Tom wrote:
Section 5.1.2 (Kalman Filtering) on page 5-4 (PDF page 48) says:
Oscillator performance is subject to two basic effects. First, changes
in environmental temperature can cause the oscillator to speed up to
slow down. Second, the oscillator has a natural tendency to drift over
time. This is called aging.
Both temperature and aging can be mathematically predicted. However, the
characteristics vary from crystal to crystal. The Kalman filtering
monitors the unique oscillator performance over time and temperature and
records this behavior.
The DS1620, perched way over in the corner of the board, cannot provide
useful crystal temperature data, even by inference. That would need to
come from within the oven at the very least, more likely from inside the
crystal envelope itself using, e.g., an IR thermometer.
Instead, Kalman filters in GPSDOs track the oscillator frequency
continuously and do their best to separate the drift effects from the
temperature effects using software algorithms (note "mathematically
predicted" in the paragraph above). You can see a hint of this by
monitoring the statistics from one of the older HP units like a Z3801.
Best regards,
Charles
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