Crystal oscilator and load capacitor?

[Jadeit]

Why is there a 3x larger capacitor on the right side of the crystal?

 

[FvM]

No idea, hardly based on manufacturer suggestions, see this Epson application circuit:




Notice that the oscillator circuit should use 74HC04U (unbuffered type), not 74HC04 which can bring up unwanted overtone oscillations.

 

[schmitt trigger]

Where did you find that circuit? Is it an actual, functional equipment or you copied it from a web page?

In addition to what FvM mentioned, the bias resistor seems way too low.

 

[d123]

Hi,

...And there is the remote possibility (worth considering before making a circuit that may need a little inevitable on-pcb fine-tuning at first to get it oscillating) that the person who did the schematic didn't have varicap symbols and forgot to mention it in an innocent sort of way that anyone could.

I've seen that on schematics for the CD4060 crystal capacitors. The few I saw showed a small varicap on one side and a fixed-value pf cap on the other like a 5 - 30 pF and 22pF or 50pF or varicaps of 5 - 30pF on both sides. Also, the resistor values are more 200k and 2k and so on, I'm not an expert at all but it does seem a bit low as basically crystals are relatively delicate.

 

[Jadeit]

Publishet picture is from internet, but I have device manufactured in USA, I have original service manual for this too.
And in this is
SN74HC04N
crystal HC49U from ECX 14.31818MHz
Cl1 24pF and CL2 75pF capacitor are expensive Silver mica,
Resisotr 100k and 100. 5% carbon
Component values are listed in Part list, on schematics and on compoment too.

And I would like to know why Cl1 Cl2 is different and what is good for

 

[d123]

Hi,

I'd guess because 24pF and 75pF works for that circuit. Unlikely to be matched capacitors on a crystal oscillator, in my experience.

"what is good for" - Do you mean silver mica? It has excellent temperature properties and is bla di bla di bla...just read yourself about when to use silver mica capacitors if that was the question.

 

[FvM]

I have no doubts that the circuit is somehow oscillating. Using buffered 74HC04 makes the exact circuit behavior unpredictable, I'm not particularly motivated to guess about it.

 

[Jadeit]

When looking for a response, I found a similarly connected oscillator
Here, C18 is also much larger than that C17, oscillator is source of sinus for class C RF amplifier .
i.e. U1 gate 1 create somehow formed clock,
this clock is amplifier on gate 2-3 and over L4 C20 drive RF amplifier transistor Q3

My question is, why the C18 is considerably larger and how much it affects the exit clock?
What is the program to simulate similar circuits?
For simulation I use Multisim, but this program mot good for crystal circuit simulation because do not include crystal library and I have not equivalent circuits for MHz crystal.

 

[LvW]

Why is there a 3x larger capacitor on the right side of the crystal?

View attachment 146830

I must admit that I do not understand the question.
Are you asking about the VALUE of this capacitor or about its general purpose?
You have a third -order feedback circuit - as required for oscillation - and, of course, the value of all elements determines the oscillation frequency.

 

[FvM]

The circuit dimensioning has to be related to the effective load capacitance specified in the crystal datasheet. The actual circuit capacitance is most likely higher than that, so expect 100 - 200 ppm negative frequency offset.

Repeating the question over and over doesn't help you on. I feel that it has been already answered as far as possible.

 

[Jadeit]

I try simulate Pierce oscillator with 10MHz crystal.
In first step with C1 18pF C2 18pF and in second step with 18pF / 84pF
In first step is positive period 45 and negative 56
In second step is positive period 58 and negative 44.

The conclusion for me, the size of the load capacitors is affected duty cycle in clock .
In other words, unequal load capacitors lead to the production of an asymmetric duty cycle signal.
which is what we supposed, but is contrary to what is said here

I'm sorry to say that, but I need to make it clear how it is
For me it means that Pierce oscillators with asymmetric load capacitors (for example as in my post #8] can not be replaced **broken link removed**
because integrated oscillator is designed for 50/50 Dutty cycle.

 

[FvM]

Did you use TTL gates? They have asymmetric output and behave completely different than 74HC gates. That's the reason for asymmetrical output, not the capacitor.

 

[Jadeit]

You have true with HC it is not works and DC is 50:50.
One more question

See on pic in my post #8
Is possible remove all to the left of the resistance R42 and and replace it with active oscillator for example Epson **broken link removed** ?
if it does, it has some disadventages against the original solution?

Thans for all

 

[d123]

Hi,


(I hope bad/ill-informed help is better than none...)

I hadn't heard of Simple Packaged Crystal Oscillators (SPXO) until the datasheet of yours I think, thanks. Interesting.

Description from SPXO, "SPXO is the simplest Crystal Oscillator with no compensation or control of temperature made by combination of a crystal unit and oscillation circuit. SPXO, especially small products used for clock purpose,"

Hopefully better performance if all components are onboard that IC already; discrete version will have hot and cold spots and corresponding drifts up and down etc. so could never be as "good" as that IC. Crystals need love and stability and ovens even sometimes and however much you put into it, otherwise they'll just be okay, like any other component. Still think that IC will outperform a discrete version.

I'm guessing you want to keep the paralleled inverters as a buffer? I'd be careful, that's a CMOS output and there will be level discrepancies into a BJT package like a 7404 - I'm assuming that package is BJT. On the other hand if you're operating it at 1.6 - 3.3V max. I suppose that won't be much of an issue.

Just in case An identical CMOS inverter is CD4049UB, specifically UnBuffered, however CD4xxx datasheets all say 15MHz max. at Vs or 5MHz at 5Vs...

Maybe the buffer as inverter isn't needed and a (dual) MOSFET totem-pole could suffice as a more powerful buffer instead, anything fast enough really.

 

[LvW]

Hi,
(I hope bad/ill-informed help is better than none...)

....worth to be discussed....perhaps in another thread?

 

[FvM]

Referring to the circuit in post #8, yes you can use an industry standard crystal oscillator. Looking again at the schematic, I notice that 74HC04 is operated beyond its specified supply voltage of 6V. Thus I would suggest to keep hands off this questionable design.

 

[optimatech]

Crystal oscillators can be used both in series resonance (impedance between pin to pin is zero) and in parallel resonance (impedance between pin to pin is infinite).

So capacitor are used with crystal oscillators

 

[LvW]

Crystal oscillators can be used both in series resonance (impedance between pin to pin is zero) and in parallel resonance (impedance between pin to pin is infinite).
So capacitor are used with crystal oscillators

Yes, correct - however, for oscillator applications the crystal is often used as a high-quality inductance (as in the circuit under discussion, see first post).
Here, the feedback circuitry forms a third- order R-C-L-C lowpass.