If the varactor you have has a larger capacitance variation than you need there are 2 things you can do. As I think you mentioned you can limit the voltage variation. This might be good as it could give you more linear operation using only part of the C vs V curve. On the other hand using a limited voltage variation could put you at different places on the curve from one unit to another requiring some adjustment.?
The second method is to put a capacitor in series with the varactor (as is done in the NC40A). Then you can use the full capacitance swing of the diode. A combination of both of these methods can also be used.?
73/72 - Mike WA8BXN
From:
[email protected] <
[email protected]> on behalf of Alun VK4LUN <alunandwendy@...>
Sent: Saturday, November 25, 2017 4:09 AM
To:
[email protected]Subject: Re: [qrp-tech] Substituion of Varactors in 2N2 40+
?
Soooo... I looked up the specs for the BB910 and got a Vr of 40v and capacitance of 40pf
Reverse voltage (VR):?? 30 V
Forward current (IF):?? 20 mA
Capacitance (VR max):?? 2.5 pF
Capacitance (VR = 0V):? 40 pF
So I'm looking good to replace the MV209? with a bb910 ??????
The 1SV149 is 300pf from 2v and up to 9v
There seems to be a gap in capacitance of varactors, with the am radio ones having high capacitance 400pfs and up, and the VHF UHF designs which have from 40pfs and down in capacitance.
So i am guessing it will be all about putting the appropriate voltage across the 1SV149 at the appropriate "time".
Am I on the right track????
Regards
Alun VK4LUN
