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> Please Help, to build a 21 mhz oscillator
Sumermagor
Posted: August 16, 2010 11:14 pm
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hello, as you can see im new on this forum, and i have very little knoweledge about electronics yet... doh.gif
i would like to ask your help to design a circuit which will give me a stable 21 megahertz Ac sinewave signal, i would use it for one of my projects.
i really appreaciete your help!!
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CWB
Posted: August 17, 2010 02:48 am
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and the project is ?


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779219
Posted: August 17, 2010 05:52 am
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QUOTE (Sumermagor @ August 17, 2010 12:14 am)
hello, as you can see im new on this forum, and i have very little knoweledge about electronics yet...  doh.gif
i would like to ask your help to design a circuit which will give me a stable 21 megahertz Ac sinewave signal, i would use it for one of my projects.
i really appreaciete your help!!

You can use an LC oscillator like this:
user posted image
I haven't tested it but it should be moderately stable. If it fails to oscillate, the value of the 2.2pF capacitor can be increased. Also reducing the value of the 1.5k emitter resistor can be useful. The oscillator is based on the Vackar circuit, output signal appears at the collector of the BF240 transistor.
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tekwiz
Posted: August 17, 2010 05:51 pm
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QUOTE (779219 @ August 16, 2010 08:52 pm)
QUOTE (Sumermagor @ August 17, 2010 12:14 am)
hello, as you can see im new on this forum, and i have very little knoweledge about electronics yet...  doh.gif
i would like to ask your help to design a circuit which will give me a stable 21 megahertz Ac sinewave signal, i would use it for one of my projects.
i really appreaciete your help!!

You can use an LC oscillator like this:
user posted image
I haven't tested it but it should be moderately stable. If it fails to oscillate, the value of the 2.2pF capacitor can be increased. Also reducing the value of the 1.5k emitter resistor can be useful. The oscillator is based on the Vackar circuit, output signal appears at the collector of the BF240 transistor.

Oscillators like this are only relatively stable if enclosed in a shielded, temperature controlled enclosure.
For decent stability without the extra precautions, a crystal is required. A crystal can be added to just about any single transistor LC or RC oscillator with little trouble.

This page explains most simple crystal oscillators, & includes circuits:
http://hem.passagen.se/communication/txo.html


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To him, mere toys of the moment, to be overturned at the flick of a finger.

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Sumermagor
Posted: August 17, 2010 11:42 pm
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Thanks 779219 and Tekwiz, im trying different options too, because i dont have a crystal resonator at hand, i could try with what i have at hand, but dont know where to start..... i have downloaded National instruments program once, but it was only a 30 day demo dry.gif
so, i have a chip at hand which is 74HC4538E and several Npn transistors , i have plenty of this chip, and do you guys know if i could make it with this? and so if, how? blink.gif
thank you very much from all of you guys smile.gif

This post has been edited by Sumermagor on August 18, 2010 12:21 am
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Sch3mat1c
Posted: August 18, 2010 05:03 am
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Would you happen to have any specs?

Like, is it 21MHz for how long?

I can give you a noise generator which will look like 21MHz to some frequency counters. Somehow I'm guessing that's not good enough.

The other extreme, make a PLL locked to a Hewlett Packard 10MHz reference, disciplined with a 1Hz GPS tick. Now you get 21.000000000000MHz, locked to the best atomic clock on Earth. Maybe that's a little overkill.

So if it can vary some, we can give you a simpler circuit. How much depends on how much it can change, and under what conditions (voltage, temperature, position of planets, ...).

Tim


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Village Idiot
Posted: August 18, 2010 07:05 am
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QUOTE (779219 @ August 16, 2010 10:52 pm)

You can use an LC oscillator like this:
user posted image
I haven't tested it but it should be moderately stable. If it fails to oscillate, the value of the 2.2pF capacitor can be increased. Also reducing the value of the 1.5k emitter resistor can be useful. The oscillator is based on the Vackar circuit, output signal appears at the collector of the BF240 transistor.

This may be fine for your purposes, depending on how stable you need it to be. As Sch3matic mentioned, specs would be useful. Another important point is what kind of test equipment do you have? Do you have a frequency counter so that you can set the frequency once it's built? If not, you may be better off spending a few dollars and get something that you know is actually oscillating at 21 MHz, such as Fox FXO-HC730-21. Though this puts out a square wave, a simple filter would turn it into a sinewave.
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tekwiz
Posted: August 18, 2010 06:08 pm
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QUOTE (Sumermagor @ August 17, 2010 02:42 pm)
Thanks 779219 and Tekwiz, im trying different options too, because i dont have a crystal resonator at hand, i could try with what i have at hand, but dont know where to start..... i have downloaded National instruments program once, but it was only a 30 day demo dry.gif
so, i have a chip at hand which is 74HC4538E and several Npn transistors , i have plenty of this chip, and do you guys know if i could make it with this? and so if, how? blink.gif
thank you very much from all of you guys smile.gif

It wouldn't surprise me if you could salvage a 21mhz resonator from a junked computer accessory...resonators are very common in these & 21mhz is one of the frequencies commonly employed.
21mhz crystals(not resonators; there's a difference) are also common in old CB radios.

