Inexpensive low phase noise high speed stabilized time base
A low phase noise high speed stabilized time base uses a crystal resonator that operates directly at a desired high frequency of one hundred to several hundred MHz. An inverted mesa AT-cut quartz crystal meets this criteria. To promote frequency stability the crystal and its oscillator circuit are thermally clamped to a convenient temperature that need be only loosely regulated, say, at about room temperature. In an exemplary ATE setting that already provides a water flow heat removal system whose supply side is 26° C.,±0.5° C., a 400 MHz inverted mesa AT-cut crystal is simply given its own loop within that water supply. Other temperature stabilization techniques for loose regulation, such as Peltier cells, may be used. The result is a high frequency time base having adequate frequency accuracy and stability, but with the extremely low timing jitter of just the crystal resonator, since there is no contributory timing jitter from a frequency multiplying PLL.
1 . A circuit for generating a clock signal, the circuit comprising:
an inverted mesa AT-cut quartz crystal resonator;
an oscillator circuit coupled to the inverted mesa AT-cut quartz crystal resonator, oscillating at the fundamental frequency of the inverted mesa AT-cut quartz crystal resonator, and having an output stage producing a clock signal at the fundamental frequency of the inverted mesa AT-cut quartz crystal resonator; and
the inverted mesa AT-cut quartz crystal resonator and the oscillator circuit thermally coupled to an environment having a loosely controlled temperature and that is different from an external ambient environment containing the circuit.
2 . A circuit as in claim 1 wherein the clock signal is at least 100 MHz.
3 . A circuit as in claim 1 wherein the environment having a loosely controlled temperature comprises a water flow heat removal system.
4 . A circuit as in claim 3 wherein the water flow heat removal system is part of an ATE system and also removes heat from DUTs.
5 . A circuit as in claim 1 wherein the environment having a loosely controlled temperature comprises at least one Peltier cell.
6 . A circuit as in claim 1 wherein the loosely controlled temperature is about 26° C.
7 . A circuit as in claim 1 wherein the external ambient environment is allowed to vary over a range of about 40° F.
8 . A circuit as in claim 1 wherein the change in clock signal frequency as a function of the loosely controlled temperature is in the range of about ±20 ppm to ±10 ppm.
9 . A circuit as in claim 1 wherein the circuit further comprises thermal insulation disposed between the circuit and the external ambient environment containing the circuit.