Electronics World articles Popular Electronics articles QST articles Radio & TV News articles Radio-Craft articles Radio-Electronics articles Short Wave Craft articles Wireless World articles Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs LinkedIn Crosswords Engineering Humor Kirt's Cogitations RF Engineering Quizzes Notable Quotes Calculators Education Engineering Magazine Articles Engineering software RF Cafe Archives RF Cascade Workbook 2018 RF Symbols for Visio - Word Advertising RF Cafe Forums Magazine Sponsor RF Cafe RF Electronics Symbols for Visio RF Electronics Symbols for Office Word RF Electronics Stencils for Visio Sponsor Links Saturday Evening Post NEETS EW Radar Handbook Microwave Museum About RF Cafe Aegis Power Systems Anritsu Alliance Test Equipment Amplifier Solutions Anatech Electronics Axiom Test Equipment Berkeley Nucleonics Bittele Centric RF Conduct RF Copper Mountain Technologies Empower RF everything RF Exodus Advanced Communications Innovative Power Products ISOTEC KR Filters Lotus Systems PCB Directory Rigol RF Superstore San Francisco Circuits Reactel RFCT TotalTemp Technologies Triad RF Systems Windfreak Technologies Withwave LadyBug Technologies Wireless Telecom Group Sponsorship Rates RF Cafe Software Resources Vintage Magazines Thank you for visiting RF Cafe!
Innovative Power Products Couplers

Colpits Oscillator help - RF Cafe Forums

RF Cafe Forums closed its virtual doors in 2010 mainly due to other social media platforms dominating public commenting venues. RF Cafe Forums began sometime around August of 2003 and was quite well-attended for many years. By 2010, Facebook and Twitter were overwhelmingly dominating online personal interaction, and RF Cafe Forums activity dropped off precipitously. Regardless, there are still lots of great posts in the archive that ware worth looking at. Below are the old forum threads, including responses to the original posts.

NOTICE: The original RF Cafe Forum is available again for reading, and the new RF Cafe Blog is an active board.

-- Amateur Radio

-- Anecdotes, Gripes, & Humor

-- Antennas

-- CAE, CAD, & Software

-- Circuits & Components

-- Employment & Interviews

-- Miscellany

-- Swap Shop

-- Systems

-- Test & Measurement

-- Webmaster


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

Anatech Electronics - RF Cafe
Lotus Communication Systems Modular RF Component Building Blocks - RF Cafe
Berkeley Nucleonics Corporation - RF Cafe
RF Cascade Workbook 2018 by RF Cafe
Anritsu Test Equipment - RF Cafe

Please Support RF Cafe by purchasing my  ridiculously low−priced products, all of which I created.

These Are Available for Free


About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

Copyright: 1996 - 2024


    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 World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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: