Hi,

Anyone can explain to me the difference in configuration require for a serie 
or parallel crystal pierce oscillator. I wish to use a SB74LVC1404 chip to 
build a oscillator using a 3.000Mhz crystal. The application sheet use a 
parallel load crystal, but the only 3Mhz crystal I can found is serie. I did 
try it, it look to work properly, but I am not sure it will work all the 
time. In the past, I observe that using a serie crystal often result in 
instable oscillator. So what can I do to make sure the oscillator will alway 
start at proper frequency. I wish I can find a proper crystal, but I can not 
at this point, and further more, the right frequency for me should be 
1.5mhz.

Bye
Jacques

Jacques St-Pierre wrote:
> Hi,
> 
> Anyone can explain to me the difference in configuration require for a serie 
> or parallel crystal pierce oscillator. I wish to use a SB74LVC1404 chip to 
> build a oscillator using a 3.000Mhz crystal. The application sheet use a 
> parallel load crystal, but the only 3Mhz crystal I can found is serie. I did 
> try it, it look to work properly, but I am not sure it will work all the 
> time. In the past, I observe that using a serie crystal often result in 
> instable oscillator. So what can I do to make sure the oscillator will alway 
> start at proper frequency. I wish I can find a proper crystal, but I can not 
> at this point, and further more, the right frequency for me should be 
> 1.5mhz.
> 
> Bye
> Jacques
> 
> 
There are two issues here.  One is the series/parallel distinction.  All 
crystals looks like a capacitor below its series resonance and also 
above its parallel resonance.  In the very narrow region between, it 
looks like an inductor.  So if you make an ordinary LC oscillator, and 
replace the inductor with a crystal, you'll get a crystal oscillator 
running at a frequency somewhere between the two resonances.  (This 
assumes that the capacitors are in the right range of values--20 to 50 
pF usually.)

The frequency marked on a 'parallel resonant' crystal is the resonant 
frequency of the tank circuit built from the crystal and the specified 
capacitance.  (Parallel resonant crystals always have a capacitance spec 
as well as a frequency spec, for this reason.)

The second issue is startup.  It's very possible for a poorly-designed 
crystal oscillator not to start reliably, or to start up at the wrong 
frequency.  There are two classes of wrong frequencies: (a) overtones, 
and (b) LC resonances.

Crystals, like guitar strings, have more than one resonance.  Generally 
the higher overtones are harder to use, because the parallel capacitance 
of the electrodes in the crystal tends to swamp the inductive reactance 
more and more at higher and higher overtones.  So if you design your 
oscillator so that it needs a decent amount of inductance, and doesn't 
have too much gain, it'll usually work reliably at the fundamental.

But you have to really design it.  Calculate how inductive your crystal 
gets (minimum and maximum Q specs apply); choose the capacitor values 
accordingly; and calculate the losses, so you can choose the right 
amount of gain in the active device: enough to start reliably from zero 
signal, but not much more than that.

If you don't do the design carefully, you're liable to find that your 
gizmo can oscillate at many frequencies--a few crystal resonances, the 
LC resonance of a poor layout, or the (much higher) frequency where the 
propagation delay and phase shift due to the capacitive load add up to 
180 degrees.  (Low and stable gain is your friend.)  Crystal oscillator 
startup is one place where SPICE isn't that much help, so do the 
algebra.  If you don't know where to begin, find the crystal parameters 
from the data sheet and post them.  One of us will probably be able to help.

If you have a 3 MHz crystal, the easiest way to get 1.5 MHz is to use a 
flipflop as a divide-by-2.

