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Can an FPGA be used in combination with an external crystal oscillator as an amplifier?
People familiar with the 3000 series onboard oscillator may want
to try to make an amp with a Spartan or similar FPGA by attaching
a crystal oscillator externally.
It is not recommended to use a crystal oscillator with a XC9500, XC5200,
XC4000E/EX/XL/XLA, Virtex/VirtexE or Spartan/SpartanXL/Spartan-II to
create a clock signal.
Using an FPGA or CPLD to act as a crystal oscillator is wasteful in power,
and also in pins. A ready-made oscillator is more beneficial as it is put together
with the appropriate circuitry to maintain a valid clock signal. It starts up reliably
and consumes a minimum of power.
For all frequencies:
Using two IOBs, one as input, the other as output, results in a multi-stage linear
amplifier with too much gain, too fast a gain roll-off and thus high phase distortion,
all resulting in instability, on top of high power consumption.
Regarding very low frequencies:
A 32 kHz crystal ( used in every modern watch ) has relatively high impedance
and needs more gain than the high-frequency crystals we normally work with. So
don't try to use it with an XC3000 type FPGA.