[time-nuts] DAC for OCXO disciplining

[Poul-Henning Kamp]

--------

In 1974 the "39A Precision Oscillator" for the L5 Coax system
boasted a 14-bit DAC to set the frequency.

The article about it is quite interesting:

        https://archive.org/details/bstj53-10-2097

They spec sort-of-an-avar:

        "This 5.12-MHz precision oscillator is characterized by
        long-term stability of < 1e-10 per day, and by short-term
        stability of <1e-8 for a 1-millisecond sampling time or
        <2e-11 for a 10-second sampling time."

They linearize the EFC with computer-picked resistors:

        "Figure 7 is a simplified schematic of the divider excluding
        the control logic.  Resistors RV1, RV2 and RM1 (or RM2 if
        required) are selected by computer program at time of
        manufacture to precisely give the required nonlinear binary-
        voltage allowing a linear binary-frequency function of
        oscillator output.

And their 14-bit DAC is implemented with TTL driven relays as
"a constant-impedance voltage divider".



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