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I need to replace a crystal component (16.257 MHz, one of its legs broke off and cannot solder it back on) from an old video card that is wired up like the schematic below (the resistor / capacitor / frequency values differ, it's just for reference on how it is connected) :

schematic

Now I read that there is such a thing as a series-resonant oscillator and a parallel-resonant oscillator, but I don't know under what category this falls. However given the schema above, I would like to find a drop-in replacement.

When I search for a replacement part on Mouser, by providing the frequency:

https://www.mouser.be/Passive-Components/Frequency-Control-Timing-Devices/Crystals/_/N-6zu9f?P=1z0wnq5Z1z0z7l5

I noticed that some of them say "load capacitance = 20 pF" others say "load capacitance = series".

I also noticed different form factors. The ones listed here all have a very low form factor,

photo of a crystal in a smaller form factor

while my broken component had a larger profile:

photo of a crystal in a larger form factor

What kind of replacement could I choose? Does the housing (form factor matter) matter, and what about the load capacitance difference or other properties I should be aware of?

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You should use the larger profile case (HC-49) because it is more tolerant of overdriving. If you can't get it, you can use the smaller HC-49/U or HC-49/US, but pick the one with the lowest ESR. The allowable drive level is about half.

For a video card, probably any of them will work. I would pick something with a 15-20pF load capacitance.

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  • \$\begingroup\$ Crystals with rated load capacitance are meant for parallel oscillators. That is a series oscillator, and series crystals do not have load capacitance rating, they just read series in place of load capacitance. \$\endgroup\$ – Justme Mar 14 '20 at 9:12
  • \$\begingroup\$ @Justme The difference in resonant frequency is pretty small so I think it will be fine. I don't see any series resonant crystals in stock at Digikey, for example. \$\endgroup\$ – Spehro Pefhany Mar 14 '20 at 9:15
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Electrically, a crystal looks like this:-

schematic

simulate this circuit – Schematic created using CircuitLab

C1 and L1 produce a series resonance, while C2 and L1+C1 produce a parallel resonance. The two resonant frequencies are very close, but not exactly the same. A crystal specified for operation in one mode will produce a slightly different frequency if operated in the other mode. For many applications the difference (typically less than 0.1%) is not significant.

Here's an LTspice plot of impedance in ohms (left scale, solid line) and phase (right scale, dotted line) for the circuit above:-

enter image description here

Lowest impedance (equal to R1) is achieved at the series resonant frequency (10.002 MHz). This suits an oscillator circuit with a non-inverting amplifier that needs a feedback element with low impedance and small phase shift. External parallel capacitance has little effect because it doesn't change the series resonant frequency (which is determined by the physical properties of the crystal) and phase varies steeply in this area.

Parallel operation is possible in the higher impedance area where phase shift is ~+90°, which suits an inverting amplifier with high input impedance and large phase shift. In this mode the exact frequency can be 'tuned' over a small range with external capacitance. Crystals designed for this usually have the external 'load' capacitance specified.

Your circuit is designed to operate the crystal in series resonance, so a 'series' crystal should oscillate closer to its marked frequency. However a 'parallel' crystal should also work provided its resistance is low enough.

The larger profile case has a bigger crystal in it which can handle higher power without being overloaded (which might cause misoperation and/or reduce its lifespan). You should replace it with one of the same size.

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That is a series resonant oscillator circuit, so you need a series type crystal which runs at the given frequency in series operation. Using incorrect crystal will still work but at slightly wrong frequency, since those crystals run at their rated frequency in a parallel circuit with the rated load capacitance.

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