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Need help building oscillators using crystal

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patrickian01

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Hello,

I want to build oscillators using crystals and transistors, however, I can't get them to work and I don't know why. can anyone help me with this? with the circuit, complete with the components and values as well as the math involved. If it helps, the frequency that I'm trying to obtain are ~72MHz, ~46MHz, ~63MHz, ~18MHz, ~39MHz. Now, I've tried them using an 74LS04 IC with the inverters and they work, the only problem is that all my crystals oscillate at around 1/3 of the frequency, I understand that these are overtones and the oscilloscope reports it as a square wave and I need a sine wave because I will be using it for transmission. please help me, i really really need to finish this project. thank you. :)
 
Google 'overtone crystal oscillator'.
 
I would have expected the 18 MHz crystal to be a fundamental one.

You will need inductors to tune the oscillators so that they have a lot less gain at the fundamental frequency than the overtone frequency.

In most designs of oscillators, the current in the transistor turns completely on and off each cycle. Trying to design an oscillator that is completely linear is very difficult, and will need an automatic gain control. Where the transistor is turning on and off, the current will be non-sinusoidal, so further tuned circuits are needed to get a sinewave output.
 
thank you so much for the link. I'm going to take a look at it later. I have tried googling it and gave me circuits that didn't work and had no explanation as to why the circuit looked like it. the 18MHz is fundamental and I just need a simple circuit that would operate all my crystals on fundamental mode. I just need to produce sine waves for transmission, since I know that square waves generated by the 74LS04 would not be effective for transmission. :)
 
see the basic need for oscillation and / or sustaining oscillation appears to be positive feedback
insufficiency.
you have to have a close study of the transistor as amplifier and the amount of feed back between emitter and collector or collector and base with needed phase shift.

perhaps this article from TI can enlighten you better.
https://www.electro-tech-online.com/custompdfs/2013/06/sloa060.pdf
 
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Let's say I start with this transistor configured as a common emitter amplifier:
CE Amp.PNG
ignore the values in there first and have a look at my computations below (since multisim won't allow me to place components without values)

Since V1 is 6V, if I want a voltage drop across R3 to be 3V and Ic (current through R3) to be 1mA, then R3 = (V1 - 3V)/1mA = 3k ohms
Since Ic ~ Ie (Current through R4), assuming that R4 = 100 ohms, Ve (voltage accross R4) = Ie*R4 = 0.1V
Assuming that Vbe (base-emitter voltage is 0.7V) and assuming that B (beta) = 100, Zin (input impedance) = B*R4 = 10k ohms
Calculations with R1 and R2 could be ignored since I will be using crystals (AC), am i right? If so, what values could I use for R1 and R2? and I understand that the crystal goes through the collector and the base, If the crystal is now added, what other things do i consider and where do i place the capacitors to take into account the phase shifts that i need?
 
you might use 10K each.R4 could be 1K and R3 needs to be less perhaps. you might select it
to make collector sit at 2.5V approximately the emitter bypass cap could well be .01uF
 
ok thanks. i will be setting R1 and R2 to be 10k each and R4 to be 1k. If I would the collector voltage to be at 2.5V, and I would still like the collector current to be 1mA, R3 should be set at 3.5k, which is more than the original 3k. If I would set it to be lower, then I should set the collector current to be lower, what value would you suggest the collector current to be and why? Also, the gain of the transistor originally was -30 while the new gain would be lesser, at most -3 when R3 is 3k and R4 is 1k. why would i need to lower the gain?
 
I just had an idea... so, i built an oscillator using inverters with the 74LS04 IC and it produces square waves as said by the oscilloscope although they appear to be sine waves. since the crystal acts as a filter for a specific frequency, what if I use the same crystal on a transistor oscillator and use the square waves generated by the 74LS04 IC to kickstart oscillation in the transistor, similar to what was done here: **broken link removed**

i tried the circuit using different crystals and it works, although the Vp-p decreases overtime (i suspect that by the 1s mark, it would have gone down to near 0), that is why i would like to use transistors to give it the needed gain to sustain oscillation. do you think it would work? :)

update---

So, i tried this configuration and got it to work!
here's what the circuit looks like: kickstart using square wave generator.PNG
i first connected the function generator and then opened the switch and it started to oscillate at sine waves even if the function gen fed square waves to the circuit. the only problem is, it started to die out the moment it started oscillating and at around 0.01ms, it's nearly in the microvolts level. any idea on how i can keep it oscillating at probably the millivolt level?
 
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it means that the transistor gain is insufficient. before of making one, and suffering, you better read and appreciate the philosophy behind sustaining the oscillations once started. hFE plays a picture, in determining the base bias for a specific collector current.
 
So i decided to go back and check my oscillators using the 74LS04 inverters and found out that the output was actually closer to a triangle wave than a square wave. I also inspected the FFT and found out that it is indeed a triangle wave with 2-3 harmonics, the strongest of which is at most half the strength of the fundamental frequency. the circuit looks like this:
IC.PNG
i could probably get a snapshot of the FFT and the wave produced by tomorrow so i can also upload it. I've also been trying to adjust the feedback potentiometer so that the gain would be just right to produce sine waves, however, the closest i can get to a sine wave is a triangle wave that LOOKS LIKE a sine wave but FFT suggests that there are odd harmonics, probably up to the 3rd odd harmonic. Is there a way to smoothen it further down to a sine wave so that i wouldn't resort to transistor oscillators? by the way, the output is 20mVp-p and assuming that it has already gone through impedance matching, is it already good enough to transmit via a quarter wave monopole and i would only transmit it for a few meters?
 
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