Cheers,

Phil Hobbs

>>
>>
> There are two issues here.  One is the series/parallel distinction.  All 
> crystals looks like a capacitor below its series resonance and also above 
> its parallel resonance.  In the very narrow region between, it looks like 
> an inductor.  So if you make an ordinary LC oscillator, and replace the 
> inductor with a crystal, you'll get a crystal oscillator running at a 
> frequency somewhere between the two resonances.  (This assumes that the 
> capacitors are in the right range of values--20 to 50 pF usually.)
>
> The frequency marked on a 'parallel resonant' crystal is the resonant 
> frequency of the tank circuit built from the crystal and the specified 
> capacitance.  (Parallel resonant crystals always have a capacitance spec 
> as well as a frequency spec, for this reason.)
>
> The second issue is startup.  It's very possible for a poorly-designed 
> crystal oscillator not to start reliably, or to start up at the wrong 
> frequency.  There are two classes of wrong frequencies: (a) overtones, and 
> (b) LC resonances.
>
> Crystals, like guitar strings, have more than one resonance.  Generally 
> the higher overtones are harder to use, because the parallel capacitance 
> of the electrodes in the crystal tends to swamp the inductive reactance 
> more and more at higher and higher overtones.  So if you design your 
> oscillator so that it needs a decent amount of inductance, and doesn't 
> have too much gain, it'll usually work reliably at the fundamental.
>
> But you have to really design it.  Calculate how inductive your crystal 
> gets (minimum and maximum Q specs apply); choose the capacitor values 
> accordingly; and calculate the losses, so you can choose the right amount 
> of gain in the active device: enough to start reliably from zero signal, 
> but not much more than that.
>
> If you don't do the design carefully, you're liable to find that your 
> gizmo can oscillate at many frequencies--a few crystal resonances, the LC 
> resonance of a poor layout, or the (much higher) frequency where the 
> propagation delay and phase shift due to the capacitive load add up to 180 
> degrees.  (Low and stable gain is your friend.)  Crystal oscillator 
> startup is one place where SPICE isn't that much help, so do the algebra. 
> If you don't know where to begin, find the crystal parameters from the 
> data sheet and post them.  One of us will probably be able to help.
>
> If you have a 3 MHz crystal, the easiest way to get 1.5 MHz is to use a 
> flipflop as a divide-by-2.
>

Thanks for the explaination.
I used the ECS-30-S-1X
Here a link for the crystal:

http://www.ecsxtal.com/store/pdf/hc49ux.pdf

I am not sure I am familiar with the computation require to match the 
crystal to the 74LVC1404

http://focus.ti.com/lit/ds/symlink/sn74lvc1404.pdf

For test, I plan to use:

Rs= 2.2m
Rf = 1K
C1 & C2 = 22pf

I was planing to use a simple flipflop to divide the 3Mhz by 2.