Like Tim mentioned, your requirements will determine which circuit you can use, with the most important being your required frequency stability.


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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Sumermagor
  Posted: August 18, 2010 07:09 pm
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Hi Everyone, thanks for all of your inputs, i do have measuring equipment, the only thing i dont know is where to start...
The specifications of the oscillator would be that it have to be stable in at least 0.1hz resolution,and it has to be the same all the time , do planets orbiting really change its frequency? shock.gif
do anyone has a parts list of what i will need? still looking where can i get those critical capacitors of a low picofarad values,
thank you v much guys smile.gif
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Village Idiot
Posted: August 18, 2010 09:17 pm
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QUOTE (Sumermagor @ August 18, 2010 12:09 pm)
The specifications of the oscillator would be that it have to be stable in at least 0.1hz resolution,and it has to be the same all the time...

Well, that eliminates the LC circuit. There's a remote chance you could do it with a crystal and an oven. But, you're in the realm of rubidium frequency standards now.
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Sumermagor
Posted: August 18, 2010 10:08 pm
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i guess i have to try any 21 mhz oscillator then rolleyes.gif , what would be my best choice? i will have to keep myself to the opinion of yours, because im not very familiar with these yet. if i could have a good schematic, i could buy the components and build it smile.gif
thank you:)
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Sumermagor
Posted: August 19, 2010 10:28 pm
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So is there anyone out there who would help me? wink.gif
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CWB
Posted: August 19, 2010 11:46 pm
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weeelll ... your requirements are a little steep for the average person to construct ...
.1HZ resolution would require a stability at least a magnitude (or two) greater than the frequency you need to ensure/monitor .
this is indicated by/in these replies:

tim :
"The other extreme, make a PLL locked to a Hewlett Packard 10MHz reference, disciplined with a 1Hz GPS tick. Now you get 21.000000000000MHz, locked to the best atomic clock on Earth. Maybe that's a little overkill."

VI :
"Well, that eliminates the LC circuit. There's a remote chance you could do it with a crystal and an oven. But, you're in the realm of rubidium frequency standards now."

in the method tim mentioned ... i have seen/used a similar scheme to ensure the phase delay in time/distance of a couple of on channel FM boosters ...
the price for the unit to accomplish this was around 35K USD .

maybe a place to start is with more information about what you are doing ?


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Sumermagor
Posted: August 20, 2010 12:46 am
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hello:) now i have to watch what im writing here because i said my requirement is of 0.1 hz resolution, but it is unnecessary, because im just experimenting here, i need to build an ordinary 21 mhz oscillator circuit, the one which my measuring equipment will recognize as 21 mhz,
maybe a place to start is with more information about what you are doing ?
ihave frequency generators at home but none of these high frequencies, im experimenting with electronics at home for a hobby. i also would like to know more about amplifiers and other circuits. but first i have to build this oscillator.
Thank you
Sumermagor
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Sch3mat1c
Posted: August 20, 2010 01:00 am
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As RC oscillators go, this will run 35MHz+ depending on values. It's adjustable, so you can tweak it arbitrarily close to 21.0MHz. Don't expect great stability (in fact, expect a large amount of drift).
http://webpages.charter.net/dawill/tmoranw...08/Triangle.gif

The conventional LC oscillator (in myriad forms; I'm partial to Hartley, Armstrong and Colpitts) is easy to set up and get working. Offhand, 100pF will resonate with 0.57uH, which is a couple turns of wire. Less C and more L is probably more typical (~22pF, 2.2uH). A 2N3904 or similar transistor will do a fine job.

Tim


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Sumermagor
Posted: August 20, 2010 01:49 pm
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what about the Chip that i have? the 74HC4538E can i make an oscillator with it? because i read that this is a kind of chip thats been used for oscillators, http://pdf1.alldatasheet.com/datasheet-pdf...D74HC4538E.html

i dont really understand the schematic here, may be you can help
thank you
Sumermagor
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Sch3mat1c
Posted: August 21, 2010 06:29 am
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Yes, you can make an oscillator with that. It's a one-shot timer, so as timer 1 ends, it triggers timer 2, and when it ends, timer 1 triggers...

Tim


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Sumermagor
Posted: September 11, 2010 05:21 pm
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i just heard it it is illegal to operate these frekvencies and i need to shield it?! i dont want trouble. can somebody provide some info about this?
thank you guys
Sumer
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Sumermagor
Posted: September 11, 2010 05:53 pm
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if i send 21 megahertz pulses to a coil at about 10 watts, do i have to shield it? because it becomes a transmitter then, what material and what thickness would it be good to may be make a box of it then put the device in it.
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ChipUser
Posted: September 11, 2010 06:04 pm
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Actually, it will be best to shield the whole circuit as any length of conductor carrying this frequency will radiate like an antenna. Any metal should be be fine. 1 mm thick Aluminum should be enough and easy to build with. If you expect the box to be subject to any kind of mechanical stress, use a thicker gauge. Ground the sheet metal if you are using this with some other circuits.
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CWB
Posted: September 11, 2010 09:34 pm
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google : "hf (or) rf shielding techniques" .
you will need to bypass/choke the power leads into the box in order to keep them from radiating the signal .


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