[time-nuts] Low cost high resolution software sawtooth error correction

[Dr Bruce Griffiths]

Software correction of the sawtooth timing error of a GPS timing 
receiver can be used to discipline an OCXO at a similar parts cost and 
performance to the hardware sawtooth correction method.
The OCXO sinewave output is sampled by the leading edge of the PPS 
signal and corrected for the sawtooth error in software. The LTC1412 is 
a suitable ADC (~$US12 (1-99) from Linear technology).
The ADC output is 12 bit parallel (for those who want a serial output 
just add a couple of parallel input serial output shift registers) and 
it is best to use a differential input signal (use an RF transformer).
The input sinewave amplitude only has to be known to within a few (5%) 
percent to keep the effective sawtooth correction error due to the phase 
detector gain uncertainty under 1ns.
In principle the ADC can be used to measure the amplitude of its input 
sinewave, such calibration conversions being interleaved between 
successive PPS pulses.
If the OCXO has a wider EFC range than 1E-7 for a 10MHz OCXO then the 
frequency can be divided down to say 1MHz and low pass filtered before 
being used as the analog input to the ADC. The phase detector resolution 
is still less than 1ns.

Alternatively if one has an older OCXO which has drifted so that it can 
no longer be adjusted to its nominal frequency, then using the OCXO 
output frequency as the ADC input frequency can be useful in that the 
OCXO can then be locked to a multiple of 1Hz. Such a OCXO frequency is 
still very useful, however you need to know which multiple of 1Hz the 
OCXO has locked to. Once the OCXO has locked  it is easy to measure the 
frequency using either an external counter or one built into the 
GPSDOCXO control circuitry.

If one wants to produce an exact 10MHz output when the OCXO has locked 
to a different multiple of 1Hz, then an offset generator using a DDS can 
be employed.

The hardware sawtooth can also be used to lock an OCXO to a multiple of 
1Hz if the frequency of the D flipflop input is equal to the OCXO frequency.

Bruce