Hi i wish to create a carrier signal of 27mhz using a crystal. Unfortunately, i have no ideas on how to build an oscillator circuit that is suitable for use in 27mhz. At first i tried a Colpitts Oscillator diagram i found in a book but later found out that it is limited to 20Mhz only... :cry:

So does anybody know how to build a simple one for 27mhz??

 

Physical limitation of Crystals are about 15MHz, to get a higher frequency you must multiply or use harmonic overtones, so a colpits will not work since it is a LC atuned OSC.

A overtone crystal oscillator can be made to gain only on 27MHZ with a 13.5MHz crystal.

__________________
Your website has been blocked

 

It's fairly common to use an oscillator followed by frequency doublers or triplers. Worth noting - when operated in overtone mode some (maybe all) crystals are such that the overtone frequency is not an exact integral multiple of the fundamental frequency - it's close but not exact. Amateur radio publications can provide lots of detailed information.

__________________
stevez

 

huh really? how come i was able to buy a 27mhz crystal?

 

If you take a common CB transmit crystal at 27MHz and use it in an oscillator circuit you may find it wants to oscillate at 9MHz unless the circuit is tuned to 27MHz. By the correct resonant elements the phase-shift and loop gain will favor oscillations at 27MHz and suppress it at 9MHz

What a previous poster indicated is that crystals oscillating in their fundamental mode is normally limited to about 15-20MHz and higher frequencies can be obtained by oscillating in a overtone. One reason for this is that with higher frequency the quartz slice get's thinner and more fragile and it is very easy to overdrive this thinner slice in terms of mW's, resulting in serious heating and frequency shift of the crystal.

 

It's probably an overtone crystal, it will probably have a fundimental resonance either a third or a fifth of it's marked value.

A quick google search found you this http://w1.859.telia.com/~u85920178/tx/cw-am_00.htm which you may find helpful?.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

ok thanks for the information. I tried simulating the circuit below, which is a colpitts oscillator taken from a book. But the output i get is 0. May i know what have i done wrong? The program is Electronics Workbench 5.12

Attached Thumbnails

 

 

The ratio of the two 470pF caps should be larger than 1 to overcome circuit losses and still provide enough loop gain to oscillate. There should be a 0.01uF cap or so between base and +V to provide a low impedance path for feedback.

 

Here is a working circuit around 25MHz you may try. Transistor can be kept the same (2N3904) as any general purpose NPN will work

Attached Thumbnails

 

 

Try here:

http://www.quartslab.demon.co.uk/3rd_overtone.htm

Lots of info about crystals and oscillators.

JimB

 

hmm i tried simulating another circuit but still there is no output. Is it the circuit or does the simulator having problems simulating oscillators?

Attached Thumbnails

 

 

It oscillates for me after making the top 47pF a 150 pF, but then also I use another simulation program. Frequency about 2.8MHz. Also I lowered the value of the emitter resistor to about 180 Ohm to achieve better I/P and O/P matching of the transistor. Remember some simulators have a problem with oscillator circuits if you don't set it up correctly. I am not familiar with the one you using.

:idea: You can try and make the top 47pF much bigger (1500pF) just to see if it is a loop gain problem and if there's any attempt to start up.

Below is a plot of the output frequency

Attached Thumbnails

 

 

Here is a typical example if there is not enough gain in the system. From the plot you can see there's an attempt to start up but not enough gain to sustain oscillations. In this example I made the top 47pF cap about 100pF

Attached Thumbnails

 

 

Hmm must be my lousy program because it still doesn't work. But thanks for all the effort You truly are The One

 

By the way, what program did you use?