[time-nuts] Thunderbolt tuning DAC theory of operation

[Stewart Cobb]

While poking around the Thunderbolt to determine whether -5V could be
used in  place of -12V, I discovered how the OCXO tuning DAC works.
Apologies if this is old news, but I haven't seen it documented
before.

The 10MHz sine wave from the OCXO  is squared up and used to clock the
Xilinx 5200 CPLD (U22) and a 74AC174 hex D flip-flop (U14).  Inside
the CPLD (apparently) the 10 MHz clock is divided by 1024, giving a
square wave with a period of 102.4 us (about 9.7 kHz).  The duty cycle
of that square wave is modulated by the 10 MSBs of the commanded DAC
value.  The LSBs are used to offset the falling edge of the square
wave one clock cycle (100 ns) later, during a fraction of the 9.7 kHz
square waves proportional to the LSBs value.  On a modern digital
scope, you can zoom in on the falling edge of the square wave, set the
display to "average", and see that the averaged height of that clock
cycle is proportional to the DAC LSBs.  There appear to be at least 8
LSBs, perhaps as many as 10, giving a total DAC resolution of 18 to 20
bits.  (If the DAC value is averaged over one second, there are 10^7
clock cycles which can be controlled, giving a theoretical maximum
resolution of 23+ bits.  Trimble may have chosen a shorter averaging
time and fewer bits.)

The PWM square wave travels from pin 13 of the CPLD (U22) to pin 4,
the D1 input of the 74AC174 (U14).  The flip-flops in this chip are
also clocked by the squared-up 10 MHz from the OCXO.  The Q1 output,
pin 5 of U14, goes to one side of R83 in the circuitry around the
LT1014 op-amp.  The other five inputs and outputs of U14 are
constantly high or low.  They may also be fed to the op-amp circuits,
to help it handle the square wave in a purely ratiometric manner.

The inputs and outputs of the Xilinx CPLD can be programmed for many
different I/O standards.  Unfortunately, this makes their output pin
drivers far from ideal.  The purpose of the 74AC174 is presumably to
drive the analog circuitry with a input that is as close as possible
to a mathematically ideal digital signal.  Outputs in the 74AC logic
family can source or sink 24 mA and have relatively balanced raise and
fall times.  This was probably the most ideal digital output available
to the Thunderbolt's designers in the late '90s.

This DAC implementation is guaranteed monotonic, an important
consideration.  There is exactly one rising edge and one falling edge
per cycle, so that any difference between rise and fall times will
have a constant effect which can be tuned out.  Unlike a sigma-delta
DAC, this PWM DAC produces strong spectral lines at multiples of the
9.7 kHz square wave frequency.  On the one hand, it is comparatively
easy to design filters to remove a single frequency (and its
harmonics).  On the other hand, this signal is strong enough that it
may appear in phase noise plots anyway.

If you want to view the 9.7 kHz square wave for yourself, it appears
on a small square test point next to the silkscreen designator for
C78, very close to the 6-pin power input jack.  This test point is
part of the connection from the Xilinx CPLD to the hex D flip-flop.
Probing it does not affect the OCXO tuning.

Hope this helps.

Cheers!
--Stu