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Crystal Serie VS Parallele

J

Jacques St-Pierre

Jan 1, 1970
0
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
 
P

Phil Hobbs

Jan 1, 1970
0
Jacques said:
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
 
J

Jacques St-Pierre

Jan 1, 1970
0
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
 
P

Phil Hobbs

Jan 1, 1970
0
Jacques said:
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
 
F

Fred Bloggs

Jan 1, 1970
0
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.
 
J

Joop

Jan 1, 1970
0
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
 
J

Joop

Jan 1, 1970
0
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
 
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