Electronics World Cover,TOC,and list of posted Popular Electronics articles QST Radio & TV News Radio-Craft Radio-Electronics Short Wave Craft Wireless World About RF Cafe RF Cafe Homepage RF Cafe in Morse Code Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs Twitter LinkedIn Crosswords Engineering Humor Kirt's Cogitations Engineering Event Calendar RF Engineering Quizzes AN/MPN-14 Radar 5CCG Notable Quotes App Notes Calculators Education Magazines Software,T-Shirts,Coffee Mugs Articles - submitted by RF Cafe visitors Simulators Technical Writings RF Cafe Archives Test Notes Wireless System Designer RF Stencils for Visio Shapes for Word Search RF Cafe Sitemap Advertising Facebook RF Cafe Forums Thank you for visiting RF Cafe!

Colpits Oscillator Help - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

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

RF Cafe Software

   Wireless System Designer - RF Cafe
Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chartâ„¢ for Visio
Smith Chartâ„¢ for Excel

About RF Cafe

Kirt Blattenberger - RF Cafe WebmasterCopyright
1996 - 2022
Kirt Blattenberger,

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:

Try Using SEARCH
to Find What You Need. 
There are 1,000s of Pages Indexed on RF Cafe !