Bye
Jacques

Jacques St-Pierre wrote:
>>>
>> There are two issues here.  One is the series/parallel distinction.  All 
>> crystals looks like a capacitor below its series resonance and also above 
>> its parallel resonance.  In the very narrow region between, it looks like 
>> an inductor.  So if you make an ordinary LC oscillator, and replace the 
>> inductor with a crystal, you'll get a crystal oscillator running at a 
>> frequency somewhere between the two resonances.  (This assumes that the 
>> capacitors are in the right range of values--20 to 50 pF usually.)
>>
>> The frequency marked on a 'parallel resonant' crystal is the resonant 
>> frequency of the tank circuit built from the crystal and the specified 
>> capacitance.  (Parallel resonant crystals always have a capacitance spec 
>> as well as a frequency spec, for this reason.)
>>
>> The second issue is startup.  It's very possible for a poorly-designed 
>> crystal oscillator not to start reliably, or to start up at the wrong 
>> frequency.  There are two classes of wrong frequencies: (a) overtones, and 
>> (b) LC resonances.
>>
>> Crystals, like guitar strings, have more than one resonance.  Generally 
>> the higher overtones are harder to use, because the parallel capacitance 
>> of the electrodes in the crystal tends to swamp the inductive reactance 
>> more and more at higher and higher overtones.  So if you design your 
>> oscillator so that it needs a decent amount of inductance, and doesn't 
>> have too much gain, it'll usually work reliably at the fundamental.
>>
>> But you have to really design it.  Calculate how inductive your crystal 
>> gets (minimum and maximum Q specs apply); choose the capacitor values 
>> accordingly; and calculate the losses, so you can choose the right amount 
>> of gain in the active device: enough to start reliably from zero signal, 
>> but not much more than that.
>>
>> If you don't do the design carefully, you're liable to find that your 
>> gizmo can oscillate at many frequencies--a few crystal resonances, the LC 
>> resonance of a poor layout, or the (much higher) frequency where the 
>> propagation delay and phase shift due to the capacitive load add up to 180 
>> degrees.  (Low and stable gain is your friend.)  Crystal oscillator 
>> startup is one place where SPICE isn't that much help, so do the algebra. 
>> If you don't know where to begin, find the crystal parameters from the 
>> data sheet and post them.  One of us will probably be able to help.
>>
>> If you have a 3 MHz crystal, the easiest way to get 1.5 MHz is to use a 
>> flipflop as a divide-by-2.
>>
> 
> Thanks for the explaination.
> I used the ECS-30-S-1X
> Here a link for the crystal:
> 
> http://www.ecsxtal.com/store/pdf/hc49ux.pdf
> 
> I am not sure I am familiar with the computation require to match the 
> crystal to the 74LVC1404
> 
> http://focus.ti.com/lit/ds/symlink/sn74lvc1404.pdf
> 
> For test, I plan to use:
> 
> Rs= 2.2m
> Rf = 1K
> C1 & C2 = 22pf
> 
> I was planing to use a simple flipflop to divide the 3Mhz by 2.
> 
> Bye
> Jacques
> 
> 
> 

Well, they don't seem to give much info about the circuit's gain, do 
they?  They do say that the gain is a function of frequency, so for a 
one-off, you could try changing the supply voltage and adjust Rs to get 
reliable starting and frequency keeping over the whole available range. 
    A hair dryer and some freeze spray will help ensure that it works 
over the temperature range you care about.  (Don't blast it too hard 
with the freeze spray, or you're liable to cause cracks in some of the 
components.)

The crystal wants a 32-pf load capacitance, so your capacitor values are 
probably too low.  The load capacitance seen by the crystal is basically 
C1 || (C2+Cin), where Cin is the input capacitance of the oscillator 
chip, so you probably want something more like 56 pF for C1 and 68 pF 
for C2, which works out just about exactly with Cin=6 pF (typical value 
from the datasheet).  You can adjust the gain by trading off the C1/C2 
ratio.

The datasheet says to try the circuit with the crystal replaced by the 
equivalent series resistance (200-300 ohms in your case), which is a 
good idea.

You might very well need to increase the value of Rs since you're 
running at a low frequency, where the chip's gain is high.

If you need to adjust the frequency to be exactly right, you can reduce 
the tank capacitor values a little (say 56 and 56 pF) and put a 1-5 pF 
trimmer cap in parallel with the crystal.  Watch out for the capacitance 
of the pads and traces, which also have to be factored in, and don't use 
a trimmer that's anywhere near the value of the tank capacitors, or 
you'll have problems.

Cheers,

Phil Hobbs

 > Anyone can explain to me the difference in configuration require for
 > a serie or parallel crystal pierce oscillator. I wish to use a
 > SB74LVC1404 chip to build a oscillator using a 3.000Mhz crystal. The
 > application sheet use a parallel load crystal, but the only 3Mhz
 > crystal I can found is serie. I did try it, it look to work properly,
 > but I am not sure it will work all the time. In the past, I observe
 > that using a serie crystal often result in instable oscillator. So
 > what can I do to make sure the oscillator will alway start at proper
 > frequency. I wish I can find a proper crystal, but I can not at this
 > point, and further more, the right frequency for me should be 1.5mhz.

