Crystal Oscillator Problem

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Is this all homework? Please tell us a bit more about yourself and what you are doing.

Well found trail of problems, or should I say postings Bruce. I just might suspect that finals week has descended upon us. The question is, should we ignore or produce a plausible yet bogus answer to guarantee failure?

GA

sir,

can you please help me answering that what type of pulse shape does a crystal oscillator give?

if it gives different types of shape, then should we mention that required shape when we will buy it (to avoid the use of waveform shaping ckt)?

It all depends on the ancillary circuitry attached to the crystal itself. This is why somebody here recommended that you use a fully contained oscillator package that has a buffered output. But if you really wish to do an analog analysis of a quartz crystal, from Electronic Design, From Concept To Reality by Martin S Roden and Gordon L. Carpenter, Third Edition page 630 Chapter 11.

The equivalent circuit for a crystal is shown {Rs,L,C_s in series paralleled by C_p} Since the Q is so high we can neglect Rs. We write the equation for (crystal) impedance as

Z(s)={s²+1/(LC_s)}/{sC_p[s²+(1/C_s+1/C_p)/L]}

Two frequencies exist, one for the zero and one for the pole.

ω_z=1/√(LC_s)

ω_p=√{(1/L)(1/C_s+1/C_p)}

C_p is much larger than C_s, so the two frequencies are approximately equal.

Now this means that only at the resonant frequency will the impedance be a low impedance. The crystal acts like a very narrow bandwidth bandpass filter. What I didn't put the effort to show was the 74HC04 oscillator circuit that this is from that has a 60 pF adjustable trim capacitor to tweak this into oscillation. Circuit board impedance changes from uncleaned flux and oscilloscope probes will change the circuit operation.

Now that I've given you all of the math, the oscillating wave shape depends on the circuitry around a crystal. The buffered output signal could be a square wave, sine wave or even with some clever circuit topology a triangular wave.

sir,

we have very high Q factor of crystal oscillators as an advantage.

I want to know that do we have any other pursuits compared to crystal oscillators like LC tuned oscillators--colpitts or hartley? ( the cost effectiveness is also an issue here, sir)

If you are having a problem with the oscilator take a look at the "NXP"data sheet for the 74HC4060. It has good information regarding the design of the oscilator ckt which is built into the counter. You could also use a seperate NAND gate. I have used both. Check out Fig 13 on page 16 it shows typical ckt. I used a 11.288MHz crystal, a bias resistor of 4.7M ohms R2= 2.2K and both Caps = 22pF. The oscilator is solid as a rock and the Rs & Cs are not very critical so you should get the same performance in both your breadboard and PCB lay out. One word of caution, Use Vcc bypass cap right at the IC ( I used a ceramic .1uF & Tan 10uF in parallel) Rember you are working at 11MHz but the rise and fall times could put you close to 50MHz so a clean glitch free Vcc - Gnd is critical. Vcc and Gnd layers in your PCB would be advisable. ----- Good luck

Make sure your crystal is CLOSE to the pins on the micro c where it connects... also the components associated with the crystal should be close. I don't know what kind of breadboard you are using, but if it is one which is wired with plug in wires... I would take a different approach. Personally, I still wire wrap.

Bill

My soldering iron and "stash" of real live 60/40 solder are treated like gold. Nobody knows where I keep them hidden and nobody uses but me then only in an emergency...

I strongly agree that close connections are required for all components associated with the oscillator. That having been said (my total track length for crystal, capacitors, and grounding, is generally less than 1.5 inches), in my 35 years of hands-on MPU/MCU design I've never had a well-connected oscillator fail. I do use good domestic crystals!

Don't use the crystal connections to provide external clocks. It doesn't work. My personal preference is to use an external crystal module for any application where external loading might be a factor. By the same token, use lower clock frequencies where possible to minimize radiation/RFI and to save power. My favorite is 3.6864 MHz for '51-based designs.

For that breadboard, since the other holes are there as load, whether you need them or not, I would use a crystal module and take the hit on cost. It saves on worry about the most important signal in the design.

Finally, look more carefully at the programmng around your LCD module! Good luck.