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Colpits Oscillator help - RF Cafe Forums

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Post subject: Colpits Oscillator help. Posted: Sat Oct 06, 2007 9:26 pm


Joined: Wed Sep 19, 2007 6:58 pm
Posts: 3
I am designing a Cplpits oscillator for my senior design project. I want it to oscillate at two frequencies so I plan on switching in and out a variable capacitor within the tuning ckt. below is my circuit. The two frequencies are 15.5 and 16.5 Mhz. I was planning to simply use a fet as an analog switch but I am having a very hard time finding a NFET with a low enough drain to source capacitance. Does anyone have a specific NFET in a non surface mounted package that could work for this application or another suggestion as to how I can easily switch in and out this capacitor? In the schematic below I have a black box as the switch. the varicaps are to compensate for the discrete components I will be using. Thank you.


Post subject: Posted: Mon Oct 08, 2007 5:42 am


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Don
You might be better off using a forward-biassed Schottky diode rather than an NFET. The diodes are fast, and the Scottky has a very low on-resistance so the Q is not badly effected. Your circuit has a path to ground for the switching current, and you just need an RF choke towards the bias supply. When OFF, the circuit just sees the one capacitor, and when ON it sees both in parallel. You might also consider PIN diodes, but I seem to recall that most are poor performers at lower frequencies such as 16MHz.
A more elegant solution is to use a varactor diode switched between the voltage bias that gives the two values of capacitance required. Perhaps the frequency stability with this method would not be as good as the diode switch however.


Post subject: Posted: Mon Oct 08, 2007 11:51 pm


Joined: Wed Sep 19, 2007 6:58 pm
Posts: 3
Thank you I will investigate the diode idea further.

Posted  11/12/2012

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