[time-nuts] Lars GPSDO on EEVblog

[Jim Harman]

Magnus said,

It would be interesting to test the linearity of the TIC separately for

instance.

I have done some testing of the TIC. It works quite well for the Arduino
Uno with its 1 V full scale ADC setting, but the exponential shape of the
RC charging from the 5 V is quite evident if you  use a processor like the
Micro's 32u4 with a 2.56 V sensitivity. This circuit is also sensitive to
any noise on the 5 V supply. The linearity is not too important for this
application as long as it does not affect the loop gain so much that the
loop becomes unstable.

However I have modified the circuit to use a simple 2 ma current source as
shown in the attached schematic, with significantly improved linearity. H/T
to Horowitz and Hill's The Art of Electronics for the idea of using an LED
as the voltage reference for the current source. The diode-connected 2N3904
has less leakage at a small forward voltage than any small-signal diodes I
tested, so the output voltage with a very narrow input pulse is close to
zero.

I could run some tests on my version by setting the oscillator slightly
off-frequency and capturing the resulting sloped TIC output, and would be
happy to share the results if you are interested.
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