Crystal Oscillators / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Crystal Oscillators

Move Thread

LAN_403

Dave Marshall

Sat Dec 02 2006, 11:24PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Ok, I'm completely and thoroughly frustrated. I've spent 2 days working on a VLF upconverter to bring signals from 10-500Khz up to a frequency my HF radio can hear.

I've got the mixer, filters, power supply, everything worked out, but the oscillator has completely stumped me. I've spent hours on google, gone through multiple ARRL handbooks, 3 EE textbooks and assorted electronics circuit books and math books.

How the bloody hell do you actually DESIGN a crystal oscillator? I've got a 6Mhz crystal here, but the single schematic I found that could be applied was a sort of universal circuit using a 7404 chip that just refused to oscillate. Every other circuit I've found in the last two days has been for some arbitrary frequency with absolutely no information on calculating values for a different frequency. It seems if you aren't using a frequency everyone else uses, you're just SOL.

I'm not looking for anything complicated. 6Mhz, 200mV peak to peak, and without any fancy ICs that I haven't got. Seems like a colpitts or pierce oscillator would do the trick. I've got several little fets to work with.

So, who wants to school me in how these things are supposed to work?

Dave

Hazmatt_(The Underdog)

Sun Dec 03 2006, 03:50AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

You should be able to make any frequency you want.

After the semester ends I'm going to play with a PLL frequency synthesizer with thumbwheel switches. Should be fun.

ShawnLG

Sun Dec 03 2006, 04:12AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

Gees Dave. I ususally get something to oscillate and then try to get a clean signwave out of it. Try this circuit and put a low pass butterworth filter on the output to get a good signal.

Steve Conner

Sun Dec 03 2006, 12:37PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The circuit values aren't really critical, except if there's a tuned LC circuit, which of course should be changed so it resonates at the same frequency as the crystal. Some circuits (the Colpitts springs to mind) use a capacitive divider to give the feedback signal to make it oscillate. So, you should keep the ratio of the two capacitors the same if you change them to alter the frequency.

Note, the first link gives a circuit with instructions on how to change it for different frequencies. The Pierce oscillator in the second link's Fig. 5 seems nice and simple too.

Reaching

Sun Dec 03 2006, 02:00PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

or you just buy a ready to use crystal oscillator, available in every frequency and worth the cost of around 2$, cause you only need to add 5volts on them and you get exact the freqeuncy printed on.no need for external components

its the size of a 14 pin dip IC . dont know if they are very common in the states but here in germany they are.

Dave Marshall

Sun Dec 03 2006, 03:48PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Those 4 pin oscillators put out a pretty ugly square wave that would require a substantial bit of filtering to get a usable sine wave. It seems they tend to be lower tolerance also, the one I've got here is 1Mhz, and has a stability of 100ppm.

I'm probably going to go with a 32Mhz crystal. I've ordered a 74HC4060 to divide it down for added stability, then I'll filter it to clean it up. I was hoping to get this done by the time I left for the sandbox next Sunday, but I'm not sure thats going to happen now.

I found several sites with some general purpose pierce and colpitts oscillators. Haven't gotten one to actually oscillate yet, but I'll work on that today.

Thanks for the advice guys,

Dave

Steve Conner

Sun Dec 03 2006, 03:52PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Reaching, those things work great, but they don't come in all the frequencies you might need. You can buy custom cut crystals for ham radio projects, but you can't get them packaged in oscillators, AFAIK.

Dave: If you use a switching mixer, they actually run better off a square wave LO. The odd harmonics of the LO will produce image responses, but you can easily remove them by filtering since they're so far from the wanted signal.

Sulaiman

Sun Dec 03 2006, 08:22PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

If you don't mind an 'odd' frequency,
the XTO for lifejackets etc is cheap and stable
Typ. 12.688375 MHz +/- 2 ppm over temp/volt range.

Carbon_Rod

Sun Dec 03 2006, 11:49PM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

For fixed frequencies (1 to 48MHz) in a jam I have used a OTP PIC and piggy backed the clock driver pin. I needed a way to insure low standby power and recover from crystal lockup with the RC WDT.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob