# crystal series resonant circuit

Discussion in 'Electronic Design' started by Johnny Chang, Nov 25, 2007.

1. ### Johnny ChangGuest

i'm trying to create a standalone crystal circuit to create a square
wave but all I can find are parallel resonance circuits. the series
resonant circuits don't seem to have a resistor and it is my
understanding that you change the frequency of the crystal by having
different series resistances, listed here:

http://rocky.digikey.com/WebLib/Epson/Web Data/C-2-TYPE,C-TYPE.pdf

does anyone have an example circuit I can use with this crystal? i
dont think i can use a parallel resonance circuit with it because of
the 6pF to infinity statement.

this is the only place i've seen with series resonance:
http://www.geocities.com/SiliconValley/Peaks/3938/uexosc.htm

but i don't understand how to select my frequency of oscillation or
the purpose of the op amps at the end.

2. ### Tom BruhnsGuest

So why, with those crystals, do you think you want to use series
resonance? They are specified for a particular load capacitance,
which to me implies parallel resonance. It's usual to order a crystal
for a particular load capacitance, and if you want to adjust the
frequency, provide a variable capacitor in the load circuit.
Actually, it's not too different for a circuit that uses series
resonance, since a capacitor in series with the crystal will also
adjust the frequency of oscillation, as the net resonant circuit
equivalent capacitance is the series combination of the motional
capacitance and the external capacitance...though you must also
account for the holder capacitance. And be aware that the frequency
of oscillation will depend to some extent on all the external parts as
well as on the crystal itself.

In your second link, I don't see any "op amps at the end." I see only
logic gates. Their purpose is to buffer the crystal oscillator output
so that the oscillator is isolated from changes in load impedance.

Cheers,
Tom

3. ### Tam/WB2TTGuest

Regardless of what your links may say, you do not change the frequency of a
crystal oscillator by changing the resistance. You change it by varying the
capacitor. You can only change the frequency of a crystal about +/-.01% or
so. Also, no crystal oscillator inherently has square wave output. You get a
square wave by having enough gain in the system so that the sine wave gets
squared up.

Tam

4. ### Johnny ChangGuest

i'm confused, the link i gave provides series resistance for frequency
ranges while load capacitance is 6pF to infinity, which doesnt give
any information for getting the right frequency between 20 - 165 KHz.
am i supposed to buy a bunch and experiment? i did not know there were
variable capacitors? for series, they list series resistance, so i
assumed i need some kind of resistor to vary the frequency. but i
suppose it could just be the amount of resistance it looks like.

well, if i can use that crystal in parallel mode, then i will. i was
just confused by the lack of data on the sheet as i was only familiar
with parallel resistance and specific #'s.

as for the circuits, im referring to the other 74LS04 inverters.

are there specific specs i should be looking for inverters based on
frequency? output current?

5. ### Johnny ChangGuest

thanks for the information, i actually want a sine wave output and was
considering running it through a low pass filter afterwards, but with
what you're saying I just need to find a low gain inverter?

6. ### Johnny ChangGuest

thanks for the information, i actually want a sine wave output and was
considering running it through a low pass filter afterwards, but with
what you're saying I just need to make the circuit low gain ?

7. ### Johnny ChangGuest

thanks for the information, i actually want a sine wave output and was
considering running it through a low pass filter afterwards, but with
what you're saying I just need to make the circuit low gain ?

8. ### Guest

If you are wanting a frequency range of between 20 - 165 KHz then you
don't want to use crystals at all unless like you say you'll need a
bunch of \$them\$.

What would be so wrong with using a \$34 Elenco function generator?
http://www.elenco.com/9600K.htm

"op amps at the end." hold the secret to the desired square wave
output. Their gain is so high that on and off ramps become more or
less vertical. But those aren't op amps, they are gates - they are
just drawn like op amps some times are drawn. I'm not bad, I'm just
drawn that way...

9. ### John FieldsGuest

---
No, that's the maximum series resistance _of the crystal_ when it's
oscillating in parallel resonance mode in the various frequency
ranges indicated.

---
View in Courier:

CD4009UB

| \
+---| >O--+
| | / |
| |
+---[1M]---+
| |
+--[XTAL]--+
| |
[2Cl] [2Cl]
| |
GND GND

Where 2Cl is twice the value of the load capacitance specified by
the crystal mfg.

Also, for your perusal, I've posted RCA's ICAN-6539 to
alt.binaries.schematics.electronic.

It's a good read from the good old days, and just might help to
smart you about CMOS and Xtal oscillators.
---
---
That statement indicates that the crystal can be special-ordered
with a _customer-specified_ load capacitance of 6pF upwards.

But, if you want to buy an off-the-shelf crystal, what you'll need
to do is accept whatever load capacitance is specified, by the
manufacturer, for that crystal and make sure your circuit presents
that load capacitance to the crystal.

capacitance since they're series-connected. In other words, if the
each be 32pF.
---
---
You select the desired frequency of oscillation by ordering a
crystal which, when properly loaded, will be ground to operate at
the frequency you want.

Series resonance you don't want, and the opamps at the end are, as
someone else posted earlier, not opamps, but buffers which will
isolate the eventual load from the oscillator's output and keep it
from pulling or pushing the crystal.

10. ### EeyoreGuest

How did you get that idea ? I see nothing in the data sheet you linked to that
even remotely suggests such a thing. And indeed, it's not the case.

Graham

11. ### EeyoreGuest

No it doesn't.

Graham

12. ### EeyoreGuest

Why an *INVERTER* ? You just need a buffer (NOT a logic gate). Mind you, the
crystal circuits I've come across don't produce a very good sinewave.

Graham

13. ### Guest

There is a Phillips chip I ran into when I needed to pull a crystal
transconductance of a long tail pair by varying the tail current.

Since you're building a discrete design, look around for the varactor
based designs.

14. ### Tam/WB2TTGuest

You may not need it. The signal at the output side of the crystal will be a
sine wave. Capacitively couple that to an emitter follower. Bias the base of
the EF at half of VCC.

Tam

15. ### Tam/WB2TTGuest

I have seen TV 3.58 MHz PLLs that work something like that. Changing the
current causes a change in the phase shift of the oscillator.

Tam

16. ### Fred BloggsGuest

Actually it is the xystal ESR at resonance and is a minimum. 50K is
pretty much the standard for that 32KHz thing. Off resonance ESR goes to
infinity and is the main cause of start-up failure. That ESR value only
holds for a small band of frequencies about Fresonant, and the same goes
for the ESC and ESL.

17. ### Tom BruhnsGuest

You need to separate the crystal parameters from the circuit
parameters. The "load capacitance" is a circuit parameter, and is the
capacitance which the crystal "sees" from your circuit. You can
You (or the crystal manufacturer) specify the desired capacitance, and
the manufacturer guarantees oscillation within the stated tolerance
when the crystal is operated in a circuit in which it "sees" that load
capacitance. The parameters in the data sheet you linked to
immediately below that, from "series resistance" to "shock
resistance," are all parameters of the crystal and its holder. Series
resistance is the electrical equivalent circuit resistance for the
crystal. You should be able to find info through a Google search on
the generally-accepted equivalent circuit of a quartz crystal; a first-
order equivalent is a series R-L-C, shunted by the C of the holder.
In actuality, there are other series RLC's resonant at other
frequencies that correspond to minor resonances, but you probably

I'm QUITE sure you can find a LOT more info on crystal oscillators on
the web, through a Google search. You can also find a lot of info in
From a combination of thinking about the circuit, looking at the
available info, and experimenting with some circuits, you should be
able to learn a lot and become much more comfortable with how crystals
and crystal oscillators behave, and what the important parameters are.

18. ### Guest

Bingo. A very clever design.

Most people go with the varactor if you can use one of the cheap types

I don't know why I didn't reference VXCO in the first place when the
topic came up.
http://www.sss-mag.com/cvcxo.html
The M E Ferking reference is quite a book. You need an IEEE membership