AR# 4071

Can an FPGA be used in combination with an external crystal oscillator as an amplifier?

Description


Urgency: Standard



General Description:



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.

Solution


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.
AR# 4071
Date 11/16/2011
Status Archive
Type General Article