That doesn't make any sense, the parallel mode crystals are far more 
popular than series. Brush up on your search skills. The two types are 
identical in construction but tuned differently. Series mode crystals 
are designed for circuits that resonate at the series frequency, where 
the crystal appears as a pure resistance of low value and is loaded by a 
low resistance load at its output. There is 0o phase shift through the 
crystal at this frequency, meaning the buffer is non-inverting. The 
parallel mode crystal is intended to be loaded by a high impedance, it 
is combined with other reactances to produce 180o phase shift and 
requires an inverting buffer. You can go ahead and stick the series 
resonant crystal in the parallel mode circuit, but the frequency will be 
off. Otherwise, stability and immunity to spurious oscillations should 
be the same, if you're having these problems with a series mode crystal, 
you will also have them with the parallel mode type, in the same 
circuit. The usual method of eliminating the spurious and overtone 
response is to RC filter the gate drive into the crystal, hence the 
popularity of the Pierce configuration.

> Thanks for the explaination.
> I used the ECS-30-S-1X
> Here a link for the crystal:
> 
> http://www.ecsxtal.com/store/pdf/hc49ux.pdf
> 


Apparently you have a knowledge gap here, those are parallel resonant 
mode crytals, this is inferred from the "load capacitance" specification.

On Thu, 20 Mar 2008 14:11:54 GMT, "Jacques St-Pierre"
<m...@globetrotter.net> wrote:
>Thanks for the explaination.
>I used the ECS-30-S-1X
>Here a link for the crystal:
>
>http://www.ecsxtal.com/store/pdf/hc49ux.pdf
>
>I am not sure I am familiar with the computation require to match the 
>crystal to the 74LVC1404
>
>http://focus.ti.com/lit/ds/symlink/sn74lvc1404.pdf
>
>For test, I plan to use:
>
>Rs= 2.2m
>Rf = 1K
>C1 & C2 = 22pf
>
>I was planing to use a simple flipflop to divide the 3Mhz by 2.
>
>Bye
>Jacques
>
>
There is technically no difference between a parallel and series
crystal. Each crystal operates at or near its (series) resonance
point. If the oscillator circuit is designed to look capacitive to the
crystal, it will operate on the high, inductive side of it resonance
point.
The (bad) terms parallel mode crystal or parallel mode oscillator
refer to this capacitive load aspect. The crystal manufacturer
incorporates this by manufacturing the crystal a bit lower in
frequency so it will operate at the rated frequency using a certain
capacitive load. The proper load is specified in the data sheet.

The popular Pierce circuit is such an oscillator. Like Phil said,
start with about double the specified load capacitance as values for
C1,C2 and tune (one of) them if you really need to be exactly on
frequency. Your Rs of 1K is a good starting point to keep the crystal
power below the 1mW of the data sheet.

Joop

On Thu, 20 Mar 2008 12:33:47 GMT, "Jacques St-Pierre"
<m...@globetrotter.net> wrote:

>Hi,
>
>Anyone can explain to me the difference in configuration require for a serie 
>or parallel crystal pierce oscillator. I wish to use a SB74LVC1404 chip to 
>build a oscillator using a 3.000Mhz crystal. The application sheet use a 
>parallel load crystal, but the only 3Mhz crystal I can found is serie. I did 
>try it, it look to work properly, but I am not sure it will work all the 
>time. In the past, I observe that using a serie crystal often result in 
>instable oscillator. So what can I do to make sure the oscillator will alway 
>start at proper frequency. I wish I can find a proper crystal, but I can not 
>at this point, and further more, the right frequency for me should be 
>1.5mhz.
>
>Bye
>Jacques
>
If it works, I would not worry too much. The crystal types are
basically the same. The only difference is perhaps that the frequency
of a series crystal might be a bit high in a Pierce circuit. How much
accuracy do you need?
About your problems in the past, this might have been with cheap
computer crystals that may have had a bit high series resistance.
Increasing the drive level could probably have fixed that. In general
keeping the drive level low enough makes sure the crystal survives
forever and does not drift much. Making it too low might give trouble
starting or make the oscillator a bit noisier. What is your
application?

Joop