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

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?

I am not sure how the transistor gets its base current?
Frank

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: http://www.adv-radio-sci.net/4/21/20...4-21-2006.html

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

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from here: http://www.ask.com/web?q=amw9908+dro...rc=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
(

) harmonic choosen by microstrip length and band-pass filter, and the second one (

) is coupled to DRO, so frequency is defined by DRO =)

yes, you are right. The 1st is by BPF and microstrip length and the 2nd is by DRO.

I would suspect 5~20 pF 402 cap with SRF>< operating freq.

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!