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Old 2nd Dec 2022, 3:12 pm   #1
dmowziz
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Default Crystal Oscillator

Hi...
Please any wisdom you can share will be appreciated.


Following this nice thread. 6 gang FM Tuner
https://www.vintage-radio.net/forum/...d.php?t=189678


I've been learning about crystal oscillators for a week now trying to calculate the power output and compare with simulation but in one week, what progress did this thread starter make (I don't know)


Want to make 2 crystal oscillators to use for mixer IMD measurement.
100 MHz and 101 MHz






The first image (gotten online) is an almost accurate crystal response

My first tested crystal's (20 MHz, 5th overtone) fs is 99.9 and the fp is 100.005.

I want to use a 2N3904. Operating it at fs (The crystal is just it's resistance) because in the inductive region, the ESR of the crystal is increased, greater than the negative resistance created...

Texts say crystal oscillators operate at fs or in the inductive region (between fs and fp)
Is it possible to make it oscillate at 101 MHz, operating the crystal in the capacitive region? (I tried but nothing yet)






Can the tones for IMD be 5 MHz apart?
the 100 MHz crystal and a (35 *3 ) crystal








The first tone schematic is shown. Output power is about 3dBm in simulation

But my board does not oscillate until emitter current is about 13mA which produces a -12.7dBm output

I also attached image a pic of the board..
Noticed from my previous oscillator that a big wire gauge compared well with simulation (but I can change to a smaller )
The inductors are 230nH and 74.27nH

Thank you very much for your input
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Last edited by dmowziz; 2nd Dec 2022 at 3:39 pm.
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Old 2nd Dec 2022, 3:32 pm   #2
G6Tanuki
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Default Re: Crystal Oscillator

Crystals can be 'pulled' by adding series or parallel reactances; however the amount of pulling available is very low - certainly not enough to make a 20MHz 5th overtone one generate a 101MHz output.

Also note that the frequency-stability degrades the further you pull a crystal from its natural resonance.

[For reference, I have a 5.5MHz xtal in a transmitter here which is operated at its fundamental mode and I can reliably pull it about 1.2KHz either side of its natural resonsnt frequency].
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Old 2nd Dec 2022, 5:51 pm   #3
Al (astral highway)
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Default Re: Crystal Oscillator

Sure you can get a calibration-standard Xtal oscillator going with some help from folks on this forum - lots of specialist expertise around.

It does sound you'd also like to learn along the way, and are open to a bit of experimentation. And I sense a smidgeon of frustration already ? so I hope you keep things light and fun on the way. So much easier to keep going if it's enjoyable.

So I'm thinking, how about starting with an established circuit or two to begin with, rather than trying to design something from scratch?

Maybe look up a circuit or two that uses tuning to the 2nd harmonic of a crystal, and how that's done? (Not saying this will fit your intended purpose, btw). Or find other ways of generating a stable signal at the frequency you're aiming for?

That will also help you get some confidence with practical layout and RF ground principles, which will help you as you aim for a stable, calibration-grade bit of kit.

I see that you're trying to keep things tight and organised, using SMD passive components, but on the other hand, you have these giant inductors that may be coupled in a way that you didn't intend, and you've etched away most of the ground plane on the board. What was your intention there?

Heads up... don't be too hung up on the precision of home-made inductor measurements, especially tiny ones! (You say 74.27nH, but that's too precise).
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Last edited by Al (astral highway); 2nd Dec 2022 at 6:08 pm.
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Old 2nd Dec 2022, 8:11 pm   #4
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Default Re: Crystal Oscillator

The pullability of a crystal oscillator reduces with increasing overtone number.

Roughly, the amount you can pull an overtone oscillator is proportional to the reciprocal of the overtone number squared. So if you want to go pulling far, you get forced into using a fundamental crystal oscillator followed by a frequency multiplier chain.

Even for a fundamental oscillator on a favourably low frequency, getting 1000ppm pulling is an achievement. 100ppm is more reliably successful.

For this reason, many of the crystals used in 10MHz reference oscillators, where fundamentals are easily made, are actually overtone units, used to reduce the pulling from strays in the rest of the circuit.

There is general lack on anything written about the running amplitude and stability of oscillators, even less on crystal oscillators. You're straying into the darker areas of RF design. Rules of thumb from past experience get used. Quartz manufacturers sometimes have to use quite different designs of crystal surface profile for resonators just a few kHz different in Fs.

There are parallel resonance oscillator circuits. All crystals have the full family of fundamental and overtone modes. Each of these is a pair with one series, one parallel. Ordering up a crystal with a specified mode (and loading) just means that the crystal is ground to put that mode on the specified frequency. All the other modes are still there, usually with the fundamental the most active, but they do try their best to get activity on the specified mode. It won't necessarily be the most active. So some LC resonance is essential in any reliable overtone oscillator.

Modelling can give you a rough idea of running amplitude. If amplitude is important, then a detector and ALC loop is needed.

David
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