstrip length and the 2nd is by DRO.
 

Thank for your suggestions! I soldered one 20pF capacitor in 805 package. A little big package dimensions, but it worked! No frequency tuning though, only oscillation. Please suggest some varactor i can put in there, 402 or 805 package. I hope there are some big ones, because i lost the original one tiny thing while soldering it out. Thanks!
